There are some people you will always end up envying. These are the kind who can end up eating anything and not have it show. They may be few and far between, but they look like they never age; their body weight remains the same, and yet, they are eating to their heart’s content. What is going on?
The energy burnt in your body is entirely mathematical. Given a certain weight, height, gender et al., your body needs a certain amount of energy as calories known as basal metabolic rate. The basal metabolic rate can vary to the extent of the ratio of muscle and fat in your body. If you are more muscular, your body requires a larger number of calories even at rest. If you carry a lot of fat, shockingly, fat needs fewer calories at rest. However, the difference is minor and can perhaps make a difference of say a hundred calories. The composition of your body is not determining your metabolism. So then what is?
If someone appears to be digesting food better, you may be better off looking at their lifestyle. Do they sleep well? Are they getting their calories from better quality sources? Do they live relatively stress-free life? Does their gut have good bacteria? If anything, these could be the causes for better metabolism.
Remember that weight is a symptom. If you know someone who does not appear to put on weight, there may be a secret hidden within. The person could be something called skinny fat. The person could have poor insulin production resulting in the body consuming its fat for energy production. In the end, there is no free lunch. In fact, if you know someone who is eating unhealthy and still manages to remain thin, please worry. Weight or the lack of it is only a secondary outcome of poor nutrition. The real challenge is a disease.