Title: Body fat

Key words: MRI scanner, vegans, obesity, skin fold thickness, coronary heart disease, fat distribution, saturated vs unsaturated fats, adipose tissue, polyunsaturates, babies, nerve cells, myelin, milk, breast milk, genetic, environment, omnivores, linoleic acid, anorexia

Date: April 2001

Category: The body

Type: Article

 

Body fat

Vegans, "apples" and babies are being turned inside out by scanners that are revolutionising the study of body fat

AT FIRST, the image on the computer screen looks like some kind of futuristic dummy, an empty shell of a body suspended in space. But soon the depths of its flesh become apparent. With a few quick keyboard strokes, the researcher at the console transects first the legs and then the lower and upper parts of the torso, in each case tilting the dismembered body this way and that to expose two-dimensional slices. Skin, bones and inner organs all reveal themselves. So, too, in dazzling white layers, does the fat.

This "dissection" is happening at the Robert Steiner MRI Unit at Hammersmith Hospital in London, where researchers led by Jimmy Bell and Maria Barnard are showing off a state-of-the-art technique for pinpointing fat deposits in the body: magnetic resonance imaging.

Of course, MRI has been around for a while but it only recently occurred to anyone that it could be used to look at body fat. Before then, researchers had to rely on crude methods such as measuring skin fold thickness and ratios of height to weight and of waist to hip. These only reveal the total amount of fat in the body - they cannot show how that fat is distributed. Most significantly, they say little about the fat stored around the internal organs - the very tissue that may be crucial to the development of coronary heart disease and other illnesses linked to obesity.

Spying on flesh

With MRI, by contrast, there is no place for fat to hide. In a typical scan, the Hammersmith team pushes the subject slowly through the magnetic field, pausing every 3 centimetres to "spy" on the composition of a slice of flesh up to 10 millimetres thick. This spying involves looking at the magnetic properties of protons attached to water molecules, which vary distinctively from tissue to tissue.

The researchers take between 60 and 80 slices through a body and then process the slices using brain-imaging software that has been adapted to pick out fat contours. The result is a three-dimensional image that can be "dissected" to reveal how much fat there is in the subject's body and where it is deposited.

These MRI techniques tend to back up the notion that people are either "apples", with their fat concentrated around their stomachs, or bottom-heavy "pears". Among obese people, apples are more likely to be the heart attack candidates. But most of the fat in obese people is stored around their internal organs. "In our study, fat subjects have 2.7 times as much overall fat as thin people, but four times as much visceral fat," says Bell.

Bell's study also confirms findings made two years ago when researchers at the Wageningen Agricultural University in the Netherlands showed what happened to obese men and women as they lost weight. From MRI scans, it was clear that weight loss caused not only a loss of fat, but a change in its distribution, with internal deposits shrinking most sharply. Interestingly, these changes could not be predicted from measurements of waist-to-hip ratios.

But mapping fat deposits is not the only goal. Researchers also want to know about the composition of these deposits and how they vary from person to person. This calls for a different spying tactic: homing in on the magnetic properties of carbon-13 nuclei in fat molecules. The approach, known as magnetic resonance spectroscopy (MRS), produces peaks in an electromagnetic spectrum that correlate with the amounts of saturated and unsaturated fatty acids present in the tissue. Researchers can use these peaks to estimate the proportion of saturated to unsaturated fats - a measurement that previously meant removing and analysing samples of adipose tissue.

For the first time, the way seems open to ask questions crucial to understanding the links between body fat and disease.

Researchers are even starting to ask about the role of fat in the way our bodies and brains develop. One preliminary study by the Hammersmith team has already shown that the composition of adipose tissue in infants alters as they grow older, with babies gradually acquiring more unsaturated fats. Although their sample was small, the researchers showed that premature babies have about five per cent less unsaturated fat and five per cent more saturated fat than full-term ones. Babies aged between six and eight weeks still have more saturated and less unsaturated fat than their mothers. But according to Louise Thomas, a member of the Hammersmith team, it's possible that the amount of polyunsaturates peaks at about 10 months before dropping back to adult levels. Even if adults have a diet rich in polyunsaturates, these fats are in the minority in their adipose tissue. But the researchers still don't know exactly when the baby's fat profile begins to look like that of an adult.

Nor is it yet clear why babies start life with this balance of saturated- versus-unsaturated fat at all. One theory is that very young babies may need more saturated fat, which is a richer energy source than unsaturated fat, to meet the energy needs of a growing brain, says Forrester Cockburn, professor of child health at the Royal Hospital for Sick Children in Glasgow. In the first year of life, 60 per cent of a baby's total energy intake fuels the developing brain.

Fat also provides the brain with some of the raw materials to construct nerve cells. Saturated and monounsaturated fats are used to make the insulating sheaths of myelin that coat all nerve fibres. And the first 18 months after birth are a critical period for laying down myelin. Although present in lower concentrations, polyunsaturated fats are also vital in the developing brain, particularly in forming the nerve fibre membranes. But in the first four months of life, babies do not produce the enzymes needed to make certain long-chain polyunsaturated fatty acids. Their only source of these acids during this time is milk. And here, it seems, human milk is better than synthetic formula milk.

Vegan breast milk

The Hammersmith team now has evidence to suggest that the kind of food eaten by breast-feeding mothers can affect the balance of fats in their infants. When one of the seven breast-feeding mothers in their study turned out to be vegan, the researchers noticed that the levels of polyunsaturates in her baby's adipose tissue were almost twice as high as those fed by omnivorous mothers. According to Thomas, babies breast-fed by vegan mothers may have better brain development because of the role of polyunsaturates in the growth of neural membranes.

"If you can extrapolate from what is happening in the adipose tissue to what is happening in the brain, the composition of that tissue could even act as a marker for intelligence," is Thomas's controversial conclusion. Cockburn is among those who disagree. He points out that intelligence depends on genetics and environment as well as biochemistry. But the debate may be hard to settle. To do so would go beyond the current scope of MRI and require a picture of the developing brain showing the presence of specific fatty acids and the type of nerve cells that are growing.

What the MRS work at Hammersmith Hospital has already highlighted is a link between diet, obesity and disease. In a study last year, the team compared scans of 38 vegans with those of 39 omnivores. The vegans had 10 per cent more unsaturated fats than the omnivores, and about 12 per cent less saturated fats. Vegans tend to have a high intake of linoleic acid - a precursor of a long-chain polyunsaturated fatty acid - because it is plentiful in vegetables. High levels of polyunsaturates in adipose tissue are thought to ward off coronary heart disease, and on this evidence a vegan diet looks healthier.

But there is a drawback. Polyunsaturates are susceptible to attack by energetic oxygen-guzzling molecules in the body known as free radicals, and such attacks are thought to be a risk factor in cancer. "Antioxidants like Vitamin E usually mop up those free radicals," says Thomas. "Vegans may also have higher levels of antioxidants in their diet, but they may not."

The same study included 11 vegetarians and the results for them were far more disappointing. The scans showed that vegetarians and omnivores have a lot in common when it comes to the levels of saturated fats in their bodies. Thomas says the findings suggest vegetarians are substituting saturated fats in meat with those in dairy products, so in terms of the saturated-versus- unsaturated ratio they are equally prone to heart disease.

MRI has its limitations. "At the moment it is fiendishly expensive," says Ian Baird, medical spokesman for the British Heart Foundation. "More importantly, it is not sensitive enough to identify individual fatty acids." Despite this, the Hammersmith team plans to expand its work to include studies of fat-related disease such as anorexia.

LAURA SPINNEY

From New Scientist magazine, vol 146 issue 1974, 22/04/1995, page 38

© Copyright New Scientist, RBI Ltd 2000