Digestion

Question 1

Digestion

Answer 1

Digestion is the chemical and mechanical breakdown of food into nutrients. The function of the digestive system is controlled by the nervous system and a variety of hormones.

Question 2

Mouth and esophagus

Answer 2

We break down the food with our teeth mechanically by chewing. The ground food in the mouth is called a bolus. The saliva, secreted by salivary glands, contains enzymes which digest the food chemically. Saliva is also made of mucous which coats the bolus. This makes swallowing the bolus into the esophagus easier. With wave-like movements (peristalsis action) the esophagus transports the food into the stomach.

Question 3

Stomach

Answer 3

The stomach is a hollow and muscular organ with an inner layer of expandable folds. These folds are called rugae and the hollow space is called the lumen. The stomach expands according to how much food and fluid we have in the lumen. This allows the stomach to store food for a short time. The muscular contractions have further functions; they physically grind and mix the bolus into smaller particles (called chyme) and they regulate the emptying of the chyme into the small intestine. In the stomach the enzymatic digestion is initiated by the secretion of gastric juice by specific glands. The gastric juice contains hydrochloric acid, mucous, enzymes and hormones. The hydrochloric acid (gastric acid) activates digestive enzymes. Mucous is secreted to protect the stomach wall from damage by the acid.

Question 4

Small intestine

Answer 4

The small intestine has an upper part (duodenum), a middle part (jejunum) and a lower part (ileum). In the upper part the mixing of chyme with digestive fluids from the liver and pancreas results in further chemical digestion taking place. The absorption of nutrients begins here and this is the main function of the middle and the lower part of the small intestine. Absorption is the process where nutrients enter the bloodstream, either by diffusion or active transport. The wall of the small intestine is wrinkled and on each wrinkle there are small, finger-like structures called villi. Every single cell of the small intestine also has finger-like structures attached which are called microvilli. The function of the wrinkles, villi and microvilli is to increase the surface area that can absorb nutrients. The small intestine also transports undigested food and unabsorbed nutrients to the large intestine by characteristic contractions of intestinal muscles.

Question 5

Liver and gallbladder

Answer 5

In the process of digestion and absorption the liver has two functions:
@ the production and secretion of bile for digestion into the small intestine
@ the production of lymph for the transport of fat.
Bile is a complex fluid containing bile acid, cholesterol, bile salt, electrolytes, enzymes and fatty acids. The function of bile is to digest fat. When there is no need to digest fat, for example, when people are fasting, bile is stored in the gall bladder.

Question 6

Pancreas

Answer 6

The pancreas produces a mixture of digestive enzymes and fluids that is secreted into the upper part of the small intestine. The fluids neutralize gastric acid which enters the small intestine with the chyme, therefore the fluids protect the wall of the small intestine from acid damage.

Question 7

Large intestine

Answer 7

The large intestine is wider but shorter than the small intestine. The surface of the wall is characterized by intestinal glands instead of villi and microvilli. Digestion no longer takes place in the large intestine and most of the nutrients have been already absorbed. In the large intestine water and electrolytes from the chyme are absorbed. This contributes to the regulation of the water balance in the body. It also is the main organ in the formation of solid faeces. The large intestine can store faecal matter until it is discharged by intestinal muscle movements. The large intestine is important in the absorption of vitamin K, produced by the gut bacteria. The gut bacteria play an important role in the breakdown of undigested carbohydrates.

Question 8

Digestive enzymes

Answer 8

Enzymes are a class of proteins that support biochemical reactions, that is, they speed up or catalyse those reactions.

Digestive enzymes are essential for the breakdown of carbohydrates, fats and proteins into small, absorbable molecules. For each macronutrient there are specific enzymes. Digestive enzymes are produced and secreted by salivary glands, stomach, pancreas, liver and small intestine.

Digestive enzymes are secreted in an inactive form and are only activated at the site of function to protect the secretion organs from any damaging, premature enzymatic action. Enzymes work most efficiently when the environment is optimal in temperature and pH value. The optimum temperature and pH is different for each enzyme. For example, different parts of the digestive system have a specific pH. This determines which enzymes we can find from mouth to large intestine and where the macronutrients are digested and absorbed.
Mouth pH: 5.5-7.5. Stomach pH: 1.0-4.0. Small intestine pH: 6.0-8.0.

Carbs: Amylase (Salivary glands and stomach), Sucrase, maltase, isomaltase, lactase (small intestine).
Protein: Pepsin (Stomach), Trypsin, chymotrypsin, elastase, carboxypeptidase A, carboxypeptidase B (pancreas), peptidase (small intestine).
Fat: Lipase (Salivary glands in infants, stomach, pancreas), colipase, phospholipase (pancreas), cholesterol esterase (liver).

Question 9

Lactose intolerance

Answer 9

Lactose is the naturally occurring sugar in milk. Lactose is digested by the enzyme lactase in the small intestine. Lactose intolerance occurs when lactase is not produced in sufficient amounts and the milk sugar escapes digestion and enters the large intestine. In the large intestine the presence of lactose can cause bloating, pain, discomfort, cramps, diarrhoea and nausea. Only babies need the enzyme to ensure the digestion of breast milk. When they grow older lactase production reduces. However, the change in diet that came about with the domestication of animals and the consumption of animal milk and dairy products led to older children and adults starting to produce the enzyme. Some populations, such as Asian, Native American, African and Mediterranean people, are more lactose intolerant than northern European people. For example, 90% of the Chinese adult population and more than 50% of South Americans have problems with the digestion of milk while only about 5% of northern Europeans are lactose intolerant. The symptoms of lactose intolerance can be prevented, either by consumption of industrially modified lactose-free milk and dairy products or by taking the enzyme in tablet form before having a meal.

Question 10

Digestion and absorption of water

Answer 10

@ Water is an effective solvent and this allows it to transport nutrients to cells, remove waste products from cells and transport other metabolites produced by cells such as hormones.
@ Water allows us to redistribute heat around the body and reduce body temperature through evaporation from the surface of our skin as we sweat.
@ Water makes an excellent lubricant as it is difficult to compress. For example, it is present around sliding surfaces in the body such as joint spaces and around tendons and muscles.
@ Finally water provides the aqueous medium essential for the biochemical reactions of metabolism inside and outside cells.

Question 11

Digestion and absorption of fats

Answer 11

Fat

A challenging characteristic of fat for digestion and absorption is that it is not soluble in water, but has to be transferred through a watery environment in chyme (found in the stomach and small intestine), intestinal fluids, cytosol, lymph and blood.
Furthermore, the fat-specific digestive enzymes are all water-soluble and insoluble in fat so that they cannot access the molecules for their breakdown. The following processes overcome this problem.

@ Emulsification In the process of emulsification fat is dispersed into small globules. Stomach movements initiate emulsification which is completed in the small intestine. Bile, secreted from the liver and gall bladder into the small intestine, is required to emulsify dietary fat. Bile-coated fatty droplets can now circulate in fluids. Emulsification increases the surface area accessible to fat-hydrolysing enzymes. The products of fat digestion are free fatty acids, monoglyceride, cholesterol, cholesterol ester and lysophospholipids.

@ Micelle formation A micelle is a fat particle which consists of about 20 fat molecules. With bile acid all products of the fat digestion including fat-soluble vitamins form so-called mixed micelles. The “water-hating” (hydrophobic) part of fat digestion products is located in the center of the micelle while the “water-loving” (hydrophilic) parts form the surface. This enables the micelle to travel to the intestinal cell where the molecules diffuse into the cytosol. Fats eventually enter the blood via the lymph system.

@ Repackaging into chylomicrons The majority of fat is absorbed in the middle and lower part of the small intestine. Glycerol and fatty acid chains with less than 12 carbon atoms diffuse directly through the intestinal cell into the blood stream. Longer chain fatty acids, cholesterol, monoglycerides, and lysophospholipids are resynthesized into triglycerides, phospolipids and cholesterol esters in the cytosol. Together with specific transport proteins (apolipoproteins) resynthesized molecules are then incorporated into chylomicrons. Chylomicrons are another form of fat droplet; needed to transfer the water-insoluble molecules into the lymphatic vessel and later into the blood.

Question 12

Digestion and absorption of proteins

Answer 12

Proteins

Before proteins can be absorbed they need to be broken down into small peptides and amino acids. There are numerous enzymes involved in the digestion of proteins. Most of the enzymes act on different parts of the molecular structures depending on the chemical characteristics of each protein.

The digestion of proteins starts in the stomach where the enzyme pepsin breaks proteins into larger peptides. About 15 percent of the dietary proteins are digested in the stomach. Most protein digestion takes place in the small intestine involving enzymes (proteases) secreted by the pancreas and brush-border enzymes. The pancreatic protease breaks proteins down into smaller peptides which are further digested into amino acids and peptides with chains of two to four amino acids by the brush-border proteases.

Amino acids are absorbed in the middle and lower part of the small intestine. They are carried across the intestinal cell into the blood by a range of mechanisms (passive diffusion, facilitated diffusion or active transport) depending on the type of amino acid, that is, whether they are hydrophobic, acidic, basic, neutral or aromatic. Most of the transporters are sodium dependent co-transporters. Peptides, not longer than four amino acids, enter the cytosol with support of an H+-dependent active transporter. In the intestinal cytosol these peptides are hydrolysed by cytoplasmatic peptidases into amino acids.

Question 13

Digestion and absorption of carbs

Answer 13

Carbohydrates

Carbohydrates are digested and hydrolysed to the sugars glucose, fructose and galactose.
The digestion of carbohydrates begins in the mouth. Complex carbohydrates such as starch and glycogen are broken down enzymatically by salivary amylase.

However, once the food enters the stomach the low pH value of the stomach inhibits the enzyme.

A more important role in the digestion of carbohydrates is played by the pancreatic amylase which is secreted into the upper part of the small intestine.

Amylase hydrolyses carbohydrates into the oligosaccharides maltose, maltotriose and a-limit dextrin. The enzymes needed to digest these molecules further are located in the microvilli-rich cell membrane, also called the brush-border membrane, of the small intestine. Maltase and isomaltase hydrolyse maltose, maltotriose and a-limit dextrin to glucose molecules. Other carbohydrates such as sucrose and lactose which enter the small intestine undigested are broken down by specific brush-border enzymes. Lactose is digested by lactase into glucose and galactose molecules, and sucrose is digested by sucrase into glucose and fructose molecules.

Monosaccharides are mostly absorbed in the upper and middle part of the small intestine. They travel through the brush-border membrane and the cytosol of the absorptive cells, pass the basolateral membrane and enter the capillary blood system.

Glucose and galactose are transported actively, which means that the molecules pass the cell wall with help of a transporter located in the brush-border membrane. The transport requires energy which is generated by the transporter called sodium glucose co-transporter (SGLT).

Fructose passes the intestinal wall by a process called facilitated diffusion. Another transporter (GLUT5) supports the transfer process of fructose into the cytosol.

Glucose, galactose and fructose cross the basolateral membrane of the small intestine cells in a similar way to that used by fructose to enter the cell but facilitated by a GLUT2 transporter. All GLUT transporters transport the monosaccharides from the site with a high concentration (lumen of small intestine or cytosol) to the site with a low concentration (cytosol of intestinal cell or blood).

There are a few carbohydrates that escape digestion in the small intestine. A reason for this is that these carbohydrates have a particular chemical structure where the bonds can not be hydrolysed by the enzymes of our digestive system. Another reason is that these carbohydrates are enclosed in whole grains or are otherwise physically inaccessible for the digestive enzymes, for example, in fiber. These carbohydrates pass into the large intestine where they are digested by bacteria. In this so-called fermentation process carbohydrates are broken down to short-chain fatty acids. These two- to four-carbon fatty acids are easily absorbed by the cell of the large intestine.

Question 14

Body water and balance

Answer 14

Around 50 to 70 per cent of total body mass is made up of water. The figure can vary greatly between people depending on how much body fat they have. The reason for this is that the fat present inside fat storage cells (called adiposites) does not contain any water. Therefore in overweight people a large proportion of body mass can be made up of tissue containing little water. Fat-free tissue on the other hand is comprised of 60 to 80 per cent water, so the leaner we are, the greater the percentage of our body mass that is water.

Intracellular (ICF) and extracellular (ECF) fluids are not just defined by their different locations; they are also very different in terms of the composition of solutes. One principle difference is that in ICF potassium (K+) salts dominate while in ECF sodium (Na+) salts dominate. The resulting concentration gradients across cell membranes are maintained by active transport (requiring ATP) which results in substantial, continuous energy expenditure. Although different in composition the overall concentration or osmolarity of ICF and ECF is the same.

Day-to-day fluctuation in body mass is relatively small; even though there is a turnover of around 2.5 liters of body water per day in healthy people there is usually no substantial net gain or net loss of water. In non-exercising people the water losses of around 2.5 liters per day are replaced through ingestion of food, drinking fluids and oxidation of substrates (metabolic water).

Negative feedback regulates water balance

When water balance is threatened and there is a net loss of body water, the concentration of body fluid increases. This change is detected in the hypothalamus and it responds by doing two things:
@ activates the sensation of thirst increasing the desire to drink fluids
@ secretes anti-diuretic hormone (ADH) which causes the kidneys to retain fluids and reduce urine production.
These two mechanisms ‘gain and retain’ water; the consequence is increased water availability in ECF. The resulting dilution of solutes in ECF is detected in the hypothalamus and the response is the opposite of that described above. Thirst is ‘switched off’ and ADH secretion is reduced.
The mechanisms above are a good example of how subtle changes in a biological variable are monitored by receptors which trigger a response that corrects the detected fluctuations within a remarkably narrow range of normal functioning (set point). This process is called negative feedback.

Question 15

The kidney

Answer 15

The kidney controls retention and loss of water. Water and electrolytes are small molecules and are physically filtered from blood cells and large molecules in the glomerulus. This filtered fluid moves into the descending loop of Henlé into the medulla of the kidney.

@ The wall of the descending loop is permeable to water but not electrolytes. Since the surrounding medulla has a high osmolality water is absorbed passively into the medulla due to the concentration gradient; this increases the concentration and reduces the volume of fluid in the tubule.

@ The wall of the ascending limb of the tubule actively transports sodium chloride but is impermeable to water. Sodium chloride, but not water, is transported out of the fluid in the tubule therefore resulting in redilution of the now reduced volume of fluid.

@ In the collecting duct the reabsorption of water occurs and it is at this point that ADH is involved in regulation. The presence of ADH increases the permeability of the collecting duct wall increasing passive water reabsorption and reducing urine volume. Thus it is this phase which dictates the final urine volume and concentration.

Question 16

Hyponatremia

Answer 16

Hyponatremia is a condition where the concentration of sodium in body fluid is too low. It is classified as a plasma sodium concentration <135 mmol.L21.

One effect of this is transport of water into cells. Brain cells, confined by the skull, are adversely affected by the increased pressure and this can lead to death in severe cases. Hyponatremia can arise in a number of clinical situations. Exercise is associated with hyponatremia under certain conditions and this has been implicated in occasional deaths. Many people, however, develop hyponatremia without symptoms.

Examples of hyponatremia linked to exercise
Boston Marathon 2002
13% of a sample of 489 runners developed hyponatremia.
London Marathon 2003
12.5% of a sample of 88 runners developed hyponatremia.

How might this arise? Losing salt and fluid through sweating in hot conditions and replacing only with water, thus ‘diluting’ the blood, has been suggested as a possible cause. This makes the idea of salt-containing sports drinks seem logical. However, it is not that simple! Current data suggest that exercise-associated hyponatremia is caused principally by excessive drinking of fluids during long-duration exercise (sometimes evidenced by gain rather than loss of weight during exercise). Consuming commercial sports drinks may make little difference since the concentration of Na+ is very low. One reason for this is sales-related; salty-tasting drinks are less desirable and may reduce sales among the general population, the biggest share of the market. There has been some controversy about whether the involvement of commercial sports drink manufacturers has influenced the availability of scientific information on how common hyponatremia is and advice on drinking behaviour during exercise (Shephard 2011; Noakes 2011). Any kind of sponsorship by a commercial company represents a conflict of interest which should be examined carefully.
@ A company may deliberately or inadvertently downplay scientific evidence that contradicts what they say about their products.
@ A company may be more likely to draw attention to scientific evidence that supports their products.

Question 17

Hydration in athletes

Answer 17

The sensation of thirst is an indicator that our hydration status is not optimal and that we need to take in fluid to restore homeostasis. However, sometimes there is a need to have a more precise measure of hydration status, for example, in certain groups of patients or athletes. While becoming dehydrated is a potential health risk for all, a principle concern for athletes is that being dehydrated can impair performance in both competition and training. Current thinking is that athletes should aim to keep net fluid losses within an amount not exceeding two percent of their body mass. A number of methods exist to monitor hydration status; changes in body mass is one of the simplest.

Using body mass to monitor dehydration—a simple example
An athlete’s fluid loss and fluid replacement strategy is being monitored during one of their training sessions. Their body mass is monitored before and after training and their drinking behaviour recorded. They attend training with a typical 750ml drink bottle and consume all of this during training. Body mass prior to training 75.8kg Body mass after two hours training 74.1kg Fluids consumed during training 750ml Urine produced during training 0ml From this data we can estimate:
Total water loss from sweating (without fluid replacement)…
75.8 – 74.1 + 0.75 = 2.45L
Total water deficit remaining after fluid replacement (750ml)…
75.8 – 74.1 = 1.70L
This deficit is equivalent to…
1.70 / 75.8 = 2.2% of total body mass
Analysing urine to monitor dehydration–simple approaches
The colour of urine can be used as a subjective indicator of dehydration, with a darker colour suggesting dehydration.
Use of colour scales can assess this more objectively.
A hydrometer measures the specific gravity of urine and offers a simple way of assessing the concentration of urine. Using an osmometer to measure freezing point in urine. Increased solute concentration reduces the freezing point and this can be used to quantify the osmolarity of urine.

The analysis of urine offers another means of monitoring the hydration status of athletes. Loss of body water results in smaller amounts of more concentrated urine due to the effect of ADH. This means that concentrated urine is indicative of a dehydrated state and this is easily seen just by looking at the colour. Large amounts of pale urine are associated with a normal hydration while small amounts of darker–coloured urine indicate risk of dehydration.

Why do athletes need more fluid?
Water balance can be disrupted dramatically when we exercise or are exposed to hot environmental conditions. Much of the metabolic heat generated from muscle contraction during exercise is lost to the environment because of evaporation of sweat from the surface of the skin. Therefore sweat losses, which are small at rest, tend to be greater during exercise. Similarly, exposure to a warm climate will cause water loss due to an increased sweat rate. Both of these scenarios mean that fluid intake must be increased to compensate for losses and maintain fluid balance.

When training and exercise occur in a hot climate maintaining fluid balance is even more challenging and can mean fluid intakes of up to 10 to 15 liters of water per day. Exercising harder means increased rates of metabolic heat production which needs to be lost to the environment to control body temperature. Sweating faster helps achieve this but increases dehydration. This graph shows how faster running speeds up sweat loss; combining this with a hot environment makes the situation worse and fluid balance is harder to maintain. Dehydration during participation in sport and exercise is often unavoidable, however, the degree of fluid loss must be controlled. Exercise in a dehydrated state has health risks (e.g. heat stroke) and it can also impair sporting performance. Current evidence suggests that athletes should aim to not drop more than two percent of their body mass due to fluid losses.

Question 18

Energy intake and physical activity

Answer 18

Energy intake is confined to the chemical energy we ingest in foods. It is the macronutrient content (carbohydrate, protein and fat) of food which influences energy content.
@ Carbohydrate 1760 kJ.100g21
@ Protein 1720 kJ.100g21
@ Fats 4000 kJ.100g21
For example, foods which are high in fat are referred to as “energy dense” since fat has a higher energy content than both carbohydrate and protein. High energy content is directly linked to the ability to generate a large amount of ATP.
Energy expenditure There are three routes by which the body expends energy:
@ basal metabolic rate (BMR)
@ thermic effect of feeding (TEF)
@ thermic effect of physical activity (PAL).

The basal metabolic rate (BMR) refers to the minimum energy requirement for maintenance of biological activity in the body, in other words to stay alive!

These basic demands are the repair of body tissues, brain activity, membrane transport and the energy requirement for breathing and circulation. Since all metabolism in the body is ultimately aerobic BMR can be estimated by measuring the rate at which we consume oxygen from the air while at rest. Utilization of 1 liter of oxygen equates to expenditure of around 20 kJ of energy. Thermic effect of food This describes the energy needed to process food, that is, the energy needed to digest food and to absorb, transport and store the nutrients derived from it. This can be measured using the same approach as BMR whereby energy expenditure at rest is compared after fasting and after eating a meal.

Physical activity is defined as any muscle-driven movement which increases energy expenditure. This can include subconscious activities like fidgeting, or conscious efforts involving movement such as day-to-day activities, activity at work, walking and more vigorous efforts such as running, exercise training or participation in sports. Because there is so much variation in people’s levels of physical activity this domain of energy expenditure is the most variable.
The longer we exercise for, and the higher the exercise intensity, the greater the energy expenditure.

Question 19

Relationship between intake and expenditure

Answer 19

Energy intake only occurs intermittently throughout the day when we eat food. However, we constantly expend energy and the rate at which this occurs is very variable. Therefore between meals the only way to ensure that we have sufficient energy to meet our requirements is to have stores of energy available in the body which can be used to synthesize ATP via the energy systems.
Therefore our energy stores in the body even out the fluctuations in energy intake.

The energy stores in the body comprise fats which we store in adipose tissue under the skin and carbohydrates in muscle and the liver. Although proteins in the body are plentiful and can be used as an energy source, they are not an energy store as such. This is because all proteins present in the body have a distinct biological role—breaking these down to meet an energy requirement will always be at the expense of other biological requirements and functions.

Question 20

Energy balance

Answer 20

In the long term, energy intake and energy expenditure tend to be closely balanced in healthy adult humans, hence this relationship is referred to as the energy balance. In this state body mass does not change. If people habitually ingest food with an energy content which is greater than their total energy expenditure then chemical energy is stored in the body. Likewise if the diet has an energy content which is less than total energy expenditure then the energy stores must compensate for the deficit. Therefore disturbing the energy balance causes either a net gain in body mass or a net loss in body mass.

Question 21

Body composition

Answer 21

When body mass gets heavier or lighter we can not be certain what has changed without exploring the composition of the body. Conversely the composition of the body can change without necessarily influencing body mass in a noticeable way.

Body composition is complex as there are many different tissues and materials which make up the body. A simple approach to describe body composition involves estimating the fraction of body mass which is made up of fat mass (FM) and the remainder which is termed fat-free mass (FFM).

How can FM and FFM be measured?
Expressing the amount of FM and FFM in kilograms or as a percentage of body mass is a simple idea but the actual measurement of this is problematic. The only truly reliable means of achieving this is by directly quantifying the amounts of tissues in the body. This can be done by the dissection of cadavers but of course this cannot be used on living people! Therefore more indirect approaches are needed but these give a less accurate measurement. What are the typical levels of FM and FFM?

The amounts and relative proportion of FM and FFM vary with gender, age, genetics, diet and level of physical activity. Women tend to have a greater proportion of FM than men. If FM is either too low (<5% for men and <12% for women) or too high then health is threatened. For competitors in a range of sports the gender difference persists but there is also variation in FM and FFM across different sports

These data suggest that a low body fat may be important for weight-bearing endurance sports like running a marathon, “anti-gravity” sports like the long jump and aesthestic sports like gymnastics. However it seems even a relatively high body fat may be not a disadvantage for power sports like shot-put.

All different shapes and sizes
All aspects of shape, size and composition of the body are linked to sporting success. Observation of athletes reveals obvious differences between sports, with endurance athletes tending to be small and slender while strength and power athletes tend to be muscular. In strength and power sports where body mass is not a limiting factor, the athletes may also have high levels of adipose tissue. This suggests there is optimal body morphology for success in different types of effort.

What if body morphology differs from what is “ideal” for success in a sport?

If we are too tall or too short there is of course nothing that can be done. However, body mass and body composition can be influenced, to some degree, by a combination of training and dietary behaviours.
@ Gaining muscle mass (increasing FFM) Appropriate strength training programmes can cause muscle hypertrophy. Bigger muscles mean more FFM. On the one hand this may increase muscle strength but there is a trade off since this must be offset against an increase in body mass. Not only must the correct training take place but the athlete must be in a positive energy balance and this must include an adequate protein intake. Any change occurs slowly over a long period of time.
@ Reducing fat mass (decreasing FM) Fat mass represents “dead weight” so a reduction may benefit performance in some circumstances. However the low energy intake associated with lean athletes, particularly women, has a number of potentially serious health problems.
@ Dehydration In sports such as rowing and judo athletes may compete in weight categories. Since an official weigh-in will often take place several hours before competition, there is a tendency for some competitors to deliberately restrict food and fluid intake in order to temporarily achieve a body mass which makes them eligible to compete in a weight class below their “normal” day-to-day body mass. The aim is to attempt to gain a weight advantage over an opponent, however, the trade off here is dehydration and low energy stores, both of which threaten rather than benefit performance. But the consequences can be much more serious than poor performance (see case study).

In 1997 three collegiate wrestlers died after rapid weight loss before an official weigh-in. “On November 21, over a 4-hour period, a 22-year-old man in Wisconsin attempted to lose 4 lbs to compete in the 153-lb weight class of a wrestling tournament scheduled for November 22. His preseason weight on September 6 was 178 lbs. During the next 10 weeks he lost 21 lbs, of which 8 lbs were lost during November 17–20. On November 21 at 5:30 a.m., he wore a vapour–impermeable suit under a cotton warm-up suit and exercised vigorously in a hot environment. An hour later, he complained of shortness of breath but continued exercising. By 8:50 a.m., he had lost 3.5 lbs. He drank approximately 8 oz of water, rested for 30 minutes, and resumed exercise. At 9:30 a.m., he stopped exercising and indicated he was not feeling well. Efforts were made to cool him, and his clothing was removed. He became unresponsive and developed cardiorespiratory arrest; resuscitation was unsuccessful.”

Question 22

Nutritional strategies

Answer 22

Nutritional intake can have a profound effect on exercise performance; the intake of carbohydrate can have a particularly noticeable effect.

Carbohydrate
Carbohydrate is used by both the aerobic and anaerobic systems to synthesize ATP. This means that carbohydrate plays a key role in driving muscle contraction across a very wide spectrum of exercise intensities. This can range from short-duration high-intensity exercise lasting less than one minute to longer duration endurance-type efforts lasting several hours, such as marathon running. Despite the importance of glycogen for muscular work, the amount of carbohydrate in the body is relatively small. There is around 400 grams in muscle, 100 grams in the liver and a few grams circulating in the blood. Because the energy density is relatively low (1760 kJ.100g21 compared to 4000 kJ.100g21 in fat) it represents a limited source of energy. In healthy, active people there is sufficient glycogen to allow between one and a half and two hours of continuous activity. However, exercise intensity can affect the way glycogen is used up. During moderate intensity exercise most muscular work is due to the activation of slow twitch fibers. As intensity increases fast twitch fibers are recruited too. Since glycogen is only used for energy metabolism within the cells in which it is stored exercise intensity changes the pattern of glycogen use within a muscle. Glycogen is depleted in type I fibers but more muscle glycogen remains in fast twitch muscle fibers after a long period of moderate cycling. This is because the slower twitch fibers would have been activated. Since glycolysis is a very fast metabolic pathway it has the potential to use up glycogen very quickly! All-out cycling exercise of just 30 seconds can substantially reduce glycogen stores.

Jonas Bergström (J.B.) and Eric Hultman (E.H.) sat on different sides of the same exercise bike, the first turning the left pedal with their right foot and resting the left, and their companion doing the opposite. Measures of muscle glycogen in each leg of both cyclists were made before and after one-legged cycling which they continued until they were exhausted. During recovery over the next three days the cyclists consumed a high carbohydrate diet.
The study demonstrated several important concepts;
@ Muscle glycogen is utilized locally within the muscle in which it is stored.
@ Depletion of glycogen is connected with fatigue and exhaustion; when carbohydrate is limited, exercise is limited or prevented. The fatigue and drop in exercise capacity when carbohydrate runs out is sometimes referred to as “hitting the wall” by athletes who experience this. In addition the study suggested that:
@ high carbohydrate intake during recovery can restore glycogen stores in 24 hours
@ rest and carbohydrate intake results in a “supercompensation” where additional glycogen is stored; in this example glycogen stores have more than doubled the initial levels seen in the rested legs.
The research of Bergström and Hultman started the idea that ingestion of carbohydrates can increase muscle glycogen stores. A similar effect can be achieved simply by reducing training and increasing carbohydrate intake; this avoids the need for hard exercise to empty the muscles of glycogen. This process is known by athletes as carbohydrate loading or carbo loading. Athletes do this because a larger glycogen store represents a greater energy store; this permits exercise to continue for longer before the glycogen store becomes depleted. This delaying of fatigue may be of particular value in long-duration endurance events or tournament-type situations where glycogen might run low. If sports allow, it is possible to eat foods or drink fluids containing carbohydrate during exercise or between breaks in exercise. This is not replacing endogenous glycogen but it is providing an exogenous source of glucose in the blood which can be taken up by muscles and used as a substrate. Maintaining blood glucose helps reduce perceived level of effort and maintains concentration, both of which can benefit performance.

Types of carbohydrate

A plentiful supply of carbohydrate-containing foods is important in the lives of sportspeople. However not all carbohydrate-containing foods are equal! The physical structure of food and the chemical form of carbohydrate within it influence how efficiently carbohydrate is extracted and the rate and extent to which it increases the concentration of glucose in the blood after ingestion. One approach to classifying foods is termed the glycemic index (GI). If a range of carbohydrate-containing foods are ingested, even if they contain the same amount of carbohydrate, the rate of its appearance in the blood can be very different.

Question 23

Recovery from training

Answer 23

The same principles which allow carbohydrate loading can be applied to recovery after exercise. As athletes finish one training session they may already be thinking about the next one; this may be the following day or even later the same day. Therefore it is important to maintain high levels of muscle glycogen and replace quickly what has been used in training. It is the foods with a high GI value which provide the fastest, most efficient way of replacing glycogen.

Protein
Unlike fat and carbohydrate there is no storage capacity for protein in the body. All protein is present with a specific biological function (e.g. as enzymes or structural proteins such as muscle). All protein in our bodies is in a state of flux, that is, molecules are constantly being broken down into their constituent amino acids and resynthesized.
The steady loss of amino acids from the body along with no storage means that a regular daily intake of protein is required to sustain biological function and health. The current level of intake recommended for healthy adults is 0.8 grams per kilogram of body mass per day (0.8 g. kg21. d21).
Influence of strength training and endurance training
The basic requirement for protein intake can increase in certain groups at certain times, such as in growing children, in people recovering from illness and in breastfeeding mothers. Also when people are engaged in exercise and training the requirements increase.

Meeting the requirements
Although protein degradation yields both essential and non-essential amino acids, net losses due to oxidation and metabolism means that dietary intake must include all amino acids.

Question 24

Nutritional ergogenic aids

Answer 24

Sports drinks, bars and gels are products formulated around macronutrients and micronutrients. They are intended as a convenient means of ingesting fluid and/or macronutrients in an exercise setting when “normal” food may be impractical.
There are also numerous compounds and supplements that are not based on essential components of the diet which are marketed as commercial sports nutrition supplements. However there are few real “quick fixes” in sports nutrition and only a handful of these products are supported by good evidence

Caffeine: CNS stimulant which can reduce the sensation of discomfort and effort during continuous exercise and increase force production during strength-type exercise. 2-6mg per kg body mass before or during exercise. Increase exercise performance at a range of exercise intensities. Anxiety, insomnia, mild diuretic, weakly addictive.

Creatine: Increases muscle creatine content, facilitates rapid PCr resynthesis in the rest periods during repeated high-intensity exercise. Creatine ingestion may also augment the effects of strength training by stimulating muscle anabolism. 15-20 g per day for 4-7 days followed by a maintenance dose of 2 g per day. Benefits exercise that relies on the PCr energy system such as strength, power and sprinting sports. Increase in body mass may be detrimental for some.

Bicarbonate: Buffer which increases blood pH. This can increase tolerance to the H generated by the lactic acid system during high intensity exercise. 0.3 g per kg taken before exercise. Increases performance during high intensity exercise lasting 1-7 minutes. Bicarbonate can cause gastrointestinal upset.

Question 25

Diet manipulation prior to competition

Answer 25

Nutritional manipulation of the diet can specifically be used to modify body weight and body composition. Some sports require smaller stature to compete in a lower weight class (e.g. boxing), to improve aesthetic appearance (e.g. gymnastics) or to increase physical performance (e.g. distance running). Athletes use various strategies to reach a low body weight. For rapid body weight loss, athletes cut down their fluid and total energy intake. Other athletes try to remove the perceived weight-gaining properties of body fat by following a diet high in carbohydrates (60–70% of total energy intake) and low in fat (15–20% of total energy intake). Both diets can have a harmful effect on health and can impair performance if done poorly and over a long period of time.