对于大腹便便者来说这是个不幸的消息：研究者在仔细查阅了成千上万的老年人健康记录后作出结论，中年人腰部的赘肉可能会成为几十年后患老年痴呆症的诱因。

Scientists already know that overweight adults risk developing diabetes, heart problems and other medical conditions. But over the last few years researchers have noticed that obesity in middle age is linked with cognitive problems in the aged. Now it seems that overall body mass is not so important – it's the fat around the belly that appears to cause the problems.

Epidemiologist Rachel Whitmer and colleagues looked at the medical records of 6583 adults registered with the healthcare firm she works for – Kaiser Permanente of Oakland, California. During the 1960s and 1970s, records were taken of the diameters of the adults' bellies, which gave Whitmer a rough idea of how much fat they were carrying around the waist.

When she compared the measurements with the subjects' current medical records, the results were startling. Incidence of dementia(痴呆) increased steadily with the amount of belly fat, such that the 20% with the most belly fat were over two and half times more likely to develop dementia that those carrying the least. Levels of total fat also seemed to increase dementia risk, but not by the same magnitude.

"This is an important paper," says Sudha Seshadri, a neurologist at Boston University who has also found evidence linking belly fat to cognitive problems. "I believe the effect is real." The link may seem odd, but Whitmer notes that the fat around the belly is the most metabolically(新陈代谢地) active of all fat types.

For example, it secretes larger amounts of cytokines, molecules that carry chemical signals between cells. Some of these molecules, such as leptin, have been shown in animal studies to cross the blood-brain barrier, suggesting a mechanism by which extra weight around the belly could be affecting the brain.

Yet Seshadri also notes that factors other than fat could be responsible for the results. Whitmer's team controlled for some of these, such as education and rates of other illnesses. But other issues were not taken into account. Overweight people are less likely to exercise, for instance. Physical activity is known to decrease obesity risk, as well as being psychologically beneficial.

Researchers next need to study the impact that the molecules released by body fat have on the brain. If Whitmer's hypothesis proves right, the conclusions could be disturbing – those beer bellies may be silently damaging the brain, long before old age sets in. But she notes that there are also grounds for optimism, since moderate exercise and diet can reduce weight around the belly more easily that it can in other parts of the body." It's the least stubborn fat," Whitmer says.