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 generally 18 different ones, calcium in relatively large amounts, zinc in "trace" amounts
- vitamins - a dozen or so, small organic molecules that we cannot synthesize from other precursors in our diet
Determining what substances must be incorporated in the human diet and how much of each is incorporated even after years of research still under active study. Why the uncertainty? 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.
- scurvy: lack of ascorbic acid (vitamin C)
- beriberi: lack of thiamine (vitamin B1)
- pellagra: lack of niacin
Similarly, 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 Food and Nutrition Board of the U. S. National Academy of Sciences publishes guidelines. One of the most useful of these is called recommended daily allowances or RDAs. These provide the basis for the nutrition labels on food.
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 (19–30 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.
|Methionine (and/or cysteine)|
|Phenylalanine (and/or tyrosine)|
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. Birds, mammals, and some other animals are able to discriminate food that contains a nutrient, e.g., an essential amino acid, that they need from food that doesn't. If offered a food lacking that nutrient, they quickly stop eating it. How is this done? In rats, at least, it turns out that certain neurons in the brain detect the lack of an essential amino acid and signal the appetite centers of the brain to stop feeding on deficient food. The neurons detect the lack by the failure of their transfer RNAs (tRNAs) for that amino acid to acquire it. Rats whose tRNAs for threonine have been blocked from loading threonine cease feeding even if their food contains adequate concentrations of it.
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
- linoleic acid
- linolenic acid
- arachidonic acid
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 fewer double bonds and of those that remain, converting them from a cis to a trans configuration.
- 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 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. For this reason, 1.1 grams/day for women (1.6 for men) is recommended.
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. Blood clotting, intracellular signaling and muscle contraction need only trace amounts. 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 parathyroid hormone (PTH), calcitonin and calciferol (vitamin D) 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 cytochromes, myoglobin and hemoglobin. 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 and 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.
Because iodine deficiency during pregnancy can lead to mental retardation of the infant, it is recommended that pregnant women receive 150–250 µg of iodine daily during both pregnancy and lactation. Hundreds of supplements — both prescription and nonprescription — are sold for this purpose. However, a study of 60 of them reported in the 2/26/09 issue of The New England Journal of Medicine found that only 9 of the 60 contained an amount of iodine within 5% of the amount claimed on the label. Others ranged from only 11% of the amount claimed to almost 3 times as much. Examples: one (prescription) preparation claiming a daily dose of 150 µg actually provided only 26 µg while another (nonprescription) preparation claiming 226 µg of iodine actually contained 610 µg!
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. The rat in the bottom photo received the same diet as that in the top except that tin, vanadium, and fluorides were carefully excluded for 20 days. When tin and vanadium were then given to the deprived rat, it still did not grow normally. But adding 0.5 ppm of potassium fluoride (KF) to its diet restored normal growth and health. (Photos courtesy of Klaus Schwarz, VA Hospital, Long Beach, CA.)
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 enzymes and transcription factors. 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.
Vitamin A (Retinol)
- Functions: Multiple, including serving as the precursor to retinal, the prosthetic group of all four of the light-absorbing pigments in the eye and regulating gene expression essential for the health of epithelia.
- Sources: cream, butter, fish liver oils, eggs. Carrots and some other vegetables provide beta-carotene, which the liver can convert into vitamin A.
- Deficiency: night-blindness.
- Excess: stored in the liver, but can be toxic in large doses, especially in children. Even in adults the range between too little and too much is narrow: ingesting vitamin A in amounts not much greater than the recommended dietary allowance (RDA) leads to an increase in bone fractures later in life. High doses taken early in pregnancy have been linked to a greater risk of birth defects. (Its chemical relative isotretinoin — the acne treatment Accutane® — is such a notorious teratogen that it should not be used when there is any chance of a pregnancy occurring).
Thiamine ( Vitamin B1)
- 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.
Riboflavin ( Vitamin B2)
- Function: prosthetic group of flavoprotein enzymes, e.g., flavin adenine dinucleotide (FAD) 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.
Niacin (Nicotinic acid or Vitamin B3)
- 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 (maize) is the staple carbohydrate.
- Excess: accidental ingestion of very high doses produces a brief illness, but niacin is water-soluble and any excess is quickly excreted.
Biotin (Vitamin B7)
- Function: this member of the B vitamins is a cofactor in many essential metabolic enzymes.
- Sources: liver, egg yolks, corn (maize), intestinal bacteria.
- Deficiency: rare except perhaps during pregnancy.
- Excess: none identified.
- 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.
Folic acid ( Folacin)
- 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.
- Excess: water soluble and any excess easily excreted.
Vitamin C (Ascorbic acid)
- 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, cancer, etc. They seem to suffer no harm except, perhaps, to their wallets.
- Functions: absorption of calcium from the intestine and bone formation.
- synthesized when ultraviolet light (mostly UV-B) strikes the skin (forms vitamin D3).
- present in some fish (e.g., salmon), cod liver oil, eggs, and steroid-containing foods irradiated with ultraviolet light.
- rickets — inadequate conversion of cartilage to bone — in children;
- osteomalacia — softening of the bones — in adults.
- breast feeding and
- protecting children from exposure to the sun
Breast milk provides less than 20% of the recommended daily dose for infants. Until the infant is old enough to eat foods fortified with vitamin D, many pediatricians recommend vitamin supplements for breast-fed babies.
- Excess: However, this fat-soluble vitamin is dangerous in very high doses, especially in infants, causing excessive calcium deposits and mental retardation. So some pediatricians view the use of vitamin D supplements for infants with caution (especially since certain preparations have been found to contain amounts far higher than that listed on the label).
Vitamin E (Tocopherol)
- Function: acts as an antioxidant agent in cells.
- Sources: vegetable oils, nuts, spinach.
- Deficiency: anemia, damage to the retinas.
- Excess: high doses may be toxic.
- 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. Because
- little or no vitamin K crosses the placenta,
- the colon of newborn babies has not yet been colonized by vitamin K-synthesizing bacteria,
- breast milk is a poor source of the vitamin,
- Excess: No risk from natural forms of the vitamin (K1 and K2).
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 or whether it is still in plant or animal material or has been extracted and incorporated in a pill.
Control of Food Intake
A complex web of signals controls appetite and the intake of food. These include both nerve signals and hormones - both of which signal centers in the brain - chiefly in the hypothalamus. This table lists some of the hormonal signals that have been identified, their effect on appetite and weight gain. Such complexity probably reflects the need for redundant circuits in such a vital activity as acquiring food. But, it also frustrates the search for treatments to attack the increasing incidence of obesity.
|Appetite Stimulants||Appetite Suppressants|
|Agouti-related protein (AgRP)||α-MSH and β-MSH|
|Neuropeptide Y (NPY)||β-endorphin|
|Melanin-concentrating hormone (MCH)||Cholecystokinin (CCK)|
|Orexins (also called hypocretins)||Insulin|
|Brain-derived neurotrophic factor (BDNF)|
This diagram presents a model of how some of the chief players interact.
- After a period of fasting, secretion of ghrelin activates neurons ("X") in the hypothalamus. They release the excitatory neurotransmitter glutamate where they synapse with AgRP/NPY-releasing neurons. These set in motion the signals that induce feeding.
- A positive feedback loop strengthens the response: AgRP and NPY inhibit the activity of proopiomelanocortin (POMC) neurons whose function is to inhibit "X" neurons (a double-negative is a positive).
- When satiety is finally reached, leptin activates the POMC neurons which release α-MSH and β-endorphin where they synapse with the "X" neurons and the stimulus to continue feeding is stopped. (The precise identity of the "X" neurons remains to be determined.)