The science of food and man

The famous French gastronome Brillat Savarin said, “Tell me what a man eats and I will tell you what he is.” We rely on other biological systems to provide us with complex molecules which we break down by digestion and rebuild into the bodily material we call ourselves.

The body’s primary need is for energy. This is measured in kilocalories which are defined as the amount of heat needed to raise the temperature of 1,000 grams of water from 14 to 15 C. kilocalories (kcals) are often described as calories although the unit of kilojoules (kJs) is replacing calories. There are 4.2 kJs per kcal.


Fat is the most concentrated form of food energy giving 9 kcals (38 kJs) per gram when oxidised during metabolism. Fat may be of animal origin (butter, lard, dripping); or of plant origin (corn, olive or palm oil, or margarine). Fat is found in many foods which we would not usually think of, such as lean meat, wheat germ, egg yolk and milk. It adds flavours, textures and energy to food and reduces the bulk of food required to satisfy energy requirements.

The fats in our foods are usually triglycerides, which means that they consist of three different fatty acids combined with one molecule of glycerol. Each of the fatty acids consists of a chain of carbon atoms and the length of this chain plus the number

of hydrogen atoms attached to the chain affects the nature of the fat. If all the space on the carbon chain is occupied by hydrogen it is a saturated fat. If there are a lesser number of hydrogen atoms the fat is called unsaturated. Saturated fats are usually solid at room temperature (butter, coconut oil). Unsaturated fats tend to remain liquid at room temperature. Some solidify if they are stored in the refrigerator.

There is controversy about the quantity and quality of fat which is ideal for a healthy diet. As countries increase in affluence the percentage of energy from fat increases. We now get over 40 per cent of our energy from fats which we eat; and a recent government report recommended that we should eat less fat. There is also dispute about the relative health benefits of unsaturated versus saturated fats.

There are two unsaturated fats which the body cannot synthesize called linoleic and linolenic acid. They are known as essential fatty acids. The need for these has been shown in experiments on rats but has not been proved for humans. It is likely that man does need very tiny amounts of these essential fatty acids.


Carbohydrates contain the elements carbon, hydrogen and oxygen in the ratio 1:2:1, and yield 4 kcals (17 kJs) per gram when metabolized. The smallest unit of carbohydrate is a monosaccharide sugar which contains a ring of six carbon atoms. The basic sugars in the diet are glucose, fructose (fruit sugar) and galactose (milk sugar). They each have their own type of sweet flavour. Household sugar is a disaccharide, comprising a molecule of glucose joined to a molecule of fructose. This is the sweetest sugar of all and the most extensively used. Maltose, with two molecules of glucose, is common in the plant kingdom (e.g. in germinating barley) and lactose is a disaccharide of glucose and galactose which is found in dairy products and human milk, as it is ideal for most babies to digest.

The body absorbs and digests sugars rapidly into the blood system. Secretion of the hormone insulin from the pancreas causes the blood sugar level to be kept constant within very narrow limits and stimulates excess sugar to be converted to fat and stored in adipose tissue.

Longer chains of sugars are called polysaccharides and the most common of these is starch. Long chains of glucose, joined in a straight or branched fashion, give rise to different types of starch. Some plants manufacture starch as an energy store, and many of these are cultivated. Cereals and root vegetables make the largest contribution of starch to the diet. They are not easily soluble and only become digestible when cooked. The absorption and digestion of starch is slower than that of sugar and the effect on blood sugar and insulin levels is less marked.

The polysaccharide made by animals is glycogen. It is not an important food source but it is a shortterm energy store which is vital for active animals.

There are certain types of complex carbohydrates which are indigestible. These form the cell walls and protective parts of plants and are pectins, lignins and celluloses. They add bulk to food as it passes down the gut. They retain water and form a large part of the faeces produced. Although they have no positive food value, they are vital in maintaining a healthy digestive tract.

Another energyproducing food is alcohol which is metabolised by the liver and yields 7 kcals (29 kJs) per gram. National Food Survey figures show that we now obtain five per cent of our dietary energy from alcoholic beverages.


Proteins can be metabolised by the liver to release energy if our intake of energy foods is insufficient. The protein is split, the nitrogen excreted and the remaining molecules directed along energy producing pathways to yield 4 kcals (17 kJs) per gram. Protein is more important for its use in tissue synthesis and growth, as it contains nitrogen which is an essential component of living tissue. It features in structural and chemical functions in every cell. The smallest unit of protein is an amino acid. Thousands of types of amino acids are possible chemically, but there are only 20 in nature, from which all types of protein are made. Set sequences of amino acids give specific proteins whose qualities are determined by their chemical arrangement. Plants synthesize all their own amino acids from the elements, but mammals require the amino acid units readymade. The liver can change some of the amino acids it receives by rearranging chemical groups but there are eight “essential” ones which cannot be made.

Body proteins are constantly being broken down and renewed. We can see the growth of hair, nails and skin, but blood, muscle, bone and all other tissues are also replaced. Old protein is replaced by protein synthesis from protein we eat and it is essential to health to achieve a balance between input and output. Growing children retain dietary nitrogen and pass less out than they take in. People on starvation or after severe injury will pass out more nitro gen as tissues are being broken down more quickly than they are being replaced.

Protein requirements are highest in adult life but the need relative to body size is greatest when we are born and declines as growth rate slows. A diet which contains six to eight per cent energy as protein will satisfy minimum requirements. Daily recommended intakes for the United Kingdom are calculated assuming a ten per cent intake of protein energy as this is a more palatable level. It is doubtful whether protein requirements are raised by energetic activity unless actual muscle mass is considerably increased.

The quality as well as the quantity of protein in the diet is crucial to health. A diet which is lacking in any one of the essential amino acids is useless – all eight must be present in sufficient amounts. Animal proteins, milk, fish, eggs, cheese and meat are good sources. Vegetable proteins often lack one of the essential amino acids although nuts, peas and beans compliment the deficiencies of cereals. It is possible to obtain all the protein one needs from vegetables alone, and the most efficient use of protein is obtained by using a wide range of protein foods. Deficiencies in one food can supplement that of another. For instance bread and cheese; milk and cereal; meat and potatoes; beans on toast; all supply more valuable protein mixtures than any one of these foods alone.


Mineral elements are required by the body to make healthy tissues.

Sodium and potassium are found everywhere in nature and a balance of these elements is essential to life. Dietary deficiency is rare as most diets supply an excess.

Calcium is needed in large amounts as it is the main component of bones and teeth. Requirements are relatively high for children, who need calcium during their growing years. Dairy produce like milk, cheese and yoghurt are rich sources along with cereals and canned fish. Vitamin D is closely connected with calcium metabolism, so both nutrients are needed. Another major bone mineral is phosphorus which is present as phosphates in all cells and dietary deficiency is almost unknown.

Magnesium is a vital component of cell enzyme systems. A requirement of 350 mg daily has been estimated for adults, most of this from cereals and vegetables. Deficiency is rare in all but severe malnutrition or chronic alcoholism.

Iron is a vital constituent of body tissues and is used in the bone marrow to manufacture haemoglobin, the red compound in red blood cells. Haemoglobin combines with oxygen in the lungs and circulates it to all tissues. Insufficient iron intake leads to anaemia through decreased haemoglobin or blood cell production, but the liver store of iron helps to prevent this. Normally only ten per cent of the iron in the diet is absorbed. Iron from animal foods is more easily absorbed than the iron in vegetable foods which is often found combined with other chemicals. The best food sources are red meats, offal meats, sardines, and other sources are peas, beans, potatoes, cereals, cocoa, curry powder, dried fruits and red wine. An excess of iron is harmful as it is stored in the body and cannot be easily excreted. It can be caused by use of iron utensils in cooking or overconsumption of cheap red wines or iron tablets.

Minerals required in tiny amounts are known as trace elements. Iodine is a constituent of thyroid hormone and insuffi cient iodine causes goitre. This is an excessive growth of the thyroid gland which causes a swelling in the neck. It is remedied by an increased intake of iodine as sea salt or any sea foods. Other minerals required in trace amounts are zinc, copper, cobalt, manganese, selenium and chromium.

Metals such as lead, mercury and arsenic are not required by the body and large amounts are harmful. Industrial wastes have caused an increase of such “heavy” metals in the environment but the levels in food are kept under government surveillance.


Vitamins are essential organic chemicals which must be included in the diet in relatively tiny but critical amounts in order to maintain health. The names of the vitamins are confusing as they are related to their discovery rather than function or chemistry.

Vitamin A, retinol, is needed for the repair of membranes which line the body. It is also present in a pigment in the retina of the eye which is involved in night vision. Retinol is found only in animal fats like milk, cheese, butter, liver, fish and margarine. Plants produce a similar compound, betacarotene, which the body can convert to retinol. This is abundant in coloured vegetables like carrots, tomatoes and dark green vegetables. Excessive intakes have been reported in food fanatics who consume vast quantities of tomato and carrot juice or fish liver extracts. The vitamin is not readily excreted and accumulation in the body results in a yellowing of the skin, toxaemia and death.

Vitamin D is a fat soluble vitamin which aids the absorption of calcium from the diet, and controls blood calcium levels and the exchange of calcium between blood and bone. Deficiency retards bone growth which can be permanent in children or dis abling in adults, particularly elderly people. Good sources are egg yolks, fatty fish, fish liver oils, margarine and dairy produce. The vitamin is synthesized in the skin in sunlight but in temperate climates this is inadequate for most of the year. Vitamin D is harmful in excessive doses as high blood levels can cause unwanted deposits of calcium in tissues.

Vitamin E has been shown to be necessary for fertility in rats but its role in human metabolism has not been made clear. It is stored in the body and it is doubtful whether deficiency exists in humans except in some premature babies. Excessive intakes do not seem to be harmful. The vitamin is widespread in food but is found especially in vegetable oils, wheat germ and eggs.

Vitamin K is involved in the complex mechanism which causes blood to clot. It is found widely in vegetable products, and is synthesized in the gut. Dietary deficiency is rare.

There are several vitamins in the B group known as the B complex as many of them occur together in food. They are water soluble and are not stored in the body so regular intake is essential. They all have important functions as enzymes which assist in the chemical reactions of the body’s metabolism.

Vitamin Bl, thiamin, is involved in the breakdown of carbohydrates to yield energy so requirements are related to carbohydrate and alcohol intakes. It is abundant in milk, offal, pork products, eggs, peas, beans, cereal products. As the vitamin is destroyed by heat, cooking losses can be considerable.

Vitamin B2, riboflavin, is also involved in energy utilization. Deficiency gives rise to skin disorders and cracks at the corners of the mouth. Milk, cheese, yoghurt, offal, yeast, peas and beans are good sources. It is heat stable but is sensitive to ultraviolet light so milk should not be left standing in sunlight.

Nicotinic acid is a B vitamin without a number which has important metabolic functions. Deficiency results in pellagra which shows as dark scaley patches of skin. It is present in most meat, fish and wheat products. The body can synthesize it from the essential amino acid tryptophan, and food content is always expressed as equivalents of nicotinic acid to give a true idea of biological usefulness. It is stable when heated but soluble and can be lost in cooking water.

Vitamin B6, pyridoxine, is involved in protein metabolism and requirements are related to protein intakes. Its wide distribution in meats, fish, eggs and cereals, ensures sufficient dietary intake.

Vitamin B12, cyanocobalamin, and folic acid are needed for the bone marrow cells which make blood cells. Vitamin B12 is widespread in animal foods and requires an essential substance, secreted by the stomach, for its absorption. Folic acid deficiency results in a different type of anaemia and is found in people who eat insufficient liver, peas, beans, raw vegetables and citrus fruit juices. Folates are destroyed by heating in cooking water so fresh food and minimal cooking are advisable.

Pantothenic acid and biotin are B vitamins involved in fat metabolism. Deficiencies are rare as both vitamins are widespread in food.

Vitamin C, ascorbic acid, is vital for healthy connective tissue; skin, blood vessels and gums, and white blood cells. Scurvy results from deficiency causing unhealing wounds, bruising and bleeding which can be fatal. Vitamin C must be consumed regularly and is found in vegetables, namely brussels sprouts, cauliflower, cabbages, potatoes, tomatoes, cress and such fruits as citrus fruits, melon, berries and fresh currants. Tinned fruit juices are excellent sources. Vitamin C is destroyed by cooking so fresh foods provide better food value.