Dietary Carbohydrates Flashcards
The most accurate way to measure habitual energy (food) intake in a weight stable person is to use
Doubly labeled water
How do you calculate the number of grams of carbohydrate consumed per day by an individual in energy balance?
If in energy balance, energy expenditure:
25-35kcal/kg per day
Average diet is 45-60% of calories as carbohydrate
Carb= 4 kcal/g
So for a 70kg person:
30kcal/kg/day x 70 kg= 2,100kcal/day
If 50% of diet is carbs that’s 2,100 kcal/day x 0.5= 1,050 kcal of carb/day
1,050 kcal/day x 1g/4kcal= 262.5 g/day of carbs
Sugar chemical characteristics and examples (and what labeled as)
1-2 molecules
Monosaccharides: glucose, galactose, fructose (sugars)
Disaccharides: sucrose, lactose (sugars)
Polyols: sorbitol, mannitol, xylitol (sugar alcohols)
Glycemic index
Amount of glucose excursion equal amounts of different forms of carbohydrate (carbohydrate subtypes) cause.
Some forms of carbohydrate produce a smaller glucose excursion (low glycemic index) others a larger excursion (high GI).
Foods with a high GI produce higher insulin excursions after they are eaten
Empirically determined value reflecting the area under the glucose curve compared to a standard (glucose solution or white bread depending on the scale) following the ingestion of 100 grams of the carbohydrate source in a fasted state.
Glycemic load
The GI of a carbohydrate source multiplied by the percent carbohydrate content (by weight)
A number of studies have correlated glycemic load with adverse health effects such as the development of diabetes.
Types of studies that can be used to inform nutritional recommendations
Animal studies: Diet can be controlled absolutely. The primary limitations of these studies are their questionable relevance to humans.
Epidemiological studies: In these studies, dietary intakes are measured using self reporting tools such as diet records or food frequency records. Reported patterns of eating are then correlated with the development of a variety of diseases. This design provides tremendous power for looking for diet/disease relationships. Weaknesses of these studies are the documented inaccuracies in dietary self report, and the potential that “better dietary habits” are simply a marker for an overall healthy lifestyle and that differences seen are not attributable to the diet per se.
Small randomized controlled trials with surrogate marker endpoints: In these studies, subject’s diets are carefully controlled and changes in relevant endpoints such as insulin sensitivity, triglyceride or LDL cholesterol levels are measured following a dietary change. The weaknesses of this type of study are the small number of subjects studied and concerns over generalizability.
Large randomized controlled trials with disease specific endpoints: these are considered the “gold standard”. The limitation of these studies is that they included other interventions (such as increased physical activity) besides just dietary changes and as a result, the specific effects of diet alone are not clear. Second, the diets were not rigorously controlled and as a result it can not be known with certainty what people actually ate. Finally, these studies contain relatively few subjects when compared to the large epidemiological studies.
Biochemical pathways involved in the metabolism of fructose and galactose
Fructose is metabolized differently than glucose.
Fructose enters cells through a general hexose transporter and so its entrance is not regulated by insulin.
It does not stimulate insulin release.
It DOES NOT enter glycolysis at fructose 1,6 bisphosphate because hexokinase is typically completely used to metabolize glucose.
Fructose is typically phosphorylated by fructokinase (present in the liver) which produces fructose 1-phosphate and then following phosphorylation at the 6 position, it is converted by the action of aldolase into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate entering glycolysis at a step which effectively BYPASSES the key regulated step of PFK.
For this reason fructose much more readily cleared by the liver, and in the liver readily converted to pyruvate bypassing much of glycolysis. It is a bit like a “super glucose”. This may explain why there may be adverse health effects from fructose like insulin resistance and hypertriglyceridemia.
Clinical presentation of disorders of fructose and galactose metabolism
In these individuals, the of these suagrs is associated with gas and diarrhea due to the inability to absorb the sugar which then passes to the colon where it produces and osmotic diarrhea and is fermented by colonic flora.
Oligosaccharide chemical composition and examples (and what labeled as)
3-9 molecules
Malto- oligosaccharides: maltodextrins (other carb)
Other oligosaccharides: raffinose, stachyose (other carb)
Present in beans, onions, asparagus other vegetables and a number of probiotics (yeast containing yogurts)
Polysaccharide chemical composition and examples (and what labeled as)
9 molecules
Starch: amylose, amylopectin (other carb)
Fiber: cellulose, hemicellulose, pectins (fiber)
Starches (what is it and types)
Polysaccharides: long polymers of glucose
May be organized in a manner that either promotes rapid absorption, or slows absorption
Amylopectin: Amylopectin is a highly branched form of starch. As a result, the intestinal amylase has many “free ends” of the polymer to digest and as a result the digestion and absorption is rapid; HIGH GI (white bread, potatoes and many pastas)
Amylose: Amylose is a long unbranched chain of glucose molecules. Only 2 “free ends” that can be subjected to digestion by amylase; slowly digested and absorbed. (in basmati rice and bananas)
Resistant starch: (ex corn starch) Because of crystal structure it is slowly absorbed.
Fiber (what is it and types)
Complex carbohydrate that is not digestible by human intestinal enzymes
Pass through the GI tract largely unaltered. They increase stool volume and may lower serum cholesterol levels.
Contained in the husk of grains, oats, vegetables and fruits
Insoluble fiber: Does not absorb much water. (ex bran, whole wheat, and celery)
Soluble fiber: Absorbs water. Gram for gram create greater bulk in the gastrointestinal tract than insoluble fiber sources. Shown to lower LDL cholesterol levels and help lower post-prandial glucose excursions more effectively than insoluble fiber. (Ex. guar gum, psyllium and beta-glucan which are present in beans, oats, barley, apples, other fruits and are also available as dietary supplements)
