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<p>Inactivity is likely the main factor. Inactive people’s muscles shrink, and what little muscle they have left do not need as much glucose as active people’s muscles. So when the brain is fed, the muscles are not taking up what glucose is left in the blood.</p>
<p>Of course, when someone is inactive, any excess energy, whether it be glucose or fats, eventually gets stored as body fat. Note also that muscle is metabolically active, so even at rest it consumes more energy than body fat does.</p>