The most convincing research so far suggests that being fat in your 40s might raise your risk of developing dementia later in life.

In a study that followed more than 10,000 Californians for almost 30 years, researchers found that the fatter people were, the greater their risk for or other forms of dementia. The results were published online Friday by the British Medical Journal.

The study data showed that roughly 7 out of 100 normal-weight people developed dementia. Among overweight people, the risk was almost 8 out of 100; and for obese people, it was 9 out of 100.

Funded by the U.S. National Institutes of Health, the California study was conducted by the Kaiser Permanente Medical Foundation. The project followed 10,276 people, in their early to mid-40s, for an average of 27 years. They had detailed health checkups from the mid-1960s to early 1970s.

Between 1994 and 2003, dementia was diagnosed in 713, or about 7 percent, of the study volunteers. The scientists examined links between dementia and obesity using two different measurements — body-mass index and the thickness of skin folds under the shoulder blades and under the arm.

Adjusting for conditions such as diabetes, heart disease and other factors, the study found a higher risk of dementia for heavy people. Using the body-mass index, which measures height and weight to classify how fat people are, obese people were 74 percent more likely to develop mind-robbing dementia than normal weight people. Overweight people were 35 percent more likely to develop it.

The effect was more profound for women than men. Obese women were twice as likely as women of normal weight to develop Alzheimer’s disease or other types of dementia, while for men the risk increased by 30 percent.

However, when the researchers used skin-fold thickness, instead of the body-mass index, to measure obesity, there was no difference between the men and women; both were up to 70 percent more likely to develop dementia if they had a thick fold between the tweezers than if they had a thin fold of skin on the test. And the thicker the skin fold, the higher the chance of later dementia, the study found.

The study was not able to explain how obesity might increase the risk of dementia, but does propose several theories. One is that fat cells are known to produce harmful, inflammatory chemicals, and there is evidence that these may cross into the brain. Philip James, an obesity expert who was not connected with the research and who heads the International Obesity Task Force, suggested a dietary lack of the right kinds of fatty acids, such as those found in fish, might also be a factor.

辅助阅读：一项由美国国家卫生研究所出资在加利福尼亚进行的迄今为止最有说服力的研究表明，人在四十多岁时过于肥胖会增加日后患痴呆的风险。该研究对10,000多人进行了长达30多年的跟踪调查，发现越肥胖的人患各种痴呆的风险就越高。但该研究未能解释肥胖是如何增加患痴呆的风险的。