Vitamin A

Today's lesson is the eye saving Vitamin A! Vitamin A has many important roles besides the well known vision function. So, let's get started!

Vitamin A is fat-soluble, which means it needs fat to be absorbed by the body. Insufficient fat intake can lead to a secondary Vitamin A deficiency. Fat soluble vitamins are stored in the body for longer periods of time than water soluble vitamins.

Vitamin A, aka the retinoids, consist of 3 preformed compounds: retinol (an alcohol), retinal (an aldehyde), and retinoic acid (an acid). These active forms of Vitamin A can only be found in animal products (liver, fat of eggs and milk).

The liver plays a major role in the transport and storage of Vitamin A. Liver disease can cause secondary Vitamin A deficiencies. Fun fact: 50-80% of the Vitamin A in your body is stored in your liver!

Carotenoids, found in dark green leafy, yellow-orange vegetables (carrots, sweet potatoes, kale, spinach), can yield retinoids when metabolized in the body. Fun fact: the deeper the color of the vegetable, the higher the levels of carotenoids. The amount of Vitamin A available from dietary carotenoids depends on how well they are absorbed and converted to retinol in the body. Beta carotene, a subgroup of the carotenoids, is well known for its antioxidant capacities.

Vitamin A plays essential roles in vision, cell differentiation, recognition, and growth, bone development, immune functions, and reproduction.  Retinal is a structural component of visual pigments of the rod and cone cells of the retina. Retinoic acid acts as a hormone to affect gene expression.

Vitamin A deficiencies can be very serious. The most significant cause of blindness is due to Vitamin A deficiency.
-->Xerophthalmia: clinical eye disease consisting of atrophy of the periocular glands
-->Nyctalopia: night blindness, failure of retina to regenerate rhodopsin, cannot adjust from bright light to darkness
-->Spontaneous abortions
-->Impaired spermatogenesis
-->Phrynoderma: blockage of hair follicles with plugs of keratin, which causes "toad skin" (rough, scaly, dry skin).

RDA for Vitamin A
-adult females: 700ug
-adult males: 900ug

Toxicity of Vitamin A occurs when persistent large doses (more than 100 times the required amounts) are consumed. Toxicity can eventually lead to liver disease. The most common side effect of excessive carotene intake is hypercarotenodermia, which causes an orange tint to the skin. This is reversible by decreasing carotenoid intake.

UL for Vitamin A
-adult females: 3000mg
-adult males: 3000mg

Impact of Carbohydrates on Metabolism

Last post we talked about the basics of carbohydrates.  Now, let's delve into how carbohydrates impact your metabolism!

Through the activity of enzymes (amylase, brush border, digestive), dietary carbohydrates are digested into glucose, fructose, and galactose. Digestion is altered by factors such as: the availability of starch to enzyme action, presence of fat, which slows stomach emptying, non-absorbable oligosaccharides, and fibers. A diet rich in fruits and vegetables will slow the pace of glucose absorption in the body.

Glycemic Index
The glycemic index ranks carbohydrates based on their ability to raise blood glucose levels. The glycemic index uses of a scale of 0-100. Higher values are given to foods that cause the most rapid rise in blood glucose (Pure glucose = GI of 100). The glycemic index has been an important factor for diabetics that are trying to control their blood sugar spikes.

Glycemic Load
The glycemic load takes the glycemic index further by factoring in the type and amount of carbohydrate consumed. (Grapefruit: GL of 3, Potato Chips: GL of 30).

Regulation of Blood Lipids
A high carb diet leads to hypertriglyceridemia (high blood triglycerides). According to the NIH, "As compared with starches, sugars (particularly sucrose and fructose) tend to increase serum triacylglycerol concentrations by approximately 60%). Two hours after consuming a meal, blood glucose levels begin to drop. The body secretes counter-regulatory hormones that release free fatty acids from fat cells. The fatty acids are packaged into VLDLs and serum triglyceride levels are increased. On the other hand, a high fat diet does not translate directly into blood lipid changes.  The body regulates macronutrient levels to provide adequate supplies of fuel to body tissues.

Fiber and Blood Lipids
Fiber is an important factor in weight loss and blood cholesterol control. Fiber prevents fat and cholesterol absorption by binding bile acids of fats and lipids and excreting them.

Mahan L, Escott-Stump, S. "Food, Nutrition, and Diet Therapy." Saunders. 2004. 

www.healthbasic.org

Types of Carbohydrates

Today's lesson features carbohydrates!

Monosaccharides
-The sweet taste of edible sugars (fructose and sucrose) was an evolutionary advantage because sweetness guided humans to select plants with the highest energy value.
-Reducing Sugars: hydroxyl group can reduce (donate hydrogen); examples are glucose and galactose
-Glucose, Galactose, Fructose

Glucose
-Most widely distributed sugar in nature
-Glucose + Fructose = Sucrose (make up a large content of fruits and vegetables)
-Also referred to as "blood sugar"

Fructose
-Other names: levulose, fruit sugar
-The sweetest
-Fruits are generally 1-7% fructose
-Vegetables have 3% fructose
-Honey is 40% fructose
-As fruit ripens, enzymes cleave sucrose into glucose and fructose resulting in a sweeter taste.

Galactose
-Produced from lactose (milk sugar) by hydrolysis in the digestive process.

Disaccharides
-Monosaccharide units joined by glycosidic linkages.
-Sucrose= fructose + glucose
-Maltose = glucose + glucose
-Lactose = glucose + galactose

Sucrose
-When used in preparation of acidic foods it becomes inverted and sweeter

Honey
-More calorie dense because monosaccharides are packed more closely together than disaccharides
-1 tbsp honey = 64 kcal
-1 tbsp sugar = 46 kcal

HFCS
-Manufactured by changing glucose in cornstarch to fructose
-Added to canned/frozen fruits to preserve

Oligosaccharides
-Polymers containing 2-20 sugar molecules.
-Water soluble and often sweet
-Nondigestible oligosaccharides: resistant to stomach acid and action of amylase, enter intestine intact
-Raffinose: in sugar beets (galactose + glucose + fructose)
-Stachyose: in legumes and squash (2 galactose + glucose + fructose)

Polysaccharides
-Plants store carbohydrates as starch granules in plastids within cellulose walls
-Edible plants make 2 types of starch (amylose, amylopectin)
-In whole foods, starch is poorly digested. Moist cooking will improve digestibility.
-Carbohydrates that cannot be digested are classified as dietary fiber.

Resistant Starch
-Remains intact throughout cooking
-Limited glucose is produced for absorption

Waxy Starch
-Obtained from corn and rice strains bred to increase amylopectin
-When dissolved in water, forms a smooth paste
-Used as a thickener for frozen pies and sauces

Modified Food Starch
-Used for instant puddings, salad dressings, and baby food
-Adds viscosity and gel-like properties

Cellulose
-A nondigestible carbohydrate found in celery, broccoli, carrots, and many other vegetables
-Falls in the category of insoluble fiber

Pectins/Gums
-Contains sugars and sugar alcohols that make them water soluble
-Pectin is found in apples, citrus, strawberries, jams, and jellies
-Gums are galactose units combined with other sugars found in oats, legumes, and barley

Fructooligosaccharides
-Poorly digested in the upper GI tract
-Provide 1kcal/g, half as sweet as sucrose
-Used as a sugar replacement
-Naturally found in onions, garlic, bananas
-Increase colonic bacteria growth and have been added to yogurts marketed as a prebiotic

Lignin
-Woody insoluble fiber found in stems and seeds of fruits and vegetables and flaxseed
-Also found in the bran layer of grains

Carrageenan
-Extracted from macroalgae or seaweed.
-Used to stabilize food mixtures in pudding and infant formula
-Some studies have shown at high doses to increase cancer risk
http://www.webmd.com/vitamins-supplements/ingredientmono-710-carrageenan.aspx?activeIngredientId=710&activeIngredientName=carrageenan

Chitosan
-Sugar extracted from the shells of crabs, lobsters, and shrimp
-Hypocholesterolemic effect
-Positive charge on chitosan binds negatively charged lipids and blocks their absorption
http://www.webmd.com/vitamins-supplements/ingredientmono-625-chitosan.aspx?activeIngredientId=625&activeIngredientName=chitosan

See the next post on how carbohydrates impact health!

Mahan L, Escott-Stump, S. "Food, Nutrition, and Diet Therapy." Saunders. 2004.