Human Nutrition
Human Dietary Needs
The human diet must provide the following:
- calories; enough to meet our daily energy needs.
- amino acids. There are nine, or so, "essential" amino acids that we need for protein synthesis and that we cannot synthesize from other precursors.
- fatty acids. There are three "essential" fatty acids that we cannot synthesize from other precursors.
- minerals. Inorganic ions. We probably need 18 different ones: a few like calcium in relatively large amounts; most, like zinc, in "trace" amounts.
- vitamins. A dozen, or so, small organic molecules that we cannot synthesize from other precursors in our diet.
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?
- Vitamins. Inadequate intake of some vitamins produces easily-recognized deficiency diseases like
However, it is so difficult to exclude some other possible vitamins from the diet that deficiency diseases are hard to demonstrate.
- Minerals. Some minerals are needed is such vanishingly small amounts that it is practically impossible to prepare a diet that does not include them. However, totally synthetic diets are now available for intravenous feeding of people who cannot eat. This so-called total parenteral nutrition has revealed, unexpectedly, some additional trace element needs: chromium and molybdenum.
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.
In the future, they will be called Dietary Reference Intakes (DRIs).
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.
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.
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
- Saturated. No double bonds between the carbon atoms in the fatty acid chains. Most animal fats (e.g., butter) are highly saturated.
- Monounsaturated. Have a single double bond in the fatty acid chains. Examples are olive, peanut, and rapeseed (canola) oil.
- Polyunsaturated. Have two or more double bonds in their fatty acid chains. Examples: corn, soy bean, cottonseed, sunflower, and safflower oils.
- Trans Fats. Have been partially hydrogenated producing
- Omega-3 fats. Have at least one double bond three carbon atoms in from the end of the fatty acid molecule. Linolenic acid is an example. Fish oils are a rich source of omega-3 fatty acids.
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:
- a diet high in fat is harmful.
- mono and polyunsaturated fats are less harmful than saturated ones, except that
- trans unsaturated fats may be worse than saturated fats.
- ingestion of omega-3 unsaturated fats may be protective.
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.
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
- is absorbed from your food
- is taken from or added to bone
- is excreted in the urine.
A temporary deficit in the amount of calcium in the diet can be compensated for by its removal from the huge reserves in bone.
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
- because of the loss of blood during menstruation
- the need for extra iron during pregnancy and breast feeding.
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.
- Incorporated in the hormones thyroxine (T4) and triiodothyronine (T3).
- In regions with iodine-deficient soils, food may not contain enough iodine to meet body needs. The result is goiter: a swelling of the thyroid gland.
- The use of iodized salt (table salt to which a small amount of sodium iodide, KI, is added) has reduced the incidence of goiter in most developed countries.
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 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.
- Function: precursor to retinal, the prosthetic group of all four of the light-absorbing pigments in the eye.
- Sources: cream, butter, fish liver oils, eggs. Carrots and some other vegetables provide beta-carotene, which the body can convert into vitamin A.
- Deficiency: night-blindness.
- Excess: stored in the liver, but can be toxic in large doses, especially in children. High doses taken early in pregnancy have been linked to a greater risk of birth defects.
- Function: coenzyme in cellular respiration.
- Sources: meat, yeast, unpolished cereal grains, enriched bread and breakfast cereals.
- Deficiency: beriberi. Rarely found in developed countries except among alcoholics.
- Excess: water soluble and any excess easily excreted.
- Function: prosthetic group of flavoprotein enzymes used in cellular respiration.
- Sources: liver, eggs, cheese, milk, enriched bread and breakfast cereals.
- Deficiency: damage to eyes, mouth, and genitals.
- Excess: water soluble and any excess easily excreted.
- Function: needed for DNA synthesis.
- Sources: liver, eggs, milk; needs intrinsic factor to be absorbed.
- Deficiency: pernicious anemia; caused by lack of intrinsic factor or a vegan diet.
- Excess: none identified.
Niacin (Nicotinic acid)
- Function: this member of the B vitamins is a precursor of NAD and NADP
- Sources: meat, yeast, milk, enriched bread and breakfast cereals.
- Deficiency: pellagra (producing skin lesions); a risk where corn is the staple carbohydrate.
- Excess: accidental ingestion of very high doses produces a brief illness, but niacin is water soluble and any excess quickly excreted.
- Function: synthesis of purines and pyrimidines.
- Sources: green leafy vegetables, but destroyed by cooking.
- Deficiency: anemia, birth defects. Women who expect to become pregnant should be extra careful that they receive adequate amounts (400 µg/day). Starting 1 January 1998, any bread or breakfast cereal described as "enriched" must have enough folic acid added to it so that a single serving will provide 10% of this requirement.
Two research groups have found a statistical association between Alzheimer's disease and low levels of folic acid in blood samples taking earlier in the patient's life. Whether there is a causal connection between the two; that is, whether low levels of folic acid predispose to the development of Alzheimer's, is unknown.
- Excess: water soluble and any excess easily excreted.
- Functions: coenzyme in the synthesis of collagen.
- Sources: citrus fruits, green peppers, tomatoes; destroyed by cooking.
- Deficiency: scurvy.
- Excess: Many people take huge amounts of vitamin C, hoping to ward off colds, etc.. They seem to suffer no harm except, perhaps, to their wallets.
- Functions: absorption of calcium from the intestine and bone formation.
- Sources:
- synthesized when ultraviolet light strikes the skin (forms vitamin D3).
- Present in fish liver oils, butter, and steroid-containing foods irradiated with ultraviolet light (vitamin D2).
- Deficiency: rickets in children; osteomalacia (softening of the bones) in adults.
- Excess: This fat-soluble vitamin is dangerous in high doses, especially in infants, causing excessive calcium deposits and mental retardation.
- Function: acts as a reducing agent in cells.
- Sources: egg yolk, salad greens, vegetable oils.
- Deficiency: anemia, damage to the retinas.
- Excess: high doses may be toxic in infants.
- Function: needed for the synthesis of blood clotting factors.
- Sources: spinach and other green leafy vegetables; synthesized by intestinal bacteria.
- Deficiency: slow clotting of blood.
- Excess: high doses may be toxic in infants.
"Natural" versus "Synthetic" Vitamins
- There is no scientific distinction between them.
- The thiamine molecule (or any other molecule) is the same entity
- whether synthesized by a plant or by an organic chemist
- whether it is still in plant or animal material or has been extracted and incorporated in a pill.
24 July 1999
