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Human Nutrition

Human Dietary Needs

The human diet must provide the following:
Link to discussion of the physiology of the gastrointestinal (GI) tract.

How dietary needs are established

Determining what substances must be incorporated in the human diet, and how much of each, is - even after years of research - still under active study. Why the uncertainty?

Despite some uncertainties, the National Research Council of the U. S. National Academy of Sciences publishes guidelines. Until the summer of 1997, these were called recommended daily allowances or RDAs.

Link to table giving RDAs for men and women age 19-22.
In the future, they will be called Dietary Reference Intakes (DRIs).

Carbohydrates

Carbohydrates provide the bulk of the calories (4 kcal/gram) in most diets and starches provide the bulk of that. Age, sex, size, health, and the intensity of physical activity strongly affect the daily need for calories. Moderately active females (19-22 years old) need 1500-2500 kcal/day, while males of the same age need 2500-3300 kcal/day.

In some poor countries, too many children do not receive enough calories to grow properly. In order to maintain blood sugar levels, they attack their own protein. This condition of semi-starvation is known as marasmus.

Protein

Humans must include adequate amounts of 9 amino acids in their diet. These "essential" amino acids cannot be synthesized from other precursors. However, cysteine can partially meet the need for methionine (they both contain sulfur), and tyrosine can partially substitute for phenylalanine.

The Essential Amino Acids
Histidine
Isoleucine
Leucine
Lysine
Methionine (and/or cysteine)
Phenylalanine (and/or tyrosine)
Threonine
Tryptophan
Valine
Two of the essential amino acids, lysine and tryptophan, are poorly represented in most plant proteins. Thus strict vegetarians should take special pains to ensure that their diet contains sufficient amounts of these two amino acids.

Fats

Ingested fats provide the precursors from which we synthesize our own fat as well as cholesterol and various phospholipids. Fat provides our most concentrated form of energy. Its energy content (9 kcal/gram) is over twice as great as carbohydrates and proteins (4 kcal/gram).

Humans can synthesize fat from carbohydrates (as most of us know all too well!). However, three essential fatty acids cannot be synthesized this way and must be incorporated in the diet. These are

All are unsaturated; that is, have double bonds.

Types of fats

Many studies have examined the relationship between fat in the diet and cardiovascular disease. There is still no consensus, but the evidence seems to indicate that:

Read the label!

At present, food labels in the U.S. list the total amount of fat in a serving of the product (5 g in the example shown here) with a breakdown of the amounts of saturated (1 g), polyunsaturated (0.5 g), and monounsaturated fat (1.5 g).

What about trans fats? There is a proposal to have them included, but at present they are not. However, if you add the amounts of saturated, polyunsaturated, and monounsaturated fat, and the total does not equal "Total Fat" , the discrepancy (2 g in this example) represents the amount of trans fat. Baked goods (like the one whose label is shown here) tend to have quite a bit of trans fat.

Minerals

Calcium

Calcium is essential to almost every function in the body. For most of these, such as only trace amounts are needed. However, large amounts of calcium are needed to make bone (which is 18% calcium), So substantial amounts are needed in the diet, especially during infancy, childhood, and pregnancy. Three hormones: work together to regulate how much calcium A temporary deficit in the amount of calcium in the diet can be compensated for by its removal from the huge reserves in bone.
Link to further information of calcium in the diet.

Iron (Fe)

Iron is incorporated in a number of body constituents, notably Not surprisingly, an iron deficiency shows up first as anemia.

In developed countries like the U.S., iron deficiency is the most common mineral deficiency. It is particularly common among women

Marginal iron intake is so widespread that some nutritionists want to have iron added to common foods like bread and cereals, just as some vitamins now are. However, excess iron in the body also leads to problems, and this has made the proposal controversial.

Even iron supplement tablets pose risks: thousands of children in the U.S. are accidentally poisoned each year by swallowing too many iron tablets. In fact, iron is the most frequent cause of poisoning deaths among children in the U.S.

Link to table giving recommendation for iron intake.

Iodine

Fluoride

The value of fluoride (in ionized form, F-) was first recognized as a preventive for dental caries (cavities). This makes sense because fluoride ions are incorporated along with calcium and phosphate ions in the crystalline structure of which both bones and teeth are constructed.

But it may have other functions. In order to grow properly, a rat must consume 0.5 parts per million (ppm) of fluoride ions in its diet.

Humans get most of their fluoride in drinking water. In regions where the natural amount is less than 1 ppm, many communities add enough fluoride to bring the concentration up to 1 ppm.

Perhaps because the range between optimum and excess is more narrow for fluoride than for most minerals in the diet, water fluoridation has been controversial. Leaving aside the philosophical and political questions raised by proponents and opponents of fluoridation, the safety and efficacy of this public health measure has been thoroughly established.

Zinc

Zinc is incorporated in many:

Zinc supplements are popular for their supposed antioxidant properties and to hasten the recovery from colds. Excessive intake of zinc causes a brief illness. Its most frequent cause is from ingested acidic food or drink that has been stored in galvanized (zinc-coated) containers.

Vitamins

Link to table giving recommendations for vitamin intake.

Vitamin A (Retinol)

Vitamin B1 (Thiamine)

Vitamin B2 (Riboflavin)

Vitamin B12

Niacin (Nicotinic acid)

Folic acid (Folacin)

Vitamin C (Ascorbic acid)

However, in vitro studies show that a level of vitamin C equivalent to a person consuming 200 mg a day converts some lipids into substances that can damage DNA.
Link to table giving recommendation for vitamin C intake.

Vitamin D

Vitamin E (Tocopherol)

Vitamin K

Link to table giving recommendations for vitamin intake.

"Natural" versus "Synthetic" Vitamins

Link to discussion of the physiology of the gastrointestinal (GI) tract.
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24 July 2001