The loss of body fat can lead to a range of physiological outcomes which, in turn, affect the further loss of body fat. Physiological adaptation to weight and fat loss can be divided into predictable changes (such as the decline in RMR in response to the loss of lean mass) and adaptive changes (where the body actively works to reduce the rate of weight loss). Dr Rudy Leibel and his colleagues from Rockefeller University in New York have tried to quantify the adaptive changes in lean and obese people as they lose weight. For a 10 per cent weight loss, they found that total energy expenditure declined by about 450kcal, of which about half could be explained by the changes in body composition and half could be considered adaptive.

Studies at Cambridge University on a particular species of desert mouse for example, have shown that when compared with a ‘dry mouse—or one accustomed to plenty of food—the desert mouse is able to adapt to decreases in body weight caused by lack of sustenance by simply slowing down its metabolic processes. Humans have less facility to actively alter metabolic processes to match changes in food intake, although adaptations do certainly occur.

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Fat is either gained or lost by increases or decreases in the size of the lipid pool in large numbers of fat cells. These ‘fill up’ in good times, like a water tank after heavy rain, and ‘empty out’ in bad times, like in a drought.

Which cells fill up or empty out most, and in what order, is a function of a range of factors including genetics, gender, age, race and the number of existing fat cells. One thing is clear though: the last cells to fill up are likely to be the first to empty out. If someone gains extra fat around the waist for example, this will usually be the first place it goes from when the energy balance becomes negative.

Fat cell enlargement through expansion of existing cells is called hypertrophy. A less common way of increasing fat is through an expansion of fat cell numbers or hyperplasia. Hyperplasia is

thought to occur only at certain periods in life, in particular during growth spurts when all body cells are rapidly increasing in number, such as early infancy (1-2 years) and early adolescence (12-14 years). Inappropriate lifestyles at these times may lead to a greater increase in fat cell numbers than might otherwise occur. A third stimulus to hyperplasia which is now well recognised is a large increase in body fat. As existing fat cells ‘fill up’, new cells come into existence and form a bigger ‘maximum capacity’ which is then capable of storing a total greater volume of fat. In contrast to the 30-50 billion fat cells of the average person, an obese person may have 70-80 or even up to 100 billion adipocytes.

The basis for hyperplasia of fat cells consists of ‘baby fat cells, known as adipocyte precursors, which exist amongst other fat cells, as if waiting for the opportunity to spring into action and fill up with fat like their parent cells.

Not a lot is known about these precursors; whether they are actually small fat cells in waiting, or whether they are just other (‘stem’) cells that can be called on to make any other form of cell if needed. Some research suggests that they spring into action during the growth spurts mentioned above if conditions are right, or when increases in body weight exceed around 170 per cent of ideal or average weight. Once they mature however, they are there for life. Animal research has shown that with severe dietary restriction, fat cells can shrink to the point where they are virtually unrecognisable as adipocytes, but when overfeeding begins again, the shrunken cells rapidly fill up with fat.

With the expansion of fat cells in the waist region, some men, in particular, develop a ‘hard’ fat belly and some a ‘soft’ or ‘wobbly fat one. The reasons for this are not quite clear but it’s possible that the big, hard abdomen represents a larger degree of visceral fat within the abdomen, which then pushes out more on the inner surface of the abdomen. Fat which is more ‘wobbly’ may represent more subcutaneous fat, although the evidence for this is not currently clear.

Irrespective of the type of fat stored, it’s clear that it is easier to prevent obesity and overfatness beyond a certain level than to cure it. Obviously, the function of fat as an energy store means that the human body has a certain capacity for increases and decreases in fat stores without permanent change. In the long term and for the very obese, however, there may be more permanent changes in fat cell numbers, metabolism and other fat maintaining factors, which make it more difficult to reduce body fat levels. People in this situation and those with a strong genetic component may be structurally and functionally different to the mildly overfat and in fat loss programs their outcomes may not be as favourable. They might need different programs to achieve similar levels of success as programs aimed at mildly fat people.

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Linda O’Hanlon used a measuring cup to lose 57 pounds.

Overweight since she was a child, Linda, of Maspefh, New York, never developed that “I’m full” signal that makes most people push away their plates after eating a serving or two. “When I sat down for a spaghetti dinner, I didn’t get up until every last strand of pasta in the pot was gone,” she recalls.

Over the years, as her weight crept upward, Linda invested thousands of dollars in fad diets with hopes of slimming down. But they didn’t help. “I’d lose 5 pounds and go off the diet,” she explains. “Then I’d gain back the 5 pounds, plus another 5.”

By the time Linda reached 208 pounds, she gave up trying to lose weight, resolving to accept herself as she was. But that changed in August 1997, when she went with a friend to Weight Watchers. Through the program’s meetings, she learned how to make healthful food choices and control her portion sizes. “The lesson was that if you put junk in your body, you’ll feel like junk,” she says.

She took that lesson to heart. Realizing that she had a natural tendency to overeat, Linda became extra-vigilant about her portion sizes. When she’d sit down for pasta, she’d pull out her trusty “|measuring cup and carefully measure out 1 cup of spaghetti, 1 cup of cooked vegetables, and 1 cup of garden salad. She’d eat that and no more.

Over time, Linda became adept at eyeballing her portion sizes. By relying on her brain instead of her stomach to say “when,” she dropped three pants sizes in 7 months. Two years later, at age 30, she’s holding steady at 151 pounds. “My clothes fit great, and I’m bursting with energy,” she says.

WINNING ACTION

Learn to recognize portion sizes. For foods that are easy to overdo, especially pasta, cereal, and ice cream, measure out one serving and transfer it into the bowl you would normally use. Make a mental note of how it looks (it’s probably a lot smaller than you’re used to seeing). You’ll need some time to retrain your brain, but eventually, the smaller portions will seem normal.

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