A series of articles written by Dr. Barry Groves just blew my mind. I think it relates to the previous discussion here of fermentive bacteria and their production of short chain fats (note, original link has some picutres and diagrams not included here). I think it also brings more to light on what humans are "supposed" to eat by looking at the actual diets of other mammals.
[quote author=http://www.second-opinions.co.uk/should-all-animals-eat-a-high-fat-low-carb-diet.html]
Should all animals eat a high-fat, low-carb diet?
Part One: The Basis for a High-fat Diet
Consider cattle, sheep, horses, gorillas, rhinoceroses, koalas, all of which are herbivores eating grass and leaves; lions, wolves, polar bears, all of which are carnivores eating other animals; and humans. Before continuing, I want you to ask yourself:
Which of them in its natural habitat eats a high-fat, low-carb diet?
cows koala polar bear
lion rabbit Maasai warrior
It's likely that you will classify the herbivores as low-fat dieters and the carnivores – particularly the polar bear – in the 'high-fat diet' class. But what about humans? Where do we fit? Those are the important questions today.
In 1997 the Journal of Nutrition published a study of the dietary intake of Western Lowland Gorillas, as the authors considered that the diet of one of our nearest 'cousins' could have implications for our (human) health.[1]
gorilla
The Western Lowland Gorilla
The Western Lowland Gorilla eats leaves from around 200 species of plant. These foods are low in fat and available carbohydrate, varied in protein and very high in total dietary fibre. Macronutrients, in grams per 100 grams of dry matter, were found to be:
Fat 0.5g (range 0.1 - 1.8)
Available carbohydrate 7.7g (range 0 - 22.4)
Protein 11.8g (range 1.7 - 30.0)
Fibre made up by far the major part of the plants ranging between 52.0g and 96.5g for a mean of 74.0g.
Putting the available macronutrients in terms of calorific values we have:
fat 5.9%
available carbohydrate 37.1%
protein 57.0%
On the face of it, therefore, it looks as if gorillas eat a very low-fat, high protein diet. But things aren't as clear cut as that.
No mammal — not even the herbivores — has developed an enzyme that will digest vegetable fibre. This is why we tend to discount it when calculating our calorie intakes. However, while mammals have not developed an enzyme that will digest fibre, there are lots of micro-organisms and bacteria that can the job for them. The herbivores employ billions of these bacteria.
Comparison between Man and gorilla
You will note in the picture on the right that there is a marked difference in shape between a human and a gorilla. But the gorilla's gut is not only much larger than a human's, it also has an entirely different design.
Human
Small intestine is major organ used to extract nutrients.
Small intestine ~ 50% of the total volume.
Caecum and colon ~ 20% volume
Gorilla
Ratios exactly the opposite
Small intestine ~ 25% volume
Caecum and colon ~ 53% volume.
This difference is highly significant. In a herbivore such as the gorilla, the caecum and colon harbour huge colonies of bacteria which ferment carbohydrates, particularly fibre, and use it to produce short chain fatty acids (SCFA) — principally acetic, proprionic and butyric acids. These are then absorbed into the body to be used as a source of energy.
If we look at the gorilla's diet, we now see that the authors of the study into the western lowland gorilla's diet find that the fibre provides some 1.5 kcals of energy per gram of fibre, in the form of SCFA. As the fibre averages about three-fourths of the gorilla's diet, this energy forms a highly significant proportion of the gorilla's total energy intake.
These SCFAs must be added to the fats already present in the gorilla's diet, which gives us the following proportions:
Overall energy
(kcal) per 100g %age
Protein 47.1 24.3%
Available carbs 30.6 15.8%
Fat 4.9 2.5%
SCFA from fibre 111.0 57.7%
This gives totals of:
protein = 24.3%
carbs = 15.8%
fats = 59.8%
In other words, although the western lowland gorilla's diet, exclusively of leaves, looks like a very low-fat, carbohydrate-rich diet, it is actually a high-fat, moderate-protein, low-carb diet.
But what of other animals. We'll look at them in Part Two
Reference
1. Popovich DG, et al. The Western Lowland Gorilla Diet Has Implications for the Health of Humans and Other Hominoids. J Nutr 1997; 127: 2000-2005.[/quote]
[quote author=http://www.second-opinions.co.uk/should-all-animals-eat-a-high-fat-low-carb-diet-2.html]
Should all animals eat a high-fat, low-carb diet?
Part Two: Digestive difference between herbivores and carnivores
We know, then, that gorillas eat a high-fat, low-carb diet, but what of other herbivores? And carnivores, and humans?
Herbivores
The major digestive difference between herbivores and carnivores lies primarily in the herbivores' ability to convert vegetable fibre and other carbohydrates into short-chain fatty acids and to absorb those fatty acids, an ability that carnivorous animals and humans do not possess.
They do this in one of two ways. They are either 'hindgut digesters', or 'foregut digesters'. But firstly, no animal, whatever its diet, has developed an enzyme that will digest vegetable fibre. Vegetable fibre is always digested using fermentation by bacteria.
Hindgut digesters
The gorilla, like most primates, is a hindgut digester. Other animals in this category include horses, pigs, and rabbits. These have gastrointestinal tracts with a similar basic layout as humans. The differences are in the relative sizes and functions of the various parts. Where a human (or other carnivore) has a small caecum and colon, the caecum and colon of a hindgut-digesing herbivore are both much larger.
Hindgut digesters digest and absorb proteins, available carbs and fats, as humans do, through the stomach and small intestine. The undigested fibre in their diet then passes to the caecum and colon which house huge colonies of bacteria. It is here that the fibre, and any undigested carbohydrate, is fermented to produce SCFAs, which are then absorbed into the body to be used for energy.
Foregut digesters (ruminants)
The foregut digesters are those animals that ruminate. Ruminants evolved to consume and subsist on roughage – grasses and shrubs built predominantly of cellulose. Ruminants include the large grazing or browsing mammals such as cattle, goats, sheep, deer, and antelope and, among primates, the colobus monkey.
In ruminants, the major organ of fermentation is their stomach, or perhaps I should say stomachs as there tends to be four separate compartments: the rumen, reticulum, omasum and abomasum which, collectively, occupy almost three-fourths of the abdominal cavity.
This multiple stomach also employs bacteria. Coming first in the digestive tract, the stomach not only ferments fibre to produce SCFAs, but also available carbohydrates. This reduces the amount of available carbs to be converted and absorbed as glucose, but increases the amount of SCFAs from a given amount of plant food such that: “Volatile fatty acids are produced in large amounts through ruminal fermentation and are of paramount importance in that they provide greater than 70% of the ruminant's energy supply.”[2] In this way, ruminants have a diet that is even higher in fats and which contains practically no carbohydrates at all.
All herbivores utilise one or other of these methods of getting energy from what looks like pretty energy-deficient food sources. It seems clear, therefore, that the metabolisms of all of them are adapted to utilise SCFAs as their major energy source, rather than glucose, and that they are actually designed and adapted to live on a high-fat, moderate-protein, low-carb diet.
lions
The carnivore's diet
Carnivores, such as lions, tigers, dogs, cats, wolves and hyenas, are quite unable to use fibre as an energy source in the same way as that herbivores do. But this doesn't matter as the carnivores are adapted to eat herbivores. It is noticeable that carnivorous animals tend to go for the fattier parts of their prey. This is particularly noticeable with hyenas whose jaws and teeth are designed to break the long bones and skulls to get at the bone marrow and brain within, which are very high in fat. So, the carnivores are also adapted to eat a high-fat, no-carb diet.
The human diet
So, where do we humans fit into the picture?
The first thing to note is that we are just as much an 'animal' as any of the others when it comes to diet, and there is no reason to suppose that we need to treat ourselves any differently.
This is borne out by observations by anthropologists and medical missionaries over a couple of centuries who all related that 'primitive' human cultures also ate and preferred high fat diets.[3-8]
The only thing we need to do, therefore, is determine whether we should eat a herbivore (vegan) diet, or a carnivore diet.
And that is not difficult to determine. We have little caecum to speak of and, while we do have fermentative bacteria in our colons, their products are only poorly absorbed into our bodies if at all. Those facts put us clearly into the carnivore class of animals.
Australian Aborigine Kalahari Bushmen Inuit with polar bear
Conclusion
If we look at the various natural diets of all mammals, we find the same pattern: All of the diets are high in fat, and most of that fat is saturated as, apart from the saturated fats found in meat, all the short chain fatty acids produced by fermentative bacteria are 100% saturated. Also, all mammals' natural diets are very low in carbohydrate in the case of herbivores, and practically carbohydrate free in the case of carnivores.
There is no reason to suppose that we 'civilised' humans should eat any differently. And we do so at our peril
References
2. http://vetmedicine.about.com/gi/dynamic/offsite.htm?zi=1/XJ/Ya&sdn=vetmedicine&zu=http%3A%2F%2Farbl.cvmbs.colostate.edu%2Fhbooks%2Fpathphys%2Fdigestion%2Fherbivores%2Frumen_anat.html
3. Stefansson V. The Fat of The Land. New York: Macmillan Press, 1957.
4. Wilkins GH. Undiscovered Australia. London: G. P. Putnam & Sons, 1928.
5. Price WA. Nutrition and Physical Degeneration: A Comparison of Primitive and Modern Diets and Their Effects. Paul B. Hoeber, Inc, New York, 1939.
6. Grant GM. Ocean to Ocean. Toronto, 1873.
7. Peary RE. Secrets of Polar Travel. New York: Century Co, 1917.
8. Hanson EP. Journey to Manaos. New York: Reynal & Hitchcock. 1938.[/quote]
There is a part 3, but I didn't find it to be very relevant (it basically just sums up by saying "and this is why we're sick, and so are our pets).
As I siad, I found this really mind blowing. The idea that EVERY mammal (and maybe other species as well) eats a high fat, low carb diet. So by depending primarily on carbohydrate for a food source, without having the same hindgut or foregut fermentation abilities of other mammals, humans haven't just turned away from the ideal human diet, they've abandoned the ideal MAMMALIAN diet. Pretty fascinating stuff.
(I finally got to use the dancing banana smiley!) 
