Biochemistry and Human Nutrition

Module 4. Human nutrition

Lesson 24
HUMAN NUTRITION

24.1 Introduction

• Most foods contain a mix of some or all of the nutrient classes, together with other substances, such as toxins of various sorts. Some nutrients can be stored internally while others are required more or less continuously. Poor health can be caused by a lack of required nutrients or, in extreme cases, too much of a required nutrient. For example, both salt and water (both absolutely required) will cause illness or even death in excessive amounts.
• There are five basic components in a adequate diet that can be grouped into two category

1. Macronutrient (needed in relatively large amounts)

• Carbohydrates
• Fats
• Proteins

• The macronutrients (excluding water) provide structural material (amino acids from which proteins are built, and lipids from which cell membranes and some signaling molecules are built).
• Some of the structural material can be used to generate energy internally, and in either case it is measured in Joules or kilocalories (often called "Calories" and written with a capital C to distinguish them from little 'c' calories). Carbohydrates and proteins provide 17 kJ approximately (4 kcal) of energy per gram, while fats provide 37 kJ (9 kcal) per gram. Though the net energy from either depends on such factors as absorption and digestive effort.

2. Micronutrient (needed in smaller quantities)

• Vitamins
• Minerals and trace elements

• Micronutrients also include antioxidants and phytochemicals, which are said to influence (or protect) some body systems Their necessity is not as well established as in the case of, for instance, vitamins.

24.2 Carbohydrates

• Carbohydrates include sugars, starches and fiber. They constitute a large part of foods such as rice, noodles, bread, and other grain-based products. They are the major source of biological energy through their oxidation. They furnish base material for the biosynthesis of many cell components.
• Carbohydrates rich foods are abundant & cheap. They form a major part of the diet of the world. 4/5 world’s population rely on plant food. Carbohydrates provide 70-90% of calorie intake. But in affluent countries, where meat & dairy products are consumed it contribute 45% of total daily caloric intake. 40 % of carbohydrate is furnished by sucrose to the refined sugar.
• Dietary fiber is a carbohydrate (or a polysaccharide) that is incompletely absorbed in humans and in some animals. Like all carbohydrates, when it is metabolized it can produce four Calories (kilocalories) of energy per gram. However, in most circumstances it accounts for less than that because of its limited absorption and digestibility. Dietary fiber consists mainly of cellulose, a large carbohydrate polymer that is indigestible because humans do not have the required enzymes to disassemble it.

24.3 Fats

• A molecule of dietary fat typically consists of several fatty acids (containing long chains of carbon and hydrogen atoms), bonded to a glycerol. They are typically found as triglycerides (three fatty acids attached to one glycerol backbone).Triacylglycerols from animal and plant sources rank close behind carbohydrates as major sources of energy.
• Saturated fats (typically from animal sources) have been a staple in many world cultures for millennia. Unsaturated fats (e. g., vegetable oil) are considered healthier, while trans fats are to be avoided. Saturated and some trans fats are typically solid at room temperature (such as butter or lard), while unsaturated fats are typically liquids (such as olive oil or flaxseed oil). Trans fats are very rare in nature, and have been shown to be highly detrimental to human health, but have properties useful in the food processing industry, such as rancidity resistance.
• Most fatty acids are non-essential, meaning the body can produce them as needed, generally from other fatty acids and always by expending energy to do so. However, in humans, at least two fatty acids are essential and must be included in the diet An appropriate balance of essential fatty acids—omega-3 and omega-6 fatty acids—seems also important for health. Both of these "omega" long-chain polyunsaturated fatty acids are substrates for a class of eicosanoids known as prostaglandins, which have roles throughout the human body. They are hormones, in some respects.
• The omega-3 eicosapentaenoic acid (EPA), which can be made in the human body from the omega-3 essential fatty acid alpha-linolenic acid (LNA), or taken in through marine food sources, serves as a building block for series 3 prostaglandins (e.g. weakly inflammatory PGE3). The omega-6 dihomo-gamma-linolenic acid (DGLA) serves as a building block for series 1 prostaglandins (e.g. anti-inflammatory PGE1), whereas arachidonic acid (AA) serves as a building block for series 2 prostaglandins (e.g. pro-inflammatory PGE 2). Both DGLA and AA can be made from the omega-6 linoleic acid (LA) in the human body, or can be taken in directly through food. An appropriately balanced intake of omega-3 and omega-6 partly determines the relative production of different prostaglandins, which is one reason why a balance between omega-3 and omega-6 is believed important for cardiovascular health. In industrialized societies, people typically consume large amounts of processed vegetable oils, which have reduced amounts of the essential fatty acids along with too much of omega-6 fatty acids relative to omega-3 fatty acids.

24.4 Protein

• Most meats such as chicken contain all the essential amino acids needed for humans. Proteins are the basis of many animal body structures (e.g. muscles, skin, and hair). They also form the enzymes that control chemical reactions throughout the body.
• Each molecule is composed of amino acids, which are characterized by inclusion of nitrogen and sometimes sulphur (these components are responsible for the distinctive smell of burning protein, such as the keratin in hair). The body requires amino acids to produce new proteins (protein retention) and to replace damaged proteins (maintenance). As there is no protein or amino acid storage provision, amino acids must be present in the diet. Excess amino acids are discarded, typically in the urine.
• For all animals, some amino acids are essential (an animal cannot produce them internally) and some are non-essential (the animal can produce them from other nitrogen-containing compounds). About twenty amino acids are found in the human body, and about eight of these are essential and, therefore, must be included in the diet (HITPMTLLV). A diet that contains adequate amounts of amino acids (especially those that are essential) is particularly important in some situations: during early development and maturation, pregnancy, lactation, or injury (a burn, for instance). A complete protein source contains all the essential amino acids; an incomplete protein source lacks one or more of the essential amino acids.
• It is possible to combine two incomplete protein sources (e.g. rice and beans) to make a complete protein source, and characteristic combinations are the basis of distinct cultural cooking traditions. Sources of dietary protein include meats, tofu and other soy-products, eggs, legumes, and dairy products such as milk and cheese. Excess amino acids from protein can be converted into glucose and used for fuel through a process called gluconeogenesis.

24.5 Minerals

Dietary minerals are the chemical elements required by living organisms, other than the four elements carbon, hydrogen, nitrogen, and oxygen that are present in nearly all organic molecules. Some dietitians recommend that these be supplied from foods in which they occur naturally, or at least as complex compounds, or sometimes even from natural inorganic sources (such as calcium carbonate from ground oyster shells). Some minerals are absorbed much more readily in the ionic forms found in such sources. On the other hand, minerals are often artificially added to the diet as supplements; the most famous is likely iodine in iodized salt which prevents goiter.

24.6 Macro minerals

Many elements are essential in relative quantity; they are usually called "bulk minerals". Some are structural, but many play a role as electrolytes. Following elements with recommended dietary allowance (RDA) greater than 200 mg/day are,

• Calcium, a common electrolyte, but also needed structurally (for muscle and digestive system health, bone strength, some forms neutralize acidity, may help clear toxins, provides signaling ions for nerve and membrane functions)
• Chlorine as chloride ions; very common electrolyte; see sodium, below
• Magnesium, required for processing ATP and related reactions (builds bone, causes strong peristalsis, increases flexibility, increases alkalinity)
• Phosphorus, required component of bones; essential for energy processing
• Potassium, a very common electrolyte (heart and nerve health)
• Sodium (also see salt), a very common electrolyte; not generally found in dietary supplements, despite being needed in large quantities, because the ion is very common in food: typically as sodium chloride, or common salt. Excessive sodium consumption can deplete calcium and magnesium, leading to high blood pressure and osteoporosis
• Sulfur, for three essential amino acids and therefore many proteins (skin, hair, nails, liver, and pancreas). Sulfur is not consumed alone, but in the form of sulfur-containing amino acids

24.7 Trace Minerals

Many elements are required in trace amounts, usually because they play a catalytic role in enzymes. Some trace mineral elements (RDA < 200 mg/day) are, in alphabetical order:

• Cobalt required for biosynthesis of vitamin B12 family of coenzymes. Animals cannot biosynthesize B12, and must obtain this cobalt-containing vitamin in the diet
• Copper required component of many redox enzymes, including cytochrome c oxidase and blood protein ceruloplasmin. Copper toxicity causes Wilson’s disease.
• Chromium required for sugar metabolism
• Iodine required not only for the biosynthesis of thyroxine, but probably, for other important organs as breast, stomach, salivary glands, thymus etc. (see Extrathyroidal iodine); for this reason iodine is needed in larger quantities than others in this list, and sometimes classified with the macrominerals
• Iron required for many enzymes, and for hemoglobin and some other proteins
• Manganese (processing of oxygen)
• Molybdenum required for xanthine oxidase and related oxidases
• Nickel present in urease
• Selenium required for peroxidase (antioxidant proteins)
• Vanadium (Speculative: there is no established RDA for vanadium. No specific biochemical function has been identified for it in humans, although vanadium is required for some lower organisms.
• Zinc required for several enzymes such as carboxypeptidase, liver alcohol dehydrogenase, glutamate dehydrogenase and carbonic anhydrase

24.8 Vitamins

As with the minerals discussed above, some vitamins are recognized as essential nutrients, necessary in the diet for good health. (Vitamin D is the exception: it can be synthesized in the skin, in the presence of UVB radiation.)

Water Soluble – B1, B2, B6, B12
Fat Soluble – A, D, E K

• Serve as essential components of specific coenzymes and enzymes participating in metabolism and other specialized activities like NADPH, FAD.
• Vitamin deficiencies may result in disease conditions, including goitre, scurvy, osteoporosis, impaired immune system, disorders of cell metabolism, certain forms of cancer, symptoms of premature aging, and poor psychological health (including eating disorders), among many others Excess levels of some vitamins are also dangerous to health (notably vitamin A), and for at least one vitamin, B6, toxicity begins at levels not far above the required amount.

24.9 Water
It is not fully clear how much water intake is needed by healthy people, although some assert that 6–8 glasses of water daily is the minimum to maintain proper hydration.The original water intake recommendation in 1945 by the Food and Nutrition Board of the National Research Council read: "An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods." The latest dietary reference intake report by the United States National Research Council recommended, generally, (including food sources): 2.7 liters of water total for women and 3.7 liters for men. Specifically, pregnant and breastfeeding women need additional fluids to stay hydrated.

Water is sometimes called the “silent nutrient” and is taken for granted in nutritional consideration. A deficient intake, however, can produce death faster than that of any other nutrient. Total body water in humans varies from 55% to 65% of body weight depending on body composition. Lean body tissues contain approximately 75% of water, but adipose tissue has very little water. Therefore, the percentage of water is greater in lean than in obese individuals.

Most of the body water is found within three major body compartments: intracellular fluid (within the cells) has about 70%, interstitial fluid (e.g., lymph) has about 20%, and blood plasma has about 7%. The latter two compartments together come under the extracellular fluid category. The remaining 3% of body water is in the intestinal lumen, cerebrospinal fluid and other body compartments. The body controls the amount of water in each compartment mainly by controlling the ion concentrations in each compartment. Intracellular water volume depends primarily on intracellular potassium and phosphate concentration. Extracellular water volume depends primarily on extracellular sodium and chloride concentration.

The body has three sources of water: ingested water and beverages, the water content of solid foods, and metabolic water which is derived from the oxidation of carbohydrate, fat, and protein. The latter amounts to some 300-350 g per day in an average adult male. According to composite estimates, 100 g of starch yields 55 g of water, 100 g of fat yields 107 g of water, and 100 g of protein gives 41 g of water. Water is absorbed in the upper small intestine and is distributed by way of the lymph and blood into and from the various tissues and cells of the body. Eventually water is excreted via the kidneys, sweat, expired air, feces, and so on.

Under ordinary conditions, the water balance between the cells and the fluids of the body is maintained at a constant level. The loss of water equals the intake and endogenous formation, and is in the range of 2-4 L. Water intake is regulated mainly by “thirst” and the output is controlled by antidiuretic hormone and the kidneys. If excessive amounts of water are ingested, the kidneys excrete the excess. On the other hand, if the fluid intake is low, the kidneys excrete a more concentrated urine so that less water is lost from the body. Starvation or a carbohydrate-restricted regimen is associated with an acute loss of body water (e.g., 1-1.5L) which represents the water normally held by glycogen storage in the tissues.

Table 24.1 List of forty different substances which are essential for human

24.10 Energy provided by oxidation of bulk organic nutrient

The first requisite of an adequate diet is a source of energy- provided by oxidation of the three bulk nutrients

• Carbohydrates
• Fats
• Proteins

Unit—Kilocalorie (kcal)
The amount of energy which in the form of heat is required to raise the temperature of 1.0Kg of water by 1.0 for 15 to 16^oC.
Recommended daily energy allowances by Food & Nutrition Board.

College-age male require—2900 kcal/d
Female – 2100 kcal/d

Compare these values with basal caloric requirement i.e. the amount of energy needed by the body at complete rest 12h after a meal.

For college age males Basal Requirement-1800 kcal/d
Females-1300kcal/d

The excess energy-for physical work.

Amount of energy released by the oxidation of carbohydrates, fat, proteins has been determined by burning weighed samples in an atmosphere of oxygen in a Bomb calorimeter and total amount of heat produced is measured.

Caloric equivalent of Major Nutrient
Energy equivalent kcal/g

Carbohydrates 4.2
Fat 9.5
Protein 4.3

These foods when oxidized in body i.e. they are completely digested and absorbed yield an amount of heat equal to the heat released when they are oxidized in a calorimeter.