Having a higher BMI could reduce the risk of developing conditions such as dementia and Alzheimer's, according to a study published in the Lancet Diabetes and Endocrinology.
The study, performed by researchers from the London School of Hygiene & Tropical Medicine and OXON Epidemiology, analysed 1,958,191 patients aged over 40, measuring their BMI as the study progressed. Of this group, 45,507 people developed dementia between 1992 and 2007. The researchers found that people with a BMI under 20 carried a 34% increased risk of developing dementia compared to someone with a BMI over 20. On the other hand, people classed as 'very obese', with a BMI over 40, were 29% less likely to develop the condition than someone with a BMI under 40.
Dr Doug Brown, Director of Research and Development at Alzheimer's Society, said: 'We don't yet know enough about the link between body weight and dementia. Previous research has suggested that being overweight in midlife increases risk of developing the condition and yet this study suggests that it may actually be protective. This study of almost two million people also reports that being underweight in later years could increase risk of developing the condition by a third.'
The findings of the study contradict previous theories that being overweight increases the chance of developing dementia. The researchers concluded that being underweight in middle age and old age carries an increased risk of dementia over the course of 20 years.
Dr Brown added: ''This mixed picture highlights the difficulty of conducting studies into the complex lifestyle risk factors for dementia and reinforces the need for further research so we can identify the most important risk factors. While the evidence on body weight and dementia is unclear, we know that people can make positive lifestyle choices to keep their brains healthy by taking regular exercise, not smoking and following a healthy balanced diet.'