Carbohydrates Flashcards
What’s carbohydrate?
Provides 40-80% of the energy in majority of human diets.
Staple food is one that is eaten regularly and in such quantities, making up the dominant part of the diet and supplying major proportion of energy and nutrient needs (FAO).
Most staple foods derive either from Cereal such as wheat, barley, rye maize or rice.
Or Starch root vegetables such as potatoes, yams, taro and cassava.
Structural features of Carbohydrates
They are characterised by degree of polymerisation and type of linkages:
simple carbohydrates (sugars) which can be monosaccharides (Glucose, fructose and galactose) and disaccharides.
complex carbohydrates such as oligosaccharides and polysaccharides.
Carbohydrates functions
Energy
Not considered essential but is an important energy source for CNS, RBCs (glucose in blood) and during high intensity exercise (glycogen in muscle)
Prevent Ketones produced from fat when CHO intake is inadequate. Ketones then become an important fuel for the brain.
It synthesis of other biochemical molecules as CHO provide C atoms for synthesis of proteins, lipids, nucleic acids, etc.
form part of DNAA and RNA molecules.
CHO linked to lipids consists structural components of cell membrane.
CHO linked to protein in a variety of cell-mollecule recognition processes.
Digestion of Starch
Starch digestion starts in the mouth and salivary glands. Where the salivary glands secrete saliva into the mouth to moisten the food. The salivary enzyme amylase begins digestion.
Then, stomach acid inactivates salivary enzymes, halting starch digestion.
The pancreas produces an amylase released through the pancreatic duct into the small intestine.
Then, disaccharidase enzymes on the surface of the small intestine cells hydrolyze the disaccharides into monosaccharides.
Intestinal cells absorb these monosaccharides.
Digestion of Fiber
Starts in the mouth. The mouth’s mechanism crushes and tears fibre in food, mixing it with saliva to moisten it for swallowing.
In the stomach, fiber is not digested, delaying gastric emptying.
In the small intestine, fiber is not digested, and it delays the absorption of other nutrients.
In the large intestine, most fibre passes intact through the digestive tract to the large intestine. where the bacterial enzyme digests fiber.
Fibre holds water regulating bowel activity and biding substances such as bile, cholesterol, and another mineral, carrying them out of the body.
CHO metabolism
CHO is metabolised by cells, occurring with (aerobic) and without (anaerobic) oxygen.
Ultimately, all CHO is metabolised aerobically.
Glycaemic index
It is a relative ranking of CHO in foods according to how they affect blood glucose levels.
CHO with a low GI value (~55or less) are:
more slowly digested; and or absorbed, and
more quickly metabolised
lowering and slower rise in blood glucose
Terms used to describe CHO
Sugar-usually sucrose
Sugar/ Total sugars- naturally present in food + those which are added. EU and UK label total sugars
Free sugar is used by WHO and SaCN sugar, which is added by manufacturers, cooks, and consumers, plus sugar naturally present in honey, syrups, and fruit juice.
Added sugar is those sugars that manufacturers add during processing or by consumers before consumption.
Intrinsic and Extrinsic
NMS (non-milk extrinsic sugars)
Complex CHO
Glycaemic CHO
Available and unavailable CHO
Recommendation intakes
~50 g need to avoid ketosis
~180 g needed for the brain, rbs and renal medulla
UK SACn (2015) adivise:
Population averages ~50 total dietary energy from CHO
Population average intake of free sugar should not exceed 5% of total dietary energy for age groups from 2 years upwards.
Population average for fibre ~30g for adults
According to WHO
Intake of ~40-70% of total energy is associated with reduced mortality risk.
WHO recommends that CHO comes primarily from whole grains, vegetables, fruits and pulses.
WHO also recommends at least 400g of vegetables and fruits per day
In adults, WHO recommends an intake of at least 25g per day of naturally occurring dietary fibre as consumed in food.
