Fat cells, like all cells in the body, get replaced continuously, but the turnover rate is the same whether people are obese or lean. So what effect do diet and lifestyle have on our fat cells and how do they cells truly behave in our body?
1. How quickly does eating too much cause new fat cells to form?
"Incredibly fast," says Matthew Rodeheffer, who studies obesity at Yale University, on the basis of recent research his team conducted on mice. After just five days on a high fat diet, new fat cells had appeared. In humans the process could be even faster, he says, potentially within a day. Once the cells are present, though, it takes them several weeks to actually fill up with fat.
But Giles Yeo at the University of Cambridge Metabolic Research Laboratories says having more fat cells isn't necessarily a bad thing. Being able to spread your fat over a larger number of cells means they are less likely to overfill, so you can store more fat while staying metabolically healthy. "Imagine each fat cell is like a balloon, but there is a safe limit to how much fat it can store," Yeo says. "When they are full, the fat goes to the liver and muscle. It's one of the key causes of metabolic disorders like diabetes, and heart disease."
2. If you have more fat cells, do you put on weight more quickly?
"That's the logical assumption," says Rodeheffer. "People who have been obese struggle to maintain weight loss. And we think this is probably a major contributing factor."
In fact, between 75 and 95 per cent people who lose weight regain most of it within a few years. Part of the problem seems to be the way fat cells communicate with the brain. As well as passively storing fat, these cells send information -- usually via hormones -- about the level of fat stores in the body, and the body's metabolic needs. One of these hormones, leptin, signals satiety. In basic terms, if you have a lot of leptin circulating, you feel more full and probably eat less.
But obese and overweight individuals often become leptin-resistant -- they have very high levels of it, but it doesn't work, says Rodeheffer. There's a short circuit in their satiety system, and having a certain amount of fat is what causes it.
3. Is it true that you can never get rid of fat cells -- they just grow or shrink?
Yes, says Rodeheffer. "All the data basically says that once you gain more fat cells you can't lose them." So when people become overweight it is principally because their existing fat cells have got bigger, although they also produce additional fat cells. When people lose weight the cells shrink, but their number remains the same.
Fat cells, like all cells in the body, get replaced continuously, but the turnover rate is the same whether people are obese or lean. It's just that your body maintains a larger number of cells if you are or have been obese, Rodeheffer says.
The exception might be brown fat. Animal studies, and tentative findings in humans, suggest that white fat cells can be turned into a type of brown fat called beige fat. These cells burn, rather than store, energy from food, so might help people manage their weight.
4. Why do people who have lost weight have trouble keeping it off?
There is evidence that overweight people who manage to lose weight, especially serial dieters, put on more weight over time than people who don't diet. Putting on weight and then losing it again seems to cause long-term metabolic changes, and they have to work harder to keep the weight off.
Yeo thinks that this comes down to changes in the brain rather than the number of fat cells. We have evolved to hold on to fat, he says, so if you lose weight, causing a drop in leptin production, the brain will try and do what it can to bring that weight back up again. "The way it does this is by trying to reduce energy expenditure and increase food intake." The result is that when you compare two people who weigh the same, one of whom has just lost ten kilos, you will find that the newly lighter individual is hungrier and has a slower metabolism than the other person. Research at Columbia University in New York has shown that replacing the leptin reverses these metabolic effects, an idea that could be the focus of drug development to help people keep weight off.
5. Does eating fat make you fatter than eating other foods?
The jury is still out on this one. Not all calories are equal, says Rodeheffer. Eating more calories as fat while maintaining the same overall energy intake makes you store more off the stuff. In animal studies, mice fed a high-fat diet became obese, whether another group fed less fat but the same number of calories did not, he says.
When it comes to humans, though, evidence is lacking as, for obvious reasons, almost all studies look at people who are already obese. Besides, some fat is crucial in everyone's diet. And because fatty food does make you feel full, some studies suggest that eating full-fat versions of foods such as yogurt results in people eating less overall.
People used to think it was a "geographical" issue, says Yeo -- that what mattered was how close the fat is to your organs, and that when fat cells become full, it might leak into the blood vessels surrounding these organs.
But research is emerging, he says, to show it's really about types of fat. Visceral fat, the kind that builds up around your middle, and which affects men more than women, produces different hormones to subcutaneous fat -- found around buttocks and thighs. The evidence for this comes from mice. "When you remove the unhealthy fat from around the organs, and give the mouse a booty - put it on the backside where it's supposed to be fine - the mice are still metabolically unhealthy," Yeo says.
Visceral fat hormones signal to liver, heart and other organs to make them more resistant to the effects of insulin. And as the cells stretch and grow, they also tear a little, triggering the macrophage immune response again.
7. Does exercising at low intensities burn more fat?
The thinking goes that to burn the most fat during exercise, it's best to stick to lower intensities at which fat, rather than carbohydrate, is used as the main energy source. "Most people would agree that if you exercise at a low intensity, or even at rest, fat is the main fuel source," says Chris Easton, at the University of the West of Scotland, in Paisley, UK. But that can be misleading. For a start, if you're exercising to lose weight, the most important thing is a calorie deficit, and you'll burn more calories faster with more intense exercise.
Also, the ideal fat burning zone is different for everyone. There is evidence suggesting that the amount of fat you burn rises until about 55 per cent of your maximum heart rate, Easton says. "That's physically demanding -- it's not a walk in the park." And it's not 55 per cent for everyone. "Sometimes it's very low in people who are sedentary or obese -- they will burn the most amount of fat at lower intensities." Those differences cause confusion. Besides, Easton says, there's a problem with thinking about exercise just in terms of weight loss -- doing more intense exercise will boost your physical fitness, and that's where most of the benefits come in. And even in terms of burning fat, the more exercise you do, and the fitter you get, the better you become at burning fat while you exercise. "Untrained and sedentary individuals, following a period of exercise training, use more fat during exercise," Easton says.
8. Does exercising first thing in the morning burn more fat than exercising later in the day?
"The theoretical principle is sound," says Easton. In a fasted state, you have less access to carbohydrate so will use fat as an energy source. Easton, who works with elite athletes, has found that forcing the body to use fat as a fuel in this way is effective even for lean athletes wishing to slim down -- for instance boxers and jockeys. "Exercising in the fasted state makes sense for them," he says. Other research shows that intense exercise before breakfast burns more fat and leads to better insulin sensitivity than eating breakfast first and consuming energy drinks throughout -- even if the overall calorie intake is kept the same.
But what about the rest of us? "For normal people, it's probably not going to make much difference," says Easton. "We're not sure yet." If you're only doing a gentle workout, you might still have enough glucose in your blood and glycogen in muscles and liver to fuel the exercise without drawing on fat stores. And if you're running on empty, you may fatigue sooner and work out less than you would with a bit more available energy.