Week 3: Carbohydrates Flashcards
What is a carbohydrate?
Organic molecules containing carbon, hydrogen, oxygen
What is a monosaccharide?
One sugar molecule
Monosaccharides
Glucose
Fructose
Galactose
What is a dissacharide?
Two sugar molecules
Disaccharides
Sucrose
Lactose
Maltose
Oligosaccharide
2-10 sugar molecules
Oligosaccharides
Raffinose
Stachyose
What is a polysaccharide?
10 or more sugar molecules
Metabolic fate of carbs
- Primary energy source for body
- Stored as glycogen in liver and muscle
- Converted to triglycerides in adipose tissue
Carbohydrate digestion
Digestion by salivary and pancreatic amylase
Digestion in small intestine where disaccharides are broken down into absorbable monosaccharides by their enzymes
What is lactose broken down in to?
Glucose and galactose by lactase
What is maltose broken down in to?
2 glucose molecules by maltase
What is sucrose broken down in to?
Glucose and fructose by sucrase
Carbohydrate absorption
- Broken down into simplest form (monosaccharides)
- Active transport of glucose by SGLT1 permits entry into enterocyte of small intestine
- Leave enterocyte by GLUT 2 to enter blood stream
Glycogenolysis
Formation of glucose from glycogen (when glucose is depleted such as during fasting or exercising)
**liver
Glycogenesis
Formation of glycogen from glucose (occurs after a meal)
Where is glycogen stored?
Liver (100-200g)
Skeletal muscle (350-750g)
Gluconeogenesis
Formation of glucose from non-carbohydrate sources such as glycerol, lactate, pyruvate, amino acids; provides glucose when dietary intake is insufficient
Factors to consider when choosing a carb source?
- Nutrient density
- Glycemic index
- Fructose content
- Fibre content
Nutrient density
Nutrients per reference amount of food, typically 100 kcal/100g serving
Example of nutrient density
Liver has very high nutrient density
Glycemic response
Change in blood glucose after eating a carbohydrate containing food
What is persistently high levels of blood glucose linked with?
Obesity and chronic disease
Glycemic index (GI)
Scale that ranks carbohydrate containing foods/drinks by how much it raises blood glucose
High GI
Quickly digested and absorbed, rapid rise in blood glucose and insulin
Low GI
Slow rise in blood glucose and insulin, slowly digested and absorbed
What is a limitation to the glycemic index?
Doesn’t consider amount of food you’re consuming
Glycemic load
Considers the amount of carbohydrate ingested
GL=GI/100 grams of carbs
What are low GI diets associated with?
Decreased risk of developing type 2 diabetes and CVD
What are high GI diets associated with?
Increased risk of developing type 2 diabetes and CVD
How does reactive hypoglycemia occur?
High GI meals produce an initial period of high blood glucose and a spike in insulin, leading to reactive hypoglycemia in which blood glucose levels drop below baseline causing a glucose crash
What does reactive hypoglycemia trigger and promote?
Promotes hunger and excessive food intake, beta cell dysfunction, dyslipidemia and endothelial dysfunction
Why is there discrepancy in research regarding GI?
- Changes in total carbohydrate and fibre intake (maybe low GI diets contain more fibre?)
- Weight loss (can impact insulin sensitivity)
- Presence of and use of treatment for diabetes
Glycemic index studies conclusions
- When calorie intake is controlled, low GI and high GI diets report similar outcomes
- Large variability btwn study findings for weight loss
- Fibre content is a large contributor
- Baseline characteristics matter
Glycemic index and baseline characteristics
A low GI diet does improve insulin sensitivity in those with high insulin to begin with (obesity)
Limitations of glycemic index
- Intra and inter variability
- Lifestyle factors
- Growing conditions and meal preparation alter GI
- GI is assessed on an empty stomach and without any other foods
Inter-variability
Variability within a single group or entity
Intra-variability
Variability or fluctuations within a single individual across diff measurements
Foods with lower GI
Protein, fibre, lipids
Degree of processing and GI
Increased processing leads to a higher GI
Cooking and GI
Increased cooking time leads to higher GI
Storage of foods and GI
If you cook a food, store it in fridge, then microwave it, it increases GI
Consumption order and GI
The order that you consume foods can impact your GI
ex. eating veggies and protein before carbs leads to a lower glycemic response than eating carbs first
CVD risk factors of fructose metabolism
Gut dysbiosis
De nova lipogenesis
Abdominal adiposity inflammation
Insulin suppression
Can individuals absorb fructose?
60% of individuals cannot completely absorb fructose when consumed in large amounts
What is different about fructose metabolism?
No negative feedback loop on fructolysis enzymes so the process keeps on happening
Where does fructose metabolism occur?
Liver
Process of fructose metabolism
- Fructose is converted to fructose-1 phosphate by fructokinase
- Then its converted to triose-phosphates
- Converted to pyruvate which can enter Krebbs cycle
What results from fructose metabolism?
Increased intracellular phosphate depletion (fatigue)
Uric acid
What results from excess fructose intake?
Lipogenesis of fat in the liver
Can lead to to non-alcoholic fatty liver disease
What can increased levels of uric acid lead to?
- Increased blood pressure
- T2D
- Obesity
What are some common metabolic outcomes in animal models?
Visceral fat
Triglycerides
Insulin resistance
Uric acid
Non-alcoholic fatty liver disease
Animal study limitations
- Unbalanced calorie intake btwn groups (is weight gain causing observed metabolic health issues?)
- Fructose intake is unrealistically elevated
Average percentage of fructose in beverages
10-30%
Typical % of administered fructose in animal studies
60-70%
Fructose survival hypothesis
- Fructose is broken down by fruktokinase
- ATP converted to ADP, fructose metabolism depletes ATP reserves
- ADP is used to create uric acid which leads to mitochondrial damage and signals activation of the SURVIVAL SWITCH
Fructose survival hypothesis - survival switch
Prepares our body for times of food insecurity such as in the winter
Why is the survival switch triggered?
Fructose was previously most accessible in the fall, thereby preparing the body for the scarcity of food in the winter
What does activation of the survival pathway lead to?
Increased foraging
Reduced REE
Insulin resistance
Systemic inflammation
Increased fat storage
What does persistent activation of the survival switch lead to?
Obesity and noncommunicable diseases
Fructose survival hypothesis -present day
Doesn’t hold true as fructose is regularly available
Present day fructose health outcomes
Stroke, heart failure
Behavioural issues
Obesity, fatty liver
Diabetes
Heart disease
What would studies that align with the fructose hypothesis show?
Differences btwn glucose and fructose regarding these adaptations
Fructose and lipogenesis
Significant increase in FFA synthesis when consuming fructose diet bc of increased lipogenesis from fructose metabolism
Fructose and metabolic health
- Fructose diet leads to increase in VAT
- Fasting ApoB increases in the fructose group
- Measures of insulin sensitivity decreased in fructose group
Fructose and hepatic insulin resistance
Both isocaloric and hypercaloric fructose diets lead to hepatic insulin resistance
Fructose and NAFLD
Risk of NAFLD increased with fructose in hypercaloric diet but not isocaloric group
Limitations of long-term study research
- Small sample size
- Narrow demographic
- Short duration
- Poor study quality
Limitations of human research
Duration of study is often less than a few weeks
Fructose intake is unrealistically elevated as a % of energy intake
Dietary fibre
Non-digestible carbs and lignin that are naturally found in plants
Dietary fibres
Cellulose
Pectins
Fructans
Resistant starches
Hemicellulose
Lignin
Gums
B-glucans
Functional fibre
Isolated, extracted, manufactured non-digestible carbs that have positive physiological effects on humans
Functional fibres
Fructans
Pectins
Psylium
Gums
B-glucans
Total fibre
Dietary fibre present within the food plus functional fibre that has been added to the food
Resistant starch
Starch that cannot be or are not easily enzymatically digested
Types of resistant starch
RS1
RS2
RS3
RS4
RS1 (dietary)
Physically inaccessible to digestion due to its location
RS2 (dietary)
Resists digestion bc it it tightly packed inside of granules within food
RS3 (functional)
Formed w moist-heat cooking and cooling of starches that has gelatinized
RS4 (functional)
Results from chemical modification of starches
Fibre requirements
25 g female
38 g males
Properties of fibres
- Solubility in water
- Viscosity and gel formation
- Fermentibility
Solubility in fibres
Water-insoluble fibres
Water-soluble fibres
Water-insoluble fibres
Dont dissolve in water
Decrease intestinal time and increase fecal weight to positively impact laxation
Types of water-insoluble fibre foods
Whole grains, bran, legumes, veggies
Water-soluble fibers
Dissolves in hot water
Delay gastric emptying, increases intestinal time and decrease nutrient absorption to positively impact blood glucose and lipid concentration
Types of water soluble fibres
Oats, barley, rye, chia, flaxseeds, fruits
Do we still use solubility of fibers?
No, theres inconsistencies
Viscosity and gel formation
Ability of fiber to both bind or hold water and form a gel
**most fibres can hold water but not all form a gel
Viscous gel forming fibres
Pectin, B-glucans, mucilages, gum
Positive effects of gel forming fibers
- Gastric distention, delayed gastric emptying, longer intestinal tract transit time
- Reduced nutrient digestion (glucose and lipids)
- Reduced micelle formation (gel traps bile)
- Decreased movement of nutrients within intestinal lumen
- Decreased nutrient effusion rates
Fermentability
How quickly and efficiently gut microbiomes can degrade fiber through fermentation
Positive effects of fermentation
Growth of bacterial pop. and short-chain fatty acid formation
Increases fecal bacteria mass which attracts water to enhance stool size
Promotes laxation and treats constipation
Health benefits of fibre
- Diabetes
- Cardiovascular disease
- Appetite or satiety and weight control
- Gastrointestinal disorders
Where does glycogenesis occur?
Liver and muscle
**requires energy
What is liver glycogen important for?
Maintaining glucose homeostasis
Why can’t glycogenolysis contribute to control of blood glucose levels?
Muscle doesn’t have the enzyme that converts phosphorylated glucose back to free glucose
What does skeletal muscle glucose do once in cell?
Doesn’t exit cell once present
Where does gluconeogenesis occur?
Liver
What happens when total energy intake is used as a confounder?
When total energy intake is used as a confounder, the association btwn glycemic index and insulin sensitivity is removed
Conclusion of study on energy intake and glycemic index
No association btwn GI and fasting insulin and insulin sensitivity
Glycemic index and weight management studies
Low GI diets are generally no better than high GI diets for reducing body weight or fat
Type 2 diabetes and fibre intake
Dose-response relationship
More fibre=greater reduction in risk of T2D
Fibre and glycemic index
Increased fibre= low GI
Decreased fibre= high GI
Effect of GI and fibre intake on RR for diabetes
High GI and low fibre diets doubled risk of diabetes
Baseline characteristics and low GI diet
A low GI diet will improve insulin sensitivity in obese/overweight individuals who have high insulin to begin with
No effect on normal weight individuals
What happens when you combine a high GI food with a low GI food?
Low GI
Animal studies: paired feeding trials
Sucrose resulted in increased uric acid, triglycerides and insulin in the blood
**didnt differentiate btwn glucose and fructose
Animal studies: increased energy intake
Fructose can accelerate metabolic syndrome, fatty liver and T2D in male breeder rats, independent of excess energy intake
Animal studies : increased energy intake and leptin
In the fructose group, leptin didn’t suppress hunger like it was supposed to
Fructose and health outcomes: hypercaloric vs isocaloric diets
Still see negative health outcomes regardless of weight gain
Soft drink consumption and energy intake- Longitudinal and cross-sectional studies
Positive association btwn soft drink consumption and overall energy intake
Soft drink consumption and energy intake- Long term experimental studies
Individuals did not compensate for the extra energy consumed
Soft drink consumption and energy intake- Short term experimental studies
Mixed results
Prebiotics
Substances that are not digested by the host but provides heath benefits by acting as a substrate for the growth and activity of healthy bacteria in the colon
