Very good question. I'd like to know as well. I've seen different
ratios recommended for protein, carbohydrate and fat, like 10-80-10,
and 20-60-20 etc. Why these numbers? Who's right?

  I'm sure the reasoning must be spelled out somewhere.  Where did you
find the figures?

  I know that when I was studying nutrition back in the seventies,
there was some concern that Americans ate too much fat.  It was thought
to contribute to heart disease, colon cancer, breast cancer, and
obesity, among other things, and we were getting over 30% of our
calories from fat, while other, healthier countries (at least with
respect to the diseases we were most concerned about) got less than 25%.

  There was also some speculation that protein might promote tumor
growth and inflammation, and concern about whether hunger throughout the
world could be eliminated if we raised less cattle, ate less meat, and
freed up grains, both for export and to eat ourselves.  It was the age
of "Diet for a Small Planet," "Two Acre Eden," "Mother Earth News," "The
Whole Earth Catalog," Rodale Press, macrobiotics, seven-grain bread, and
organic gardening.  Countries in which protein consumption was low
suffered less cancer.  On the other hand, because protein is amphoteric,
when we eat less of it, we have to pay more attention to the acid-base
balance of our diets and bodies.

  It was also thought that too much carbohydrate led to obesity,
diabetes, inflammation, and so on, and it was known that extremely low
carbohydrate diets actually helped people lose weight (inexplicably) in
spite of eating plenty of calories, so there were arguments for reducing
all three sources of energy.

  But the main emphasis was, and still is, on fat.  And the general
advice is "eat less fat," regardless of how much you're eating now.  But
the pendulum's swinging the other way nowadays, so it's hard to say
what's going on.

  The health promoters have always said everybody needs more exercise
(such as veggie juicer salesman Jack LaLanne, who said that if you
exercised enough, it didn't matter what garbage you ate), though there
were a few people (such as George Bernard Shaw, who said he never
exercised and didn't believe it was healthy -- though he actually did do
a lot of walking) and William Howard Taft (who told my mother he saw
nothing wrong with being fat; he loved being fat -- though he had gone
on an extreme, successful weight-loss diet not long before, which
probably saved his life) -- whose credibility gave the energetic ones an
uphill battle.

  But then there were arguments like the one that all vertebrates, from
hummingbirds to elephants, seem to have a fixed number of heartbeats, so
the more exercise you got, the quicker you'd run out; and the examples
of the occasional athlete dying inexplicably at the peak of his career
or health nut who died young.

  People got tired of hearing about all the things which could cause
cancer, especially smokers, whom the tobacco companies were eager to
support, and there seemed to be a serious risk of backlash.  America got
into shape for WW II, out of shape in the fifties, back into shape for
JFK, out of shape under Nixon, and so forth.  I'm glad to see young
people looking healthy nowadays, but I'm worried about their intellects.

  So I guess it's no wonder that these things are influenced by the
times and politics.  The question is whether they can be influenced by
science -- and whether the science exists.  We already have hucksters
proposing to study your genes and design a perfect lifestyle custom-made
to fit.  It'll never end.