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 afteryears of research — still under active study. Why the uncertainty?
• Vitamins. Inadequate intake of some vitamins produces easily-recognized deficiency diseases like
o scurvy: lack of ascorbic acid (vitamin C)
o beriberi: lack of thiamine (vitamin B1)
o pellagra: lack of niacin.
However, it is so difficult to exclude some other possible vitamins from the diet that deficiency diseases arehard 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 andmolybdenum.
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 thebulk 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 ownprotein. This condition of semi-starvation is known as marasmus.
Carbohydrates have the general molecular formula CH2O, and thus were once thought to represent "hydrated carbon". However, the arrangement of atoms in carbohydrates has little to do with water molecules.
Starch and cellulose are two common carbohydrates. Both are macromolecules with molecular weights in the hundreds ofthousands. Both are polymers (hence "polysaccharides"); that is, each is built from repeating units, monomers, much as a chain is built from its links.
The monomers of both starch and cellulose are the same: units of the sugar glucose.
Three common sugars share the same molecular formula: C6H12O6. Because of their six carbon atoms, each is a hexose.
• glucose,"blood sugar", the immediate source of energy for cellular respiration
• galactose, a sugar in milk (and yogurt), and
• fructose, a sugar found in honey.
Although all three share the same molecular formula (C6H12O6), the arrangement of atoms differs in each case. Substances such as these three, which have identical molecular formulas but different structural formulas, are known as structuralisomers.
Glucose, galactose, and fructose are "single" sugars or monosaccharides. Two monosaccharides can be linked together to form a "double" sugar or disaccharide.
Three common disaccharides:
• sucrose — common table sugar = glucose + fructose
• lactose — major sugar in milk = glucose + galactose
• maltose — product of starch digestion = glucose + glucose