Your uncle, who is obese, has tried every diet under the sun, yet he is still a very large man. he probably has

The answer is a slower metabolic rate.

Metabolic rate determine how much you burn your food into energy. If a person has a high metabolic rate, the burning process happens so fast which make that person more explosive and hard to gained weight.

Slower metabolic rate means that a large portion of that food will be stored as fat for future use, which make that person hard to lose weight.