3500 calories = one pound

If this were true, people who are overfed by 3500 calories
should all gain one pound, but they don't. In one interesting
experiment of intentional overfeeding, identical twins gained
weight at the same rate, but unrelated people gained anywhere
from 9 to 29 pounds - even though all had eaten the same number
of calories. This is one of those equations that holds up in the
laboratory but not in real people.