10
Explaining the Fat Layer
‘Land dwellers have relatively thin skins.
The subcutaneous fat between the subcutaneous
muscles and the muscles of the
body is rarely a continuous thick layer.’
V.E. Sokolov
Those for whom the aquatic hypothesis is unacceptable have to seek some other way of dealing with the problem. The simplest ploy is to ignore its existence. The two textbooks on physical anthropology, mentioned at the beginning of Chapter 6, which between them devote three words to the loss of body hair, devote none at all to subcutaneous fat. It is another illustration of Medawar’s dictum that scientists tend not to ask themselves a question until they have a glimmering of the answer.
One suggestion has been that the fat layer evolved as a way of storing energy, and this theory is supported by citing other animals which accumulate fat under the skin, such as bears, marmots, hedgehogs and dormice. These are all hibernators; the fat layer is seasonal; they all belong to latitudes where winter brings cold and scarcity of food. In some warmer latitudes where an animal needs to store energy it stores it in some special depot where it will least impede locomotion; examples include the camel’s hump and the tails of the fat-tailed sheep and the fat-tailed gecko. None of these features applies to Homo.
Above all, the storage theory does not explain why only one primate species on the savannah needed to store energy, and it does not explain why the fat layer is thickest in babies. At that stage of their development, when they were being fed by their mothers, the danger of suffering a food shortage surely could not have been greater for hominid babies than for the young of other apes.
In the context of the savannah theory, the baby’s fat is in some respects the most baffling of all. It might be argued that a hairless baby has special need to guard against sudden chills. But a closer examination indicates that this cannot be the explanation.
Fat comes in two kinds – brown fat and white fat. Brown fat (it is the darker in colour because it has a much larger blood supply) is a special type found in all young mammals, and its function is to supply heat quickly. It is necessary because when they are first born they are unable to raise their temperature by shivering. Brown fat responds to a drop in body temperature by burning up quickly and converting into heat. White fat is predominant in mature mammals. It is stabler, does not burn up easily, and does not respond to a drop in temperature. White fat cells are depleted only when the body’s intake of calories is insufficient to replace the energy which has been expended.
If our babies became fat because being naked made them liable to chilling, we would expect most of their extra adipocytes to take the form of additional brown fat. The reverse is the case. They have the normal allowance of brown fat, but they are extremely unusual in also possessing appreciable amounts of white fat at birth. By the time they are three or four months old, all their brown fat has already been converted into the mature white form – good for insulation in water and for buoyancy, but no good at quickly raising body temperature.
More popular than the storage idea is the theory that is currently still the front runner. It proposes that the evolution of a fat-lined skin was part of a multi-stage strategy for regulating temperature. The story goes that the ancestral ape became a hunter and his exertions made him too hot; so he lost his fur to cool down; he then became too cold, especially at night, and installed a layer of fat to keep him warm; the fat made him too hot, so he evolved the capacity for profuse sweating to keep him cool.
There are variations on the theme. In some versions he was not a hunter but a gatherer; in some accounts the sweat came first, with the nakedness following to enable the sweat to evaporate. But none of the versions has produced a satisfactory explanation of why one primate species alone needed to resort to these complex ways of replacing a thermoregulatory system which works perfectly well for the other savannah animals, including savannah primates.
All in all, the concept of fat hominids evolving on the savannah is so inherently improbable that some attempt has been made to side-step it. This is done by suggesting that the subcutaneous fat layer did not evolve on the savannah, but only appeared at a much later date, in an agricultural economy. It is reasoned that the carbohydrate intake of agricultural man would be higher, and he would not need to race across the plains in pursuit of his dinner, so he could afford to carry a little more weight.
On the other hand, he would have even less need to evolve ways of storing surplus nourishment under his skin if he already had ways of storing it in barns and receptacles. Moreover, there are extant tribes who have never practised an agricultural economy and yet their young women have rounded breasts and limbs and chubby infants like the rest of humanity. And the ‘Venus’ figurines of women with fat thighs, buttocks and bellies predate the domestication of plants and animals.
The last of the non-aquatic hypotheses about fat suggests that the fat evolved first in the females to make them more easily distinguishable from the males and more attractive to them. The females then presumably passed on the new attraction to their sons as well as their daughters, though in a modified and more meagre version.
Undoubtedly there are wide differences between the sexes both in the amount and distribution of their adipose tissue. The percentage of fat in a woman’s body is on average about twice as high as in a man’s. And apart from the characteristic sex-linked accumulations on breast and buttocks, the fat on females tends to be more evenly distributed. Overweight women tend to be fat all over, while overweight men normally develop ‘male pattern obesity’. This is the condition where the fat accumulates disproportionately in the abdomen, giving rise to ‘the beer belly’ silhouette. It is often combined, as in Dickens’s Mr Pickwick, with quite thin arms and legs.
But there is one strong argument against the subcutaneous fat deposit having evolved initially as an epigamic marker – that is, for sexual attraction. The whole point about epigamic markers is that they are a sign not only of sexual differences but of sexual maturity. Young animals do not acquire manes or antlers or breeding plumage; these things only arrive with puberty. There is no conceivable reason why human infants should have such generous deposits of fat all over their bodies if the original purpose was that of arousing the sexual interest of the male.
As for the female’s extra ration of fat, the explanation of that is more likely to be practical than erotic. The gestation and breast-feeding of a human baby make heavy demands on the mother’s physical resources. If at any time during that period her intake of food is insufficient to meet those needs the white fat cells in her own body are drawn on to ensure that the baby is not affected.
For hominids in the wild, the bearing and feeding of offspring would not have been an occasional event in the life of the female. In laboratory apes the infant is generally removed from the mother at the end of a year, or at most two years, but in the wild, breast-feeding continues longer. When an infant is weaned – or if by chance it dies – the oestrus cycle soon recommences. But it does not last for long. On average only between three and four cycles elapse before the female is again pregnant. With the exception of these brief intervals, for the greater part of her life the physical needs of a bouncing baby would have imposed on a hominid mother an ongoing need for reserves of white fat which the male did not share.
In apes, the non-stop cycle of pregnancy and feeding continues till death; in human females it ends with the menopause. No one is quite sure why women are granted this dispensation, which is very rare in nature – though it has recently been claimed that we share it with whales and dolphins.
One suggestion is that the menopause tends to prolong the active life of grandmothers by lessening the physical strain on them. The young of humans take a very long time to mature, and the success of our species depends to a unique extent on the passing on of acquired knowledge and skills to the next generation. So it would have been advantageous for a breeding population of hominids if there were
some experienced non-breeding females present to help in the task of child rearing.
It seems a little odd that just around the time of the menopause, when there are not going to be any more children to gestate and suckle, there is a general tendency in women of all races for their weight to go up instead of down, and for an increased percentage of their body tissues to consist of fat.
But it has now been learned that adipose tissue performs additional functions besides insulation and buoyancy. Besides being constantly active in the turnover of fuels in the body, it is capable of storing steroids (hormones), and it influences both the amount and the potency of the female sex hormone oestrogen circulating in the blood.
Oestrogen occurs in two forms – a relatively inactive form and a highly potent form. Thin women have elevated levels of the weak type, while fatter women have higher levels of the strong. In fatter women the oestrogen also circulates more freely in the blood stream. Until 1975 it was believed that oestrogen was produced only in the ovaries; it was then discovered that it is also produced in fat cells, by converting the male hormone androgen (found at low levels in female blood plasma) into oestrogen.
It is not inconceivable that the ‘middle age spread’ which so many women battle against evolved as a benign development. It would ensure that the body’s auxiliary source of oestrogen (adipose tissue) was in plentiful supply. When the ovaries ceased to function this would minimise the withdrawal symptoms. It would enable the body to adjust more gradually to the drop in oestrogen levels instead of quitting, as it were, ‘cold turkey’.
For most people in the developed countries today it is easier than ever before for people to eat and drink too much and take too little exercise. In these conditions the capacity of the human body to gain weight rapidly and far in excess of what is needed constitutes a problem. Because it can have adverse effects on health and morale, interest in the subject has grown rapidly in recent years.
One good thing has come out of the contemporary obsession with slimming. It has prompted active research into a subject which most scientists have previously found boring. In the ’70s a textbook on animal anatomy, over 500 pages long, devoted only one page to a description of adipose tissue before concluding with the comment that ‘… little more seems necessary to be said about the subject’.
Scientists wishing to find more to say about it were confronted with an initial difficulty: most land mammals, especially those suitable for laboratory observation, do not resemble humans in the distribution of their adipose tissue. In a normal rat, for example, only about two per cent of the body consists of fat tissue, and however much it is overfed, it fails to become obese.
For a considerable period, therefore, research concentrated disproportionately on one aberrant strain known as the ‘obese rat’; and it was thought that lessons learned from an abnormal rat could be applied to perfectly normal humans. It was found, for example, that if obese rats under the age of three months were allowed more than their share of the maternal milk supply, or weaned earlier onto solid food, they grew extra adipocytes and ended up fatter than the other rats from the same litter. Women were given stern warnings about overfeeding their babies or introducing them to solid food prematurely. Plump teenagers cast reproachful eyes on guilty mothers and blamed them for their unhappy plight.
Later it was discovered that data on obese rats did not hold true for human babies, and amendments were issued on the lines of: ‘The early introduction of solid or bottle feeding does not increase the risk of infant obesity. Anxious parents should be given reassurance on this point.’
Clearly, people are a special case. The trouble with using them for research is that it often calls for long-term planning and the results are slow to emerge. There was a general agreement among doctors that people who are significantly overweight have an increased risk of becoming ill or dying, especially from cardiovascular disease – heart attacks and strokes. So it seemed only sensible to conclude that the fatter people got, the more likely it was that they would suffer from these conditions, and vice versa.
This belief was not easy to verify. But one of the functions of science is to convert facts that we are nearly sure we know into facts that have been checked and proved. So in 1967 in Sweden a project was set up involving 782 men of the same age (54 years old) resident in Gothenburg and selected on a random basis. Records were kept, including measurements of height, weight, blood pressure, cholesterol levels and the circumference of waist and hips. There was a hundred per cent follow-up on the subsequent medical histories of the men over the next thirteen years, with repeat examinations in 1973 and 1980.
The records showed that the group with the lowest risk of premature death or coronary heart disease were the fattest men, as calculated by weight in relation to height (The Body Mass Index). The highest risk group were in the leanest category by the same height-weight ratio. It was precisely the opposite of the result that the researchers had expected.
These findings only began to make sense when the figures were analysed in greater detail. It was then found that the crucial factor was not the overall amount of body fat, but the way in which it was distributed. The high risk group consisted of men with moderately lean legs and buttocks but with body fat concentrated in the abdominal region – the type of distribution identified by the French physiologist Vague in the ’60s as ‘android’ or male type. The low risk group of men weighed the heaviest, but their fat was more evenly distributed all over the body in the gynoid (female type) pattern.
Similar studies were subsequently carried out on women in Germany and France and the results indicated that the same general rules applied. Women with more android figures were more at risk than fatter women with a more gynoid silhouette.
Explanations of why a fat belly is more unhealthy than fat thighs and buttocks are still tentative. One idea is that free fatty acids from adipocytes in the abdomen go directly to the liver via the hepatic portal vein and overload it, while fatty acids from the thighs travel around the body and reach the liver by a more roundabout route, and by the time they reach it they are in lower concentrations.
The lesson to be learned from the Gothenburg experiment would seem to be that – for women, at least – a few extra pounds are not as serious a health risk as previously supposed. In Australia not long ago a young couple in their thirties, Michael and Sue Murnane, in all other respects seen as potentially ‘perfect parents’, were forbidden to adopt a Korean baby on the grounds that they were overweight. In Korea and Sri Lanka the law apparently assumed that any adopters weighing 30 per cent more than the average would be liable to leave the baby orphaned before it had time to grow up. That seems an unnecessarily alarmist proposition.
Nowadays scientists are only responsible for a tiny percentage of the millions of words of advice and exhortation which pour from the presses on this subject. Helping people to lose weight has become a multi-million dollar industry with a massive advertising budget. Insofar as it encourages people to eat less and eat more sensibly, it is doing an excellent job and can take credit for improving health awareness and the general standard of fitness.
But there is another side of the coin. The slimming gurus, like other glamour promoters, are in the business of selling dreams. When they prescribe their norms of the ‘correct’ vital statistics for a particular age group to aim at, they have every financial incentive to pitch the figures low. The ‘target weights’ are lower than the average; lower than most people can easily sustain, and lower than health considerations alone would dictate. There is even some reason to believe that they are lower than considerations of sex appeal would dictate. In one experiment, a panel of males were shown pictures of women ranging by gradations from thin to fat, and asked to pick out the most attractive. A panel of women was then invited to guess the men’s verdict. The results consistently indicated that men are less attracted to a Twiggy-type female silhouette than women imagine they are.
The subliminal message in some of the ‘advice�
�� commercially offered is that the average female is too fat, that fat means ugly, that failure to get rid of it is something to be ashamed of. It lends credence to the Duchess of Windsor’s dotty dictum: ‘You can never be too rich – nor too thin.’ It may seem strange that in our day physically normal healthy people will pay surgeons to operate on them to bring their figures closer to an arbitrarily prescribed ideal. But it is not a new phenomenon. The same impulse once drove Chinese ladies to resort to the service of a foot binder, and English ones to demand surgery on their lower ribs to help them attain a sixteen-inch waist.
The real casualties of the ‘never-too-thin’ propaganda are for the most part young people. Some of them believe the message, and it can induce in them a debilitating loss of self-esteem leading to depression or a flight into food obsessions like anorexia.
In 1985 the gynaecologist John Studd conducted an investigation into the immediate and more long term effects of anorexia. He reported: ‘Oestrogen isn’t only produced in the ovaries. It is partly made in body fat. The less you have, the less you make … It isn’t uncommon to find an anorexic woman of 28 or 35 with the skin and bones of a 70-year-old.’
Research based on his findings suggested a correlation between low oestrogen and reduced levels of collagen. Collagen is the protein which constitutes one-third of the tissue of bones and skin. In some of his young patients Studd found not only a loss of skin tone (reversible when normal eating habits are restored) but a permanent loss of height through subsidence of the vertebrae due to reduced levels of collagen.
There has to be some evolutionary reason why Homo sapiens is the fattest as well as the sweatiest of all the apes. It is bad luck that our bodies have lost the capacity to impose an appropriate upper limit on the amount of fat they can accumulate. The problem we face is how to avoid obesity without letting the fear of it escalate into a phobia, since adipose tissue remains an indispensable part of our anatomy.
The Scars of Evolution Page 12
