Asked by: Joyce Burley, Hull
If you step on the scales as soon as you have eaten a jam doughnut for lunch, you will weigh an extra 70g, because this is the mass of the doughnut itself.
But 22g of this is water, which you’ll lose over the next few hours in your breath and urine. You’ll lose another 2g sometime tomorrow, when you poop out the small amount of dietary fibre. The other 46g is digested and if your body doesn’t immediately need it to grow or for energy, it is stored as fat.
A 2012 study at Oxford University found that the fat in your food ends up on your waistline in less than four hours. Carbohydrate and protein take a little longer, because they need to be converted into fat in the liver first and it takes nine calories of protein or carbohydrate to make 1g of fat.
Altogether, that doughnut has about 225 calories. Around 100 of those come from the 11g of fat in the doughnut, which will be on your waistline by teatime. Then the 125 calories of carbs and protein will be converted into another 14g of body fat sometime tomorrow, unless you hit the gym after work and burn it off again.
Get more fascinating Q&As from BBC Focus magazine by following @sciencefocusQA