Fat-free mass or lean body mass

Fat and fat-free mass together make up total body weight. These terms should be distinguished from adipose tissue and lean body mass which also together make up total body weight. The terms fat-free mass and lean body mass are often used interchangeably but this is not appropriate. Adipose tissue is not pure fat, but approximately 79% fat, 3% protein and 18% water. Also, tissue other than adipose tissue is not totally fat-free, since there is lipid in cell membranes and in nervous tissue. Our understanding of body composition in human subjects is based on the chemical analyses of six cadavers (five male and one female), performed between 1945 and 1956. On average fat-free tissue contains about 72.5% water, 20.5% protein and 69 mmol K/kg. However, individual tissues and organs vary in their protein, water and mineral content.

Water and electrolytes

A typical normal young adult male who weighs 70 kg with an assumed fat content of 12 kg (17%), would have fat-free mass of 58 kg (83%). By far the largest component of fat-free mass is water, on average 72.5%. Approximately two-thirds of this water is inside cells, ie intracellular fluid (ICF), and the remaining third is extracellular, ie extracellular fluid (ECF).

The total amount of water in the body, and the partition of body water between ICF and ECF, is closely regulated (see chapter 4). Body stores of protein, energy, vitamins and minerals ensure survival for many weeks without dietary intake, but deprivation of water causes death in a few days. The principal anion in ICF is K, with a small amount of Na and Mg. The principal anion in ECF is Na, and Cl is the principal cation.

Bone

An adult body contains approximately 1 kg of Ca, of which 99% is in bone. This acts as a reservoir to maintain an appropriate plasma concentration of calcium. The skeleton also contains approximately 500 g of P and more than half the collagen in the body, the remaining collagen being in skin, tendons, and fascial sheaths. For more details see chapter 9.

Muscle

There are three types of muscle: skeletal, smooth and cardiac. In a typical adult, skeletal muscle accounts for approximately 35% of body weight, and another 10% is smooth muscle. Resting skeletal muscle has a lower energy consumption per unit weight than tissues such as heart, liver, kidneys or brain, but during vigorous physical exercise the energy consumption in muscle greatly exceeds the total of all other body tissues. Skeletal muscle consists of bundles of individual muscle fibres, each constituting a muscle cell and containing hundreds to thousands of myofibrils which each contains about 1500 myosin filaments overlapping with around 3000 actin filaments. When the muscle contracts the overlap between the actin and myosin fibres increases and the bones to which these ends are attached are pulled towards each other. Skeletal muscle has two types of muscle fibres: red and white fibres. Red muscle sustains posture for long periods, while white muscle is for rapid movement. In human subjects most muscles contain a mixture of red and white fibres. The process of muscle contraction is considered in chapter 7.

Smooth muscle fibres are far smaller and lie in the walls of blood vessels, gut, bile ducts, ureters, and uterus. Contraction and relaxation of the smooth muscle in the walls of these organs alters the diameter of the tubes, or may be organized in peristaltic waves so as to move forwards the contents of the tube.

Cardiac muscle forms the chambers of the heart. Unlike skeletal muscle it is not organized as bundles of individually innervated muscle fibres, but is a syncytium of cells fused end-to-end in a latticework. The electrical potential that causes one cell to contract passes to adjacent cells so that the whole mass of muscle contracts and relaxes synchronously. Contraction expels the blood within the chamber through an exit valve, and then as it relaxes the chamber refills with blood that flows in through an entry valve.

Blood

Blood consists of plasma and cells – red blood cells (RBCs), white blood cells (WBCs) and platelets. Its main function is to transport oxygen, nutrients, and hormones to the tissues, and to remove carbon dioxide and other waste products from tissues. In a typical adult the volume of blood is approximately 5 L, of which about 55% of the volume is plasma and 45% packed cells, (the haematocrit). In a glass test tube blood solidifies as a web of the protein fibrin forms and contracts, trapping the cells to form a blood clot and a clear yellowish supernatant fluid (serum). Serum is plasma from which the blood clotting proteins have been removed. To obtain plasma fresh blood is put into a tube with an anticoagulant, and then separated from the cells by centrifugation.

RBCs contain haemoglobin in a biconcave cell wall which is freely permeable to oxygen. They enable blood to transport large amounts of oxygen. RBCs develop in the bone marrow from stem cells to reticulocytes which contain remnants of nucleic acid and are the most immature form of red cell normally seen in the peripheral circulation. Normally 99% of RBCs are fully mature, with no nucleic acid or nucleus, and survive in the circulation for about 120 days, then are destroyed in the reticulo-endothelial system. The haemoglobin is broken down to bilirubin, and the iron is recycled for the synthesis of new haemoglobin for new RBCs.

WBCs are approximately a thousand times less common than RBCs in the blood. The commonest type of WBC is the polymorph, or neutrophil granulocyte, which, like RBCs, is formed in bone marrow and rapidly increases in number in response to infection or tissue injury, surviving in the peripheral circulation for a very short time; their half-life is estimated at 6–8 hours. The next commonest WBC is the lymphocyte, which is formed in lymphoid tissue and is involved in immune responses. The remaining WBCs (monocytes, basophil and eosinophil granulocytes) occur still more rarely. Platelets are very small cells that are involved in blood clotting and the repair of damaged blood vessels.

Body fat

Body fat is a valuable store of energy during times of famine, and a thermal insulator against cold, but in affluent countries (and increasingly in developing countries) the need for protection from famine and cold is less often required, and excessive fatness is increasingly common.

The typical 70 kg adult male contains 12 kg of fat, which is 17% of body weight. This degree of fatness is within the healthy range (15–18%). Usually 90% of this fat is under the skin, but some is within the abdominal cavity, and a small amount between muscles. Not all body fat is available for energy; even in people who have died of starvation about 2 kg of fat remains. A reserve of 10 kg of fat can provide 375 MJ (90,000 kcal) of energy, which is equivalent to about 4 weeks of normal energy requirements. In fact people of normal weight undergoing total starvation survive for about 10 weeks, because energy expenditure decreases. In severely obese people the fat stores may reach 80 kg; such people could survive a year of starvation, although this is not appropriate treatment for severe obesity. The amount and distribution of body fat differs between men and women, women having a higher % body fat, the healthy range being 22–25%. At puberty, women tend to store fat around breast, hip and thigh regions, whereas men tend to accumulate fat in and on the abdomen.

Body fat is synthesized by, and stored in, fat cells, or adipocytes. There are two types, white (the majority) and brown. White adipocytes can develop at any age if the amount of fat to be stored increases. They contain a single central droplet of triacylglycerol, surrounded by a thin layer of cytoplasm containing the nucleus. White adipose tissue is a source of hormones, at least two of which, oestrogen and leptin, have important functions related to energy balance and reproductive function. In post-menopausal women adipose tissue is the only route of oestrogen production. Leptin is the product of the ob gene, which is involved in the control of both food intake and energy expenditure. Obese mice which lack this gene are deficient in leptin and infertile; administration of leptin corrects both obesity and infertility in these animals. Unfortunately obese human subjects usually have abnormally high circulating leptin concentrations, and the administration of leptin is not an effective therapy.

The brown fat cell differs from white adipocytes in having only small fat droplets in a cytoplasm rich in mitochondria. Brown fat cells are particularly important to small mammals including human babies because they generate heat, which helps to maintain body temperature in cold environments.

Throughout the lifespan

On the first day after fertilization the human embryo is a single cell, approximately 0.15 mm in diameter. After two months of intrauterine life it is about 30 mm long with recognizable head, trunk and limbs; the head accounts for half the total body length. By the end of normal gestation at 9 months, the fetus is 500 mm long and weighs about 3.5 kg, the head being one-quarter of total length. After two decades of extrauterine life the average adult weighs about 70 kg and is 1.7–1.8 m tall, with the head accounting for only one-eighth of total height.

During growth there are changes in the composition of the tissues (Table 2.1). The embryo contains a very high percentage of water, but with maturation during childhood and adolescence the proportion of water in the body, and the proportion of extracellular to intracellular water, decreases. Fat-free mass remains fairly constant in both men and women between the ages of 20 and 65 years, but then decreases by about 15% in the next two decades. Throughout adult life there is a trend for fat mass to increase in both men and women, but the increase is more rapid in post-menopausal women.

Table 2.1 Effect of growth, malnutrition and obesity on the composition of the body

Through diet and exercise

Over a given period, if the energy intake of an individual is less than energy output, then the energy stored in the body is reduced by the same amount, by losing fat, fat-free tissue, or (more probably) a mixture of both lean and fat tissue. However as weight is lost, resting energy expenditure also decreases, so for a given energy intake the energy deficit, and hence the rate of weight loss, also decreases. Weight loss is significantly faster during the first week than during the next 2 weeks because during the first week on a diet glycogen and its associated water (see chapter 4) is lost, especially on very low carbohydrate diets. During total starvation weight loss is even more rapid, partly because the energy deficit is greater, and partly because a higher proportion of the weight loss is fat-free mass to provide amino acids for glucose synthesis (see chapter 4). Total starvation has ceased to be an acceptable treatment for gross obesity because several patients died unexpectedly, due to damage to heart muscle.

Strenuous exercise causes muscle hypertrophy, and immobilization causes atrophy of muscle. Exercise also increases energy expenditure, so a combination of a low energy diet and exercise should achieve more fat loss, and less loss of fat-free mass, than diet alone.

Bone density is affected by diet, exercise, genetic and other factors (see chapter 9).

Through changes in hydration

Regulation of body water is discussed in chapter 4. Dehydration may occur if water losses are very high, as in severe diarrhoea, or with sweat loss in high ambient temperature or during prolonged vigorous exercise. Dehydration may also occur with abuse of diuretic drugs to achieve weight loss, or during the recovery phase of diabetic coma.

Water excess, indicated by oedema, is much more common than dehydration. The commonest cause in elderly people is congestive heart failure: the heart is unable to pump blood out as fast as it returns from the veins, the veins become engorged and leak fluid into the tissues. Another cause is when the kidney leaks albumin into the urine, so decreasing the plasma oncotic pressure allowing fluid to pass from the vascular system into the extracellular spaces.

Oedema is also a feature of certain types of malnutrition. It is a striking feature of kwashiorkor (see chapter 8). A marasmic child does not show obvious oedema; however in such children water accounts for an abnormally high proportion of body weight.