You may never truly beat the bulge, a new study suggests. Even when people lose weight, they don’t lose fat cells. The cells just shrink. A Cold War relic helped researchers in Sweden and their colleagues figure out the birth dates of fat cells in adults. Tracking the birthdays of belly-building cells uncovered some mixed news. First, the good news — adults lose about 10 percent of fat cells each year. But the bad news is that all those cells are replaced. Obese people have about twice the number of fat cells as normal-weight adults, and fat cells are bigger in the obese people than in their lean counterparts, the researchers reported August 4 in an advanced online publication of Nature. The results could help explain why many people have difficulty maintaining significant weight loss. But the excuse doesn’t fly with all obesity experts. Neuroscientist Kirsty Spalding of the Karolinska Institute in Stockholm, Sweden, and her colleagues set out to tackle the question of whether people gain and lose fat cells as their weight moves up or down the scale. To answer it, the team adapted a technique Spalding originally developed to figure out whether neurons replicate. During the Cold War, above-ground nuclear bomb tests caused levels of the radioactive isotope of carbon, carbon 14 or C-14, to skyrocket. Since then, the level has slowly decreased, giving each year a slightly different – and signature – level of C-14. Humans take in the isotope by eating plants (or by eating the animals that eat plants), which incorporate C-14 through photosynthesis. Thus, the level of C-14 in human cells created in a given year reflects the levels of the isotope in the atmosphere that year. The team used C-14 to determine how often fat cells turn over. Children, teenagers and young adults build fat cells as they grow, but the adults maintain a fixed number of cells throughout life, Spalding and her colleagues conclude. As cells die, the same number is replenished. “The fat cell number seems to be fairly set by adulthood and remarkably stable,” Spalding says. People did not gain fat cells as they gained weight. Rather, the fat cells in their bodies swelled as more lipids were packed inside. All of the obese people in the study said they had also been overweight as children. Spalding plans to test people who were lean as kids, but then packed on pounds as adults, to find out whether the number of their fat cells has remained constant in adulthood as well. Losing weight didn’t make the fat cells go away either. The cells merely shed lipids and shrank in size, Spalding says. Samuel Klein, director of the Center for Human Nutrition at Washington University School of Medicine in St. Louis, doesn’t buy the finding that people don’t make more fat cells as they gain weight. Fat cells in obese people are larger than in lean people, he agrees, but there is a limit — about 1.5 micrograms — to how much lipid can fit in a fat cell. But some people gain far more weight than can be accounted for by the fat stored in adipose tissue, as fat is called. “If we never made more fat cells, they’d become huge in size, and that just doesn’t happen,” Klein says. Spalding’s research feeds the idea that skinny fat cells lurk, biding their time until they can stage a new expansion campaign. But some obesity experts don’t think having more fat cells means certain defeat in the battle to stay lean. “This emaciated, hungry fat cell is just waiting to fill up again. There’s absolutely no evidence to support that,” Klein says. “You don’t become obese because your fat cells are growing or not growing. It’s because you eat more calories than you burn up. It really boils down to the boring concept of eat less and exercise more.” A person with extra fat cells isn’t necessarily unhealthy, he adds. Even someone overweight as a child could avoid many obesity-related health problems by preventing fat cells from filling up. “It’s fat cell size, not number, that causes metabolic problems,” Klein says. Stuffed fat cells leak fatty acids that can damage other cells. They may also give off hormones that trigger inflammation and lead to conditions such as high blood pressure, heart disease and diabetes. Small fat cells don’t release those kinds of troublesome molecules, he says. New therapies that block fat cell renewal might help people maintain weight loss but probably would not be an effective weight-loss remedy unless coupled with restrictions on calorie consumption, Spalding says.