LONDON — A surprising study contradicting all previous research found that being fat in middle age appears to cut the risk of developing dementia rather than increase it, the Lancet scientific journal has reported.

A study of 2 million people found that the underweight were far more likely to develop dementia, a growing problem among the elderly in the Western world.

The underweight had a 34 percent higher risk of developing dementia than those of a normal weight, the study found, while the very obese had a 29 percent lower risk of becoming forgetful and confused and showing other signs of senility.

The exhaustive study, published in the Lancet Diabetes and Endocrinology Journal, flies in the face of previous smaller studies – and much modern health advice – that what is good for the heart is also good for the head.

Researchers said that if other studies confirm the findings, the next step would be to examine if people who eat more take in dementia-fighting nutrients in the extra food they consume.