Principles of Biochemistry Volume 3 - A. Lehninger 1985

Selected Aspects of Human Biochemistry
Human Nutrition
A balanced diet must contain five essential components

Having examined the coordination system of metabolic processes in The Human Body, we shall now focus on how this intricately organized system ensures the normal course of METABOLISM through the assimilation of nutrients.

The formulation of modern scientific concepts regarding Human Nutrition stands as one of biochemistry's greatest achievements, having saved countless human lives. Not so long ago, diseases such as pellagra, beriberi, and Rickets were widespread across many countries. Today, these conditions have been eradicated, and we know how to entirely prevent their recurrence. At the same time, it is well known that roughly one-eighth of the global population currently suffers from food scarcity. Paradoxically, many people in developed nations suffer from poor nutrition not due to a lack of food, but rather from overeating and imbalanced diets. One of biochemistry's primary tasks is to provide the public with reliable, scientifically grounded nutritional information capable of dispelling irrational folk myths and exposing the fallacies of dietary quackery and charlatanism.

A complete and balanced diet must include five Major Classes of nutrients (Table 26-1), with each Class playing a distinct and vital role.

Table 26-1. Essential nutrients for humans¹

¹ Water, although not strictly a nutrient in the traditional sense, is nonetheless essential for humans to replace losses caused by perspiration, Respiration, and urination.

a. Carbohydrates

Carbohydrates are the most abundant dietary nutrients, and their oxidation in the human body generates the bulk of metabolic energy. They also serve as precursors in The Biosynthesis of numerous cellular components.

b. Fats

Triacylglycerols of animal and plant origin, much like carbohydrates, function as a primary energy source and additionally provide carbon skeletons for the Biosynthesis of Cholesterol and Other Steroids. Plant-derived triacylglycerols also serve as a source of essential fatty acids.

c. Proteins

Dietary proteins fulfill three principal Functions. First, they provide essential and nonessential amino acids that serve as building blocks for Protein Biosynthesis—not only in infants and children, but also in adults—thereby supporting continuous protein turnover and renewal. Second, amino acid carbon skeletons act as precursors for Hormones, Porphyrins, and many other Biomolecules. Third, The oxidation of the carbon skeletons of amino acids makes a modest yet significant contribution to daily Energy Expenditure.

Carbohydrates, fats, and proteins constitute the primary Macronutrients. Their daily intake depends on an individual's weight, age, and sex, and is measured in hundreds of grams.

Vitamins

Vitamins are classified into two groups: water-soluble and fat-soluble. They are organic micronutrients required in daily amounts of only milligrams or micrograms. Vitamins serve as indispensable components of specific Coenzymes or Enzymes involved in metabolism and other specialized biochemical reactions.

d. Inorganic Substances and Trace Elements

Inorganic substances essential for proper nutrition can be divided into two categories. The first category includes elements such as calcium, phosphorus, and magnesium, which the human body requires in gram quantities daily. The second category comprises iron, iodine, zinc, copper, and several other elements needed in milligram or microgram amounts. Inorganic elements perform diverse functions: they act as Structural components of bones and Teeth, serve as electrolytes in maintaining the water-salt balance of Blood and Tissues, and function as prosthetic groups in various enzymes.

For optimal nutrition, humans must ingest more than 40 distinct essential substances (Table 26-1). These include 10 amino acids, 13 vitamins, 20 or more inorganic elements (typically as soluble salts), and one or more polyunsaturated fatty acids. To these must be added dietary fiber, which consists primarily of Cellulose and other indigestible plant Cell wall polymers. Although un-digestible and thus not metabolically active, dietary fiber is essential for normal intestinal peristalsis.

The Food and Nutrition Board of the U.S. National Academy of Sciences and National Research Council has established the Recommended Dietary Allowances (RDA), which outlines the various nutrients required for optimal nutrition in infants, children, adult men and women, and pregnant women (Table 26-2). The figures presented therein do not represent minimal daily requirements, but rather intake levels designed to provide a generous safety margin.

Table 26-2. Recommended Daily Dietary Allowances, Food and Nutrition Board (Revised 1980)

¹⁾ The recommendations below apply to healthy individuals living in the United States under normal conditions. The diet should include a variety of conventional foods to ensure an adequate supply of nutrients whose requirements have not yet been fully established.