Bad news for those with bulging¹ bellies²: fat that builds up around the waist during middle age may cause dementia decades later, say researchers who examined the health records of thousands of senior citizens.

Scientists already know that overweight adults risk developing diabetes³, heart problems and other medical conditions. But over the last few years researchers have noticed that obesity⁴ in middle age is linked with cognitive⁵ 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.

Metabolically⁶ active

When she compared the measurements with the subjects' current medical records, the results were startling. Incidence of dementia increased steadily⁷ 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 active of all fat types.

For example, it secretes⁸ larger amounts of cytokines, molecules⁹ 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 mechanism¹⁰ by which extra weight around the belly could be affecting the brain.

Slimming exercise

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.

Whitmer acknowledges this short-coming, but points out that the dementia rates were greater among those who were not overweight during middle-age, but did have high levels of belly fat. These people are likely to have exercised since their weight was normal, she says, but they still went on to develop cognitive problems.

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.

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.

科学家已经查明超重的成年人更容易患糖尿病, 心脏疾病和其他病症. 然而在过去几年里研究人员发现中年肥胖与老年时发生认知障碍有关联.

如今看来似乎身体肥胖并非如此紧要 – 而是啤酒肚上的脂肪造成了这种疾患。

流行病学家Rachel Whitmer和他的同事查看了6583名成年人的医疗记录，这些患者都在她工作的保健公司——加利福尼亚奥克兰的Kaiser Permanente 公司做过登记。

20世纪60年代到70 年代，该公司测量了这些成年患者的腹部直径，Whitmer 据此大致了解了他们在这个部位上含有多少脂肪。

新陈代谢旺盛

当她把测量结果与患者现在的医疗报告进行比较后 ，发现了一个惊人的事实. 老年痴呆症的发生随着腹部脂肪的含量成正比例增长, 例如与肚子较小的人相比，20% 啤酒肚最大的病人患老年痴呆症的几率是前者的两倍半.

全身肥胖似乎也会增加患痴呆症的几率，但是远比不上腹部发胖造成的影响。

"这是一个重要的报告,"波斯顿大学的神经学家Sudha Seshadri说, 他也发现啤酒肚与认知障碍有显著的关联性. "我认为这种影响是真实的."

这种关联看起来似乎稀奇古怪, 但是Whitmer注意到在所有的脂肪类型中，肚子周围的脂肪新陈代谢是最活跃的.

比方说, 它分泌大量能在细胞间传递化学信号的细胞因子,分子。 其中有些分子, 如瘦素, 在动物实验中已经表明能够穿越脑血管屏障, 促成这样一个机制，即腹部过胖能影响到大脑.

瘦身运动

Seshadri 也注意到了脂肪以外造成痴呆的原因. Whitmer的团队控制了其他一些因素, 例如教育和其他疾病. 但是还有一些原因没有考虑进来. 比如说肥胖的人不太喜欢运动. 众所周知运动可以降低患痴呆的危险, 而且有利于心理健康.

Whitmer 承认了这个缺陷, 但是指出那些中年时虽然没有超重，但腹部脂肪含量确实过多的人到了老年痴呆症发病率更高. 她说，这些人也许锻炼过，因为他们的体重正常 ,但还是发生了认知障碍.

研究者接下来需要研究身体脂肪释放出来的分子对大脑造成的影响. 如果 Whitmer的假说成立，这个结论会让人困扰– 虽然还远未到老年，那些啤酒肚却已经悄声无息地在损伤着大脑了.

但她说仍然有值得乐观的地方, 因为适量的运动和节食可以更加容易地减消腹部脂肪，而其他的部位就没有这么容易达到减肥效果.

"这是最不顽固的脂肪," Whitmer 说.