Note that the issue is that there is a human in that system.

The problem is that we (rightfully!) don't easily allow experiments on humans that might be harmful.

And, as an aside, nutrition and weight management does have people doing real science (things like studies of hunter gatherer societies with deuterated water), it's just that the results aren't very flashy (ie. a human burns roughly the same amount of calories regardless of activity level--so the only way to lose weight is to eat less--moving more is healthy but doesn't contribute substantially to weight loss).