Suppose you're a mouse. You skitter around in the forest, nibbling at leaves, twigs, bugs-- whatever catches your fancy. You're lean and strong, and healthy. You do not have heart disease, your teeth aren't falling apart, your bones are strong. Good for you.
Now be a different mouse. The scientists have gotten a hold of you, fed you various chows with different mixes of fat, carbohydrate, protein and other nutrients which science has learned are essential to good health. They want to know what these various nutrients do in the body, how they interact, so they constantly manipulate and take notes. These scientists are studying the effect of certain foods on your body's metabolism; nutrition. At least, they might think that that's what they're doing. The reality is quite different, nutrition is a much slippier subject than most people imagine.
Be one more mouse, before I go into a little review of various actual mouses and other lab animals. Be an X-mouse, a mutant mouse altered by scientists to study the effect of genetics on body weight, lifespan, and disease. You may be altered to be particularly susceptible to heart disease. Then they can feed you various pelleted chow-type diets, with different ratios of various different nutrients, and see what the effect of those nutrients is on the progression of the disease. This is hopelessly complex. If you look at humans and protein alone, you can see how complex this really is. There are eight essential amino acids, and a number of conditionally essential amino acids, as well as numerous types of fat, and sugar. The possible dietary combinations are endless. Let's not kid ourselves. This appears to be beyond human comprehension. We may get clues from the mice in the studies that will help to decipher various disease processes, but we will never be competent to steer this machine.
Because that's what it's all about. Wild animals live in all kinds of constantly shifting environments. They run around eating. If food is plentiful, instead of growing obese and getting heart disease, they increase in population. This is what we see, so it is reasonable to believe that this is the case, although the picture is horribly incomplete. This is a cruel process; many of the young, often most, will be killed and eaten. Perhaps this is how homeostasis is achieved. Only the strong survive, survival of the fittest, and all that. Competition limits the availability of food, so the animals do not get fat.
The truth is that the interface between food and animal is an intricate hormonal dance. This is an even greater complexity than the problem with the large number of variables in nutrition itself. Insulin, growth hormone, testosterone, estrogen, amylin, glucagon, leptin, the list goes on and on and scientists haven't even completed that list, they're still discovering new hormones and peptides and enzymes. How are we to steer this thing?
The answer is of course that we are not. The mouse in the forest has absolutely no idea of nutrition. Instead, the mouse has an interface with reality, in the form of all of its senses, and especially in its senses of taste and smell. The mouse lives in the real world. We've forgotten how to do that.