BBC Earth (Asia) - - Q&a -

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. LV

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