This should be common sense, yet of the five "laws" of metabolism this is the one that seems to get the most blowback. Basically, it means that each of us are metabolically unique, psychologically varied, and have vast discrepancy in personal preferences. I think it's the "metabolic uniqueness" that bothers some people.
Sure, we humans share a metabolism that functions, by and large, the same from person to person. The best way to conceptualize this metabolic individuality is to think about appearance. There's no mistaking that the people you interact with daily are human. They almost always have two arms and two legs, they walk upright, and they interact with you in relatively predictable ways.
They are completely recognizable as "human," yet at the same time look entirely different. This is the way to think about metabolic individuality. Just as humans vary in their physical appearance, they differ in metabolic function as well.
It always amazes me that people argue this point. With the sequencing of the human genome we now know we can fluctuate substantially in how we handle and digest food, whether we're sensitive to bitter compounds, how insulin sensitive or resistant we are, our susceptibility to illness, and more.
It's simply ignorant to deny the broad differences in metabolic function from one human to the next. There's nothing at all incompatible with the idea that we each share a vast amount (and the most important aspects of metabolism), but at the same time vary in ways that make a significant difference in our health, fitness, and appearance.
This goes for our psychological make up too. Our personalities differ. We have different relationships and coping strategies for stress, hardship, work capacity etc. Some of us are more or less susceptible to addiction. Some people like chocolate and others prefer vanilla.
These psychological differences, and the personal preferences of each human, are not just somewhat important, they are HUGE. Imagine a person who loves chocolate being given a dietary regime that never allows for chocolate again? Do you believe they're going to be able to sustain this approach? Of course not! It is the height of ignorance to deny these concepts.
We have to respect these individual distinctions in ourselves. If you know that when you eat fat you get bloated, break out with acne, and feel sluggish, then you need to honor that despite what the latest health book says about adding a pound of fat to your coffee.