The studies of genotype and body fat are usually based on comparisons of body size in families and some of the most informative studies involve twins. Identical twins share 100 per cent of their genes, whereas non identical twins only share 50 per cent of their genes. By comparing the similarities in body size between the two types of twins, an estimate of the genetic contribution can be made. Three major twin studies carried out in the late 1980s showed some startling findings, in particular, amazing similarities in body fatness and body shape in identical twins who had been reared apart since birth, some never even having met! In general, genes explain about 25-40 per cent of the variation in body fatness, although it has to be remembered that these estimates are based on people living in fairly similar environments.

Some of the most significant research in this area has come from Professor Claude Bouchard and his group at Laval University in Quebec. Their experiments are carried out with pairs of twins who are kept in comfortable holiday-type accommodation for months at a time, and compare responses to various eating and exercise regimes. Their findings confirm that there is a wide range of responses to identical environmental influences and that the degree of response (such as the degree of weight gained for a given calorie excess) is largely genetically determined.

Genetic influences are unlikely to be the result of a single ‘fat gene’. Genes probably influence all aspects of energy balance including food preferences, nutrient digestion and processing, fat burning and storage and physical activity levels. In fact, to the mid-1990s, a total of 24 genes had been specifically identified as related to some aspect of obesity, but scientists believe several hundreds more are likely to be involved.

One key factor which appears to be at least partly inherited, for example, is food preference. Researchers at the University of Cincinnati examined preferences for 17 different types of foods ranging from fruit to snacks, chips and hamburgers. Comparisons were made between young (9-18-year-old) identical and non-identical twins living together. Frequency of eating and preferences for different foods were rated on a series of scales which indicated that genetic factors (e.g. in identical twins) do, indeed, appear to account for certain food preferences. The main heritable factor in preference appears to be sensitivity to, and preference for, bitter compounds in foods. Preferences for orange juice, broccoli, cottage cheese, chicken, sweetened cereal and hamburgers, for example, all appear to have a hereditary component.

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The body’s cells trap the chemical energy released from food in a high-energy compound known as adenosine triphosphate (ATP) which is stored in small quantities in every cell. When energy is needed (say to transport glucose into the cell), the ATP gets broken down to adenosine diphosphate (ADP), thereby releasing the energy needed for the cell’s processes.

Energy derived from fat, carbohydrate, a protein and alcohol is used in the body for catabolic processes which are involved in the break down of cell tissue and for anabolic processes which build up cell tissue. If protein is called upon to fuel these processes, the basic protein units (.amino acids) are first converted to glucose through a process called gluconeogenesis. In addition to supplying the energy for these processes, the four nutrients, but especially protein, can also be the building blocks for growth, such as when muscle size increases with exercise. The sum total of energetic events which occur in the body, i.e. anabolism plus catabolism, is known as metabolism.

To provide the energy for these events, the body has three major reserve energy stores; glucose which is stored in the liver and muscle as glycogen, protein which is stored primarily in muscle, and fat, the majority of which is stored as depot fat, subcutaneously, around the internal organs and intramuscularly.

Eventually all energy is reduced to heat, therefore the energy produced by living organisms is measured in terms of heat production as kilocalories (kcal). One kilocalorie is defined as the amount of heat energy required to raise 1 kilogram of water 1° Celsius (C) at 15°C. The energy values of food are measured in a similar way through direct calorimetry where a food item is placed in a chamber called a ‘bomb calorimeter’ and combusted in a vat of water. Using the conversion formula above, the rise in water temperature is recorded as kilocalories. The four basic food components have the approximate energy values per gram.

It is important to understand the abbreviations of terms as these are often confused in the popular press. What the lay person usually refers to as T calorie’ is actually 1000 calories’, or 1 kilocalorie’. One real calorie is actually a very small unit, and hence it is multiplied by 1000 to give a kilocalorie (or kcal). This is sometimes also referred to as 1 Calorie (spelt with a capital C). New metric measures also confuse the issue with 1 cal being equivalent to 4.184 joules, or 1 kcal being equal to 4.184 kilojoules (notation ‘kJ’). To round off the figures, 1 kcal is generally regarded as being equal to 4.2 kJ.

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“Those guys ate all the time, and I was right there with them,” says the 25-year-old landscaping-design supervisor from Tempe, Arizona. “Still, I couldn’t believe how quickly the pounds piled up. I went from 225 to 240 in 12 weeks. More than a pound a week!”

At 6-foot-4, Dave carried the extra weight well. But it made him feel heavy and uncomfortable. “I work outside in the heat, and I felt miserable,” he says. “Plus, I wasn’t running as fast or jumping as high when I played basketball, one of my favorite pastimes.

“My roommates and I would play basketball or do something else for a couple of hours practically every night,” he continues. “By the time we finished, we’d be so hungry that we’d eat just about anything that we could get our hands on.” Their foods of choice were pizza, burgers, and Mexican takeout, all washed down with copious quantities of soda and beer. “Sometimes, I’d eat an entire pizza and drink three or four beers, plus a couple of Cokes, before going to bed,” Dave says. “And that was on top of eating a sandwich or something else when I got home from work.”

Feeling out of shape and overweight, Dave decided his late-night eating habits had to go. “I figured that if I ate a good dinner, I wouldn’t get hungry later that night,” he says. “I’m not much of a cook, but even I can heat up a can of soup and put together a turkey sandwich.”

As he began paying more attention to his food choices, his other meals became healthier, too. He traded in his usual sausage-egg-and-cheese breakfast sandwich for a bowl of cereal, a glass of orange juice, and sometimes toast. For lunch, he still favored fast-food restaurants, but he replaced his bacon double cheeseburgers with grilled chicken sandwiches. And he carried bottles of water with him everywhere. “I have to drink a lot while I’m working,” he explains. “I used to down seven or eight sodas a day. I think that switching to water helped me lose weight.”

Indeed, Dave got rid of those 15 extra pounds, plus 8 more, in about 6 months. He has held steady at 217 pounds, a comfortable weight for his size, since 1998.

These days, Dave seldom eats after 8 o’clock at night. If he feels hungry after a couple of hours of shooting hoops, he’ll eat fruit or fat-free frozen yogurt. When his roommates order out, he helps himself to a healthy snack or goes to bed instead.

“Being around junk food and not eating any of it was hard at first,” Dave admits. “But now I feel so much better about myself that I don’t even miss that stuff.”

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