Babies are vulnerable to the cold, because they are small and can’t move themselves out of draughts or the wet. Evolution has compensated by giving them brown adipose tissue, or BAT (it’s often called ‘brown fat’ but it’s actually more like muscle). BAT generates heat by metabolising fat in a deliberately inefficient way. About 5 per cent of the bodyweight of a newborn baby is BAT, but as you grow older you don’t need this wasteful metabolic afterburner. From your teens onward the BAT gradually changes into ordinary fat tissue. In our twenties and thirties, we generally compensate with a more active lifestyle, and the strain of raising children. But by the time we reach our forties, that has begun to taper off. We suddenly don’t need to book a babysitter just to go out for the evening. Restaurant food, alcohol and disturbed sleep patterns can all contribute to weight gain. Even more importantly, we don’t have the same capacity or enthusiasm for exercise as we once did. The less muscle tissue we have, the fewer calories we need to support it, and yet somehow, no one tells our stomachs. One day we cross an invisible threshold where ‘calories in’ are greater than ‘calories out’ and the weight begins to pile on.
