biochem - nutrition exam 1 Flashcards
Carbohydrates – Quick Review
Monosaccharides:
– Glucose (glc)
– Fructose (fru) – the sweetest of the sugars
– Galactose (gal)
- Disaccharides:
– Maltose (glc-glc) – minor food constituent
– Sucrose (glc-fru)
– Lactose (glc-gal)
Polysaccharides
Glycogen
– Poly-glucose
– Highly branched
– Not a significant food source
Starch
Fiber
Starch
Poly-glucose
- Amylose: linear
- Amylopectin: branched
- Found in grains: rice, wheat, corn, etc.
two components:
- Amylose
- Amylopectin
Fiber
Structural parts of plants
- Polysaccharides other than starch (next slide) – Cellulose, hemicellulose, etc.
- Some non-polysaccharides
- Cannot be broken down by the human digestive system (missing enzymes)
– Not a source of monosaccharides
– Bacteria in GI tract may digest some
Polysaccharides other than starch
Cellulose (plant cell walls)
– Vegetables, fruits, legumes
– Linear and non-digestible, because of (β1→4) linkage
Hemicellulose(cereal fibers)
– Branched
– Some soluble, some not
long chain and linear
Carbohydrate Digestion of starch
mouth and salivary glands
- the salivary glands secrete saliva into the mouth to moisten the food. The salivary enzyme amylase begins digestion
- starch + amylase = small polysaccharide
stomach
- stomach acid inactivates salivary enzymes, halting starch digestion
small intestine and pancreas
- the pancreas produces an amylase that is released through the pancreatic duct into the small intestine
- starch + pancreatic amylase = small polysaccharide and disaccharides
not finished
Carbohydrate Digestion of fiber
Notes on Carbohydrate Metabolism
Catabolism:
– Branched chains: digested more rapidly
– Linear chains: digested more slowly
- Glucose, galactose and fructose:
– Glc and gal: move quickly into the blood (active transport); fru transport slower (passive)
– Fru and gal metabolized primarily in the liver; glc metabolized in many cells, as needed.
Fructose
- Recall that fructose is metabolized by the glycolytic pathway.
– Liver (major): converted to glyceraldehyde 3- phosphate, bypassing steps #1-5
– Muscle & kidneys (minor): converted to fructose 6-phosphate, bypassing steps #1-2 - Compared to glucose, fructose catabolism is less stringently regulated (steps #1 & 3 – chapter 15) and may lead to more fat- formation in the liver.
Fructose (continued)
Unlike glucose, fructose does not induce insulin secretion and does not reduce appetite as much as glucose.
- Same true of high-fructose corn syrup.
– HFCS is made by enzymatically converting some of the glucose in corn syrup to fructose, typically resulting in mix of ~55% fru. Because fru is much sweeter than glc, this is sweeter than a mix with 50% fru and 50% glc.
more sweeter due to fru so they can use less of it
Lactose Intolerance
Little/No lactase → lactose reaches the large intestine and is converted into toxic products by bacteria [→ cramps, diarrhea])
- Solution: reduce lactose, and add lactase
- Soy milk contains sucrose, not lactose.
- Caution: milk products contain many important nutrients (e.g., riboflavin, Vitamin D, calcium)
- Note: 20% of prescription meds and 5% of
OTC meds use lactose as a filler/sweetener.
Glycemic Effect/Response
Food containing carbohydrates leads to an increase in blood [glucose], which elicits an insulin response (secretion)
- Foods are classified according to how rapidly they lead to this increase.
- Classification of food: Glycemic Index
– Low: slow absorption., modest rise, smooth return, slow spikes
– High: rapid absorption., surge in [glc], dramatic drop, quick spikes
High
- White Bread
- Baked pot.
- Cornflakes
- Sports dr.
- Donuts
low
- Low
- Peanuts
- Cashews
- Soybeans
- Milk
- Beans
Some Nutrition Guidelines & Recommendations
- American & Canadian standards (1997-): Dietary Reference Intakes (DRI) - being continually updated
- Recommended Dietary Allowances (RDA): age-, sex- and life-stage-specific
- Daily Values (DV): Average recommended daily intake for a person on a 2000-kcal. diet (Note: average, not based on age or sex)
Dietary Recommendations: Carbohydrates
45-65% of calories from carbohydrates
- E.g., on a 2000-kcal diet, 900-1300 kcal
- 1200 kcal/4 kcal/g = 300 g (the Daily Value) - this is the conversion factor
- Includes sugar, starch, and fiber. – Sugar: slides #17-20
– Fiber: slides #21-27
Sugar Intake
where we get our sugar from:
In the U.S., ~1/2 from milk, fruits, vegetables, grains
- Other 50% is refined and added by processors (drinks, desserts, candy, cereals) or consumers: sucrose, high-fructose corn syrup, honey, etc.
- Recommendations below.
Health Effects of Sugar
A little is OK
- Honey (sucrose = glc-fru)
– Trace amounts of protein, Ca, Fe, Vit. C no need to know these Ca, Fe, etc… - The refined (= processed/purified; nothing added) sugar problem: usually means:
– Too many calories, or …
– Too few nutrients (low nutrient density)
More Sugar Health Effects
Dental caries (bacteria in or mouth, bacteria secretes acid onto our teeth)
– Worst case: sugar alone, between meals
- Obesity
- Heart disease: sugar is a minor factor (compared to smoking, genetics, lipids)
- Hyperactivity: connection not supported by research * not on exam *
Recommended Intake of Sugar
Guidelines: “Reduce intake … from added sugars.”
- Old recommendation: ≤ 10% of total calories (still a good approximation)
- E.g., 200 kcal of a 2000 kcal/day diet
- 1 tsp = 20 kcal
- Typically, Americans get about 1/6 (16%) of daily energy from simple sugars.
- Minimize consumption of HFCS (slide #12).
Fiber: sources, actions
Soluble
- Whole grains (oats, oat bran, barley, rye), fruits, legumes (beans, peas), seeds, vegetables
* *Slow upper GI transit, slow starch hydrolysis, slow glucose absorp., lower blood cholesterol, soften stools
Insoluble
- Brown rice, fruit, legumes, seeds, vegetables, wheat & corn bran, whole grains
* Accelerate colon transit, increase stool weight, relieve constipation, increase feeling of fullness
Health Effects: Fiber
Makes one feel full, reducing appetite.
- High-fiber foods help to reduce the risk of
Type-2 diabetes
– Soluble fibers slow glucose absorption, moderating glucose surge/rebound (which appears to lead to Type 2 Diabetes)
Health Effects: Fiber
May decrease risk of heart disease
– Reasons not well understood
– Soluble fiber binds and removes bile acids, lowers blood cholesterol
– Byproducts of bacterial digestion may inhibit cholesterol synthesis in the liver
– Fiber may displace fat in the diet.
Health Effects: Fiber
High-fiber diet correlated with decreased
risk of cancer, esp. colon cancer
– Esp. green & yellow vegetables, and citrus fruits
* Cause may be fiber, or other chemicals in these foods (antioxidants and phytochemicals: See Chapter 11 and 13 Highlights.)
Health Effects: Fiber
Intake of dietary fiber is associated with
increased GI health
- Insoluble fiber often helps to alleviate constipation
(Un)Healthy Effects of Excess Fiber
For a person who does not eat much (young, elderly, malnourished), consuming a large amount of fiber may mean consuming an inadequate amount of foods high in nutrients such as vitamins, minerals, and proteins.
- Fiber may decrease nutrient absorption.
- Sudden change to high fiber can lead to GI problems (discomfort, obstruction).
- Without adequate water: stool problems
- None of the above should be taken to mean that fiber in the diet is unhealthy.
