this is perhaps the best analogy so far.

the body naturally converts energy to fat for storage. some body fat is necessary to stay healthy, the bodybuilders that cut to 1% for competitions don't look like that year round, it's actually not healthy to be at that percentage at all. that said, less than .0001% of the population can do this, which means we are not in danger of this at all.

the opposite is also true. excess body fat leads to all sorts of health problems.