He is biased; however, my immensely powerful secret skill of "Logic" agrees. Organisms evolve basically to continue existing; however, the continuous reproduction of DNA and its support structures requires energy. Plants get energy from the sun, using light to excite an electron off a Chlorophyl molecule to power an endothermic reaction that strips the carbon off carbon dioxide and bonds it with water, producing sugar (shorthand CH2O, technically (CH2O)6, C6H12O6). The reverse reaction is exothermic, and thus sugar is fuel used later by the plant.

Every organism that cannot use environmental energy (heat, light, etc.) to produce chemical energy storage seeks what we call "food," but what is most basically "Fuel," which means "an energy source." Diet was never important; every animal, every protozoa, every bacteria seeks energy. Humans, like anything else, must eat to live. We don't eat because we need beta carotine and vitamin C and calcium; those happen to exist in abundance in a varied diet, and we shove whatever we find down our throats to collect the sugars and proteins and fats. As complex structure required certain things, those certain things became necessary; but the nutritional needs of an organism are satisfied by the food sources that they typically find in the environment the organism evolves in.

In other words, as long as you don't live entirely on cheesecake, you're fine.

The argument for exercise is far more complex. It has to do with homeostasis, which in turn impacts hormesis and metabolism, which impact how your body deals with toxins, damage, and foods. A very basic argument might cover something like how an abundance of food requires much less muscle, while a cycle would be fatal and thus fat storage protects you and muscle growth kicks in when you start to have to work for your food. This ... does not account for the negative health effects of becoming fat and lazy; although it doesn't much matter when life is good, predators are low, physical activity is minor, and you can breed like rats. "Survival of the fittest" only matters when you need to be fit to survive long enough to breed. You can imagine how complex this would get.

I guess animals evolved to supply variable fitness as their needs change. A fat, unhealthy human won't live long; but he only has to hit about 15-20 to breed, the girls only have to reach 14-16, pop out 2-3 babies, and the tribe (humans are community animals) raise the children. If the environment is full of predators and/or scarce on food, however, you'll need to live longer (predators cull the population, while extended life increases breeding opportunities) and you need to be stronger and faster to do that (so you can get your hands on the scarce food before others or before it runs away, and evade predators).

In the Garden of Eden, it's survival of the fattest.

Actually, fat is harder to derive energy from, but easier to store. You ever notice your arms are real flabby if you're not doing push-ups or weight lifting or whatnot? But you have good muscles under there ... and when you start working them out, your arms become lean. Working a muscle doesn't burn fat from that part of the body; six thousand sit-ups won't burn your belly fat. This is a fact. So why do you get lean and tone?

Three reasons, of course. The first and most obvious is the reduction of fat overall; the second, and also somewhat obvious if you think about it, is that the muscle becomes bigger and thus the same mass of fat will spread thinner around it. The last is more complex.

When your muscles need to work a lot, they become fat storage cells. Normally your body builds storage cells that retain a lot of water and fatty acids, giving you a saggy gut or flabby arms or packing a bunch of crap around your heart (and squeezing it instead of just cushioning it happily). Besides making you fatty and flabby, these cells... store fat. When you need to burn that fat, your body needs to find certain proteins--in your immediate diet (excess protein becomes urea and gets urinated out) or by deteriorating your muscles and recycling the proteins--to break down that fat. Lipolysis. The fat is then somehow chemically altered so it becomes a transportable energy source, which gets brought to the muscle for use. I'm unclear on the details; it is, obviously, difficult.

When your muscles store fatty acids inside them, however, you can literally "burn" the fat. Oxygen comes to the cell and combines with fatty acids to release heat, just like when you burn tallow candles or glucose. That little bit of heat is used to break down ATP, releasing more heat and activating other things in your muscles to make them contract. Direct energy source.

When you consume proteins, on the other hand, your body uses them for building structure. RNA, DNA replication process, building of new cells, lipolysis, all kinds of processes use proteins or require new proteins (cell walls contain a lot of proteins for various purposes). Proteins are basically very tiny construction tools: they have a physical effect on chemicals, stretching them to weaken chemical bonds or attracting them together to force reactions, or simply contracting from stimulation and moving certain chemicals around (i.e. sodium-potassium pump). You can break them down for energy, of course; but if not needed, your body rejects the excess protein, turning it into urea and then your pee turns yellow (yes, vitamin B pills also will turn your pee yellow ... it's not exclusive).

Glucose, of course, creates an immediate toxic blood glucose concentration spike. Your pancreas recognizes this and immediately releases insulin, causing the liver to bind glucose into glycogen. Then your pancreas releases glucagon as your blood sugar levels drop, turning the glucagon back into glucose, which your liver releases into the blood. Glucose is thus an immediate energy source which gets stored in a temporary hold for immediate use, unlike fat which gets bound up for long-term storage. Given both, your body will use glucose immediately and begin storing fat unless the demanded energy output is higher than the throughput possible by glucose regulation (even under load, you will release insulin until your blood glucose levels are sane, then start pumping out glucagon; you can only turn glycogen into glucose so fast, though).

Thus sugars (starch, sugar) are the most essential energy source because you can use sugars immediately, and you will use sugars to power the processes that turn proteins and fat into energy or store fat for long-term use (this mean that you need to intake a small amount of sugar (and protein) to burn fat; don't exercise hungry, give yourself an ignition charge at least). Protein is the least essential energy source; it is an essential structural component required for repairing existing structures and building new structures, but you can derive energy from it. Fat is an extremely essential energy source, providing an over-capacity control (when you can't physically burn sugar fast enough) and long-term storage.


You are correct in that it is silly to say that one is bad and the other is good. Cholesterol is critically important, even "bad" cholesterol; so is sugar. The interactions between these energy sources are complex, and cannot be simplified in that way.