Question: how to know how much to eat in a day?
Answer: Women: 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161 = REE. Men: 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) + 5 = REE.
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Eat more calories than presented to gain weight, eat less to lose weight, and eat the same amount to maintain your weight.