Macronutrients

Carbohydrates

The main source of energy for most Asians, Africans and Latin Americans is carbohydrates in the food they eat. Carbohydrates constitute by far the greatest portion of their diet, as much as 80 percent in some cases. In contrast, carbohydrates make up only 45 to 50 percent of the diet of many people in industrialized countries.

Carbohydrates are compounds containing carbon, hydrogen and oxygen in the proportions 6:12:6. They are burned during metabolism to produce energy, liberating carbon dioxide (CO2) and water (H2O). The carbohydrates in the human diet are mainly in the form of starches and various sugars. Carbohydrates can be divided into three groups:

· monosaccharides, e.g. glucose, fructose, galactose;

· disaccharides, e.g. sucrose (table sugar), lactose, maltose;

· polysaccharides, e.g. starch, glycogen (animal starch), cellulose.

Monosaccharides

The simplest carbohydrates are the monosaccharides, or simple sugars. These sugars can pass through the wall of the alimentary tract without being changed by the digestive enzymes. The three most common are glucose, fructose and galactose.

Glucose, sometimes also called dextrose, is present in fruit, sweet potatoes, onions and other plant substances. It is the substance into which many other carbohydrates, such as the disaccharides and starches, are converted by the digestive enzymes. Glucose is oxidized to produce energy, heat and carbon dioxide, which is exhaled in breathing.

Because glucose is the sugar in blood, it is most often used as an energy-producing substance for persons fed intravenously. Glucose dissolved in sterile water, usually in concentrations of 5 or 10 percent, is frequently used for this purpose.

Fructose is present in honey and some fruit juices. Galactose is a monosaccharide that is formed, along with glucose, when the milk sugar lactose is broken down by the digestive enzymes.

Disaccharides

The disaccharides, composed of simple sugars, need to be converted by the body into monosaccharides before they can be absorbed from the alimentary tract. Examples of disaccharides are sucrose, lactose and maltose. Sucrose is the scientific name for table sugar (the kind that is used, for example, to sweeten tea). It is most commonly produced from sugar cane but is also produced from beets. Sucrose is also present in carrots and pineapple. Lactose is the disaccharide present in human and animal milk. It is much less sweet than sucrose. Maltose is found in germinating seeds.

Polysaccharides

The polysaccharides are chemically the most complicated carbohydrates. They tend to be insoluble in water, and only some can be used by human beings to produce energy. Examples of polysaccharides are starch, glycogen and cellulose.

Starch is an important source of energy for humans. It occurs in cereal grains as well as in root foods such as potatoes and cassava. Starch is liberated during cooking when the starch granules rupture because of heating.

Glycogen is made in the human body and is sometimes known as animal starch. It is formed from monosaccharides produced by the digestion of dietary starch. Starch from rice or cassava is broken down in the intestines to form monosaccharide molecules, which pass into the bloodstream. Those surplus monosaccharides that are not used to produce energy (and carbon dioxide and water) are fused together to form a new polysaccharide, glycogen. Glycogen is usually present in muscle and in the liver, but not in large amounts.

Any of the digestible carbohydrates when consumed in excess of body needs are converted by the body into fat which is laid down as adipose tissue beneath the skin and at other sites in the body.

Cellulose, hemicellulose, lignin, pectin and gums are sometimes called unavailable carbohydrates because humans cannot digest them. Cellulose and hemicellulose are plant polymers that are the main components of cell walls. They are fibrous substances. Cellulose, which is a polymer of glucose, is one of the fibres of green plants. Hemicellulose is a polymer of other sugars, usually hexose and pentose. Lignin is the main component of wood. Pectins are present in plant tissue and sap and are colloidal polysaccharides. Gums are also viscous carbohydrates extracted from plants. Pectins and gums are both used by the food industry. The human alimentary tract cannot break down these carbohydrates or utilize them to produce energy. Some animals, such as cattle, have microorganisms in their intestines that break down cellulose and make it available as an energy-producing food. In humans, any of the unavailable carbohydrates present in food pass through the intestinal tract. They form much of the bulk and roughage evacuated in human faeces, and are often termed "dietary fibre".

There is increasing interest in fibre in diets, because high-fibre diets are now considered healthful. A clear advantage of a high-fibre diet is a lower incidence of constipation than among people who consume a low-fibre diet. The bulk in high-fibre diets may contribute a feeling of fullness or satiety which may lead to less consumption of energy, and this may help reduce the likelihood of obesity. A high-fibre diet results in more rapid transit of food through the intestinal tract and is thus believed to assist normal and healthy intestinal and bowel functioning. Dietary fibre has also been found to bind bile in the intestines.

It is now recognized that the high fibre content of most traditional diets may be an important factor in the prevention of certain diseases which appear to be much more prevalent in people consuming the low-fibre diets common in industrialized countries. Because it facilitates the rapid passage of materials through the intestine, fibre may be a factor in the control of diverticulitis, appendicitis, haemorrhoids and also possibly arteriosclerosis, which leads to coronary heart disease and some cancers.

Frequent consumption of any sticky fermentable carbohydrates, either starch or sugar, can contribute to dental caries, particularly when coupled with poor oral hygiene. Adequate intake of fluoride and/or a topical application is the best protection against caries (see Chapter 21).
Fats

In many developing countries dietary fats make up a smaller part of total energy intake (often only 8 or 10 percent) than carbohydrates. In most industrialized countries the proportion of fat intake is much higher. In the United States, for example, an average of 36 percent of total energy is derived from fat.

Fats, like carbohydrates, contain carbon, hydrogen and oxygen. They are insoluble in water but soluble in such chemical solvents as ether, chloroform and benzene. The term "fat" is used here to include all fats and oils that are edible and occur in human diets, ranging from those that are solid at cool room temperatures, such as butter, to those that are liquid at similar temperatures, such as groundnut or cottonseed oils. (In some terminologies the word "oil" is used to refer to those materials that are liquid at room temperature, while those that are solid are called fats.)

Fats (also referred to as lipids) in the body are divided into two groups: storage fat and structural fat. Storage fat provides a reserve storehouse of fuel for the body, while the structural fats are part of the essential structure of the cells, occurring in cell membranes, mitochondria and intracellular organelles.

Cholesterol is a lipid present in all cell membranes. It has an important role in fat transport and is the precursor from which bile salts and adrenal and sex hormones are made.

Dietary fats consist mainly of triglycerides, which can be split into glycerol and chains of carbon, hydrogen and oxygen called fatty acids. This action, the digestion or breakdown of fats, is achieved in the human intestine by enzymes known as lipases, which are present primarily in the pancreatic and intestinal secretions. Bile salts from the liver emulsify the fatty acids to make them more soluble in water and hence more easily absorbed.

The many fatty acids in human diets are divided into two main groups: saturated and unsaturated. The latter group includes both polyunsaturated and mono-unsaturated fatty acids. Saturated fatty acids have the maximum number of hydrogen atoms that their chemical structure will permit. All fats and oils eaten by humans are mixtures of saturated and unsaturated fatty acids. Broadly speaking, fats from land animals (i.e. meat fat, butter and ghee) contain more saturated fatty acids than do those of vegetable origin. Fats from plant products and to some extent those from fish have more unsaturated fatty acids, particularly polyunsaturated fatty acids (PUFAs). There are exceptions, however. For example, coconut oil has a large amount of saturated fatty acids.

These groupings of fats have important health implications because excess intake of saturated fats is one of the risk factors associated with arteriosclerosis and coronary heart disease (see Chapter 23). In contrast, PUFAs are believed to be protective.

PUFAs also include two unsaturated fatty acids, linoleic acid and linolenic acid, which have been termed "essential fatty acids" (EFAs) as they are necessary for good health. EFAs are important in the synthesis of many cell structures and several biologically important compounds. Recent studies have also shown the benefits of other longer-chain fatty acids in the growth and development of young children, and arachidonic acid and docosa-hexaenoic acid (DHA) should conditionally be considered essential during early development. Experiments with animals and studies in humans have shown definite skin and growth changes and abnormal vascular and neural function in the absence of these fatty acids, and there is no doubt that they are essential for the nutrition of individual cells and tissues of the body.

Fat is desirable to make the diet more palatable. It also yields about 9 kcal/g, which is more than twice the energy yielded by carbohydrates and proteins (about 4 kcal/g); fat can therefore reduce the bulk of the diet. A person doing very heavy work, especially in a cold climate, may require as many as 4 000 kcal a day. In such a case it is highly desirable that a good proportion of the energy should come from fat; otherwise the diet would be very bulky. Bulky diets can be a particularly serious problem in young children as well. A reasonable increase in the fat or oil content of the diets of young children raises the energy density of predominantly bulky carbohydrate diets and is highly desirable.

Fat also functions as a vehicle that assists the absorption of fat-soluble vitamins (see Chapter 11).

Thus fats, and even specific types of fat, are essential to health. However, practically all diets provide the small amount required.

Fat deposited in the human body serves as a reserve fuel. It is an economic way of storing energy, because, as mentioned above, fat yields about twice as much energy, weight for weight, as does carbohydrate or protein. Fat is present beneath the skin as an insulation against cold, and it forms a supporting tissue for many organs such as the heart and intestines.

All fat in the body is not necessarily derived from fat that has been eaten. However, excess calories from the carbohydrate and protein in, for example, maize, cassava, rice or wheat can be converted into fat in the human body.
Proteins

Like carbohydrates and fats, proteins contain carbon, hydrogen and oxygen, but they also contain nitrogen and often sulphur. They are particularly important as nitrogenous substances, and are necessary for growth and repair of the body. Proteins are the main structural constituents of the cells and tissues of the body, and they make up the greater portion of the substance of the muscles and organs (apart from water). The proteins in different body tissues are not all exactly the same. The proteins in liver, in blood and in specific hormones, for example, are all different.

Proteins are necessary

· for growth and development of the body;

· for body maintenance and the repair and replacement of worn out or damaged tissues;

· to produce metabolic and digestive enzymes;

· as an essential constituent of certain hormones, such as thyroxine and insulin.

Although proteins can yield energy, their main importance is rather as an essential constituent of all cells. All cells may need replacement from time to time, and their replacement requires protein.

Any protein eaten in excess of the amount needed for growth, cell and fluid replacement and various other metabolic functions is used to provide energy, which the body obtains by changing the protein into carbohydrate. If the carbohydrate and fat in the diet do not provide adequate energy, then protein is used to provide energy; as a result less protein is available for growth, cell replacement and other metabolic needs. This point is especially important for children, who need extra protein for growth. If they get too little food for their energy requirements, then the protein will be diverted for daily energy needs and will not be used for growth.

Amino acids

All proteins consist of large molecules which are made of amino acids. The amino acids in any protein are linked together in chains, called peptide linkages. The various proteins are made of different amino acids linked together in different chains. Because there are many different amino acids, there are many different possible configurations, so there are many different proteins.

During digestion proteins break down to form amino acids much as complex carbohydrates such as starches break down into simple monosaccharides and fats break down into fatty acids. In the stomach and intestines various proteolytic enzymes hydrolyse the protein, releasing amino acids and peptides.

Plants are able to synthesize amino acids from simple inorganic chemical substances. Animals do not have this ability; they derive all the amino acids necessary for building their protein from consumption of plants or animals. As the animals eaten by humans initially derived their protein from plants, all amino acids in human diets have originated from this source.

Animals have differing abilities to convert one amino acid into another. In the human this ability is limited. Conversion occurs mainly in the liver. If the ability to convert one amino acid into another were unlimited, then the question of the protein content of diets and the prevention of protein deficiency would be simple. It would be enough merely to supply sufficient protein, irrespective of the quality or amino acid content of the protein supplied.

Of the large number of amino acids, 20 are common in plants and animals. Of these, eight have been found to be essential for the adult human and have thus been termed "essential amino acids" or "indispensable amino acids", namely: phenyl-alanine, tryptophan, methionine, lysine, leucine, isoleucine, valine and threonine. A ninth amino acid, histidine, is required for growth and is essential for infants and children; it may also be necessary for tissue repair. Other amino acids include glycine, alanine, serine, cystine, tyrosine, aspartic acid, glutamic acid, proline, hydroxyproline, citrulline and arginine. Each protein in a food is composed of a particular mixture of amino acids which might or might not contain all eight of the essential ones.

Protein quality and quantity

To assess the protein value of any food it is useful to know how much total protein it contains, which amino acids it has and how many essential amino acids are present and in what proportion. Much is now known about the individual proteins present in various foods, their amino acid content and therefore their quality and quantity. Some have a better mixture of amino acids than others, and these are said to have a higher biological value. The proteins albumin in egg and casein in milk, for example, contain all the essential amino acids in good proportions and are nutritionally superior to such proteins as zein in maize, which contains little tryptophan or lysine, and the protein in wheat, which contains only small quantities of lysine. It is not true, however, to say that the proteins in maize and wheat are not valuable. Although they contain less of certain amino acids, they do contain some amount of all the essential amino acids as well as many of the other important ones. The relative deficiency of maize and wheat proteins can be overcome by providing other foodstuffs containing more of the limited amino acids. It is therefore possible for two foods with low-value protein to complement each other to form a good protein mixture when eaten together.

Humans, especially children on diets deficient in animal protein, require a variety of foods of vegetable origin, not just one staple food. In many diets, pulses or legumes such as groundnuts, beans and cowpeas, though short of sulphur-containing amino acids, supplement the cereal proteins, which are often short of lysine. A mixture of foods of vegetable origin, especially if taken at the same meal, can serve as a substitute for animal protein.

FAO has produced tables showing the content of essential amino acids in different foodstuffs, from which it can be seen which foods best complement each other. It is also necessary, of course, to ascertain the total quantity of protein and amino acids in any food.

The quality of the protein depends largely on its amino acid composition and its digestibility. If a protein is deficient in one or more essential amino acids, its quality is lower. The most deficient of the essential amino acids in a protein is called the "limiting amino acid". The limiting amino acid determines the efficiency of utilization of the protein present in a food or combination of foods. Human beings usually eat food in meals which contain many proteins; they seldom consume just one protein. Therefore nutritionists are interested in the protein quality of a person's diet or meals, rather than just one food. If one essential amino acid is in short supply in the diet, it limits the use of the other amino acids for building protein.

Readers who wish to become familiar with the methods used for determining protein quality are advised to consult comprehensive textbooks on nutrition, which describe them in detail (see Bibliography). One method uses experiments on growth and nitrogen retention in young rats. Another involves determination of the amino acid or chemical score, usually by examining the efficiency of utilization of proteins in the foods consumed by comparing their amino acid composition with that of protein known to be of high quality, such as that in whole eggs.

The chemical score may thus be defined as the efficiency of utilization of food protein in comparison with whole egg protein. Net protein utilization (NPU) is a measure of the amount or percentage of protein retained in relation to that consumed. As an example, Table 16 gives the chemical score and NPU of the protein in five foods.

It is not usual or easy to obtain NPU values in people, and in most studies rats are used. Table 16 suggests that there is a good correlation between the values in rats and in children, and that chemical score provides a reasonable estimate of protein quality.

For the professional involved in nutritional activities to help people - be it a dietitian in a health facility, an agricultural extension worker or a nutrition educator what is important is that the protein value differs among foods and that mixing foods improves the protein quality of the meal or the diet. Table 17 gives the protein content and the limiting amino acid score of some commonly eaten plant-based foods. Because Iysine is most commonly the limiting amino acid in many foods of plant origin, the Iysine score is also given.

Protein digestion and absorption

Proteins consumed in the diet undergo a series of chemical changes in the gastrointestinal tract. The physiology of protein digestion is complicated; pepsin and rennin from the stomach, trypsin from the pancreas and erepsin from the intestines hydrolyse proteins into their component amino acids. Most of the amino acids are absorbed into the bloodstream from the small intestine and thus travel to the liver and from there all over the body. Any surplus amino acids are stripped of the amino (NH2) group, which goes to form urea in the urine, leaving the rest of the molecule to be transformed into glucose. There is now some evidence that a little intact protein is taken up into certain cells lining the intestines. Some of this protein in the infant may have a role in the passive immunity conveyed from the mother to her newborn child.

A little of the protein and amino acids released in the intestines is not absorbed. The unabsorbed amino acids, plus cells shed from the intestinal villi and acted upon by bacteria, together with gut organisms, contribute to the nitrogen found in faeces.