I don't know how scientific this is, but in my younger days, we were told that because humans have the characteristic teeth of both carnivores and herbivores, that is, canines and grinding molars, we are omnivores.

���What is the proper diet for human beings?” What is the diet that allows us to look, feel, and function at our best? Not just to survive or lose weight. Is it vegetarian? Does it contain meat? How much meat? There must be a correct answer. Just like there is one diet best for horses, one for cats, one for dogs, and one for each kind of bird – there must be one diet best for people. Why have we not discovered this diet? It is certainly not because of lack of interest. Russell Henry Chittenden, the father of American biochemistry and professor of physiological chemistry at Yale Medical School, wrote, a century ago (1904), “We hear on all sides widely divergent views regarding the needs of the body, as to the extent and character of food requirements, contradictory statements as to the relative merits of animal and vegetable foods; indeed, there is a great lack of agreement regarding many of the fundamental questions that constantly arise in any consideration of the nutrition of the human body.” You would think that after so many years of investigation using the latest scientific methods and employing modern technology that this matter of such grave importance would have been settled beyond a doubt. Coexistence today of enthusiastic advocates of “all meat” and “no meat” diets, and everything in between, proves this matter is far from settled. Our dentition evolved for processing starches, fruits, and vegetables, not tearing and masticating flesh. Our oft-cited "canine" teeth are not at all comparable to the sharp teeth of true carnivores. If you have any doubt of the truth of this observation then go look in the mirror right now – you may have learned to call your 4 corner front teeth, “canine teeth” – but in no way do they resemble the sharp, jagged, blades of a true carnivore – your corner teeth are short, blunted, and flat on top (or slightly rounded at most). Nor do they ever function in the manner of true canine teeth. The lower jaw of a meat-eating animal has very little side-to-side motion – it is fixed to open and close, which adds strength and stability to its powerful bite. Like other plant-eating animals our jaw can move forwards and backwards, and side-to-side, as well as open and close, for biting off pieces of plant matter, and then grinding them into smaller pieces with our flat molars. From our lips to our anus our digestive system efficiently processes plant foods. Digestion begins in the mouth with a salivary enzyme, called alpha-amylase (ptyalin), whose sole purpose is to help digest complex carbohydrates found in plant foods into simple sugars. There are no carbohydrates in meats of any kind (except for a smidgen of glycogen), so a true carnivore has no need for this enzyme – their salivary glands do not synthesize alpha amylase. The stomach juices of a meat-eating animal are very concentrated in acid. The purpose of this acid is to efficiently break down the muscle and bone materials swallowed in large quantities into the stomachs of meat-eaters. Digestion of starches, vegetables and fruits is accomplished efficiently with the much lower concentrations of stomach acid found in the stomachs of people, and other plant-eaters. The human intestine is long and coiled, much like that of apes, cows, and horses. This configuration makes digestion slow, allowing time to break down and absorb the nutrients from plant food sources. The intestine of a carnivore, like a cat, is short, straight, and tubular. This allows for very rapid digestion of flesh and excretion of the remnants quickly before they putrefy (rot). There are also marked sacculations (many sac-like enlargements that bulge out along our large intestine), like those found in all apes, which strongly supports the view that we are primarily plant-eating animals. Overall, the intestines of meat-eaters are noticeably simpler than ours. Cholesterol is only found in animal foods – no plant contains cholesterol. The liver and biliary system of a meat-eating animal has an unlimited capacity to process and excrete cholesterol from its body – it goes out, in the bile, passing through the bile ducts and gallbladder, into the intestine, and finally, out with the stool. For example, you can feed a dog or cat pure egg yolks all day long and they will easily get rid of all of it and never suffer from a backup of cholesterol. Humans, like other plant-eating animals, have livers with very limited capacities for cholesterol removal – they can remove only a little more than they make for themselves for their own bodies – and as a result, most people have great difficulty eliminating the extra cholesterol they take in from eating animal products. The resulting cholesterol buildup, when people eat meat, causes deposits in the arteries (atherosclerosis), in the skin under the eyes (xanthelasma), and in the tendons. Bile supersaturated with cholesterol forms gallstones (over 90% of gallstones are made of cholesterol). About half of all middle-aged women who live on the Western diet have cholesterol gallstones. Ascorbic acid – found preformed and ready to use in plant foods – is called vitamin C in the diet of people. Insufficient amounts of this vitamin cause scurvy. Vitamins are essential micronutrients that cannot be synthesized by the body; and therefore, must be in the food. Meat-eating animals have the ability to synthesize ascorbic acid from basic raw materials found in their meat diet – therefore, it is not a vitamin for them. (In other words it is not “vital” or essential to be preformed in their food supply.) Humans, and other plant-eating animals, have the ability to make vitamin A from a precursor found in large quantities in plants, called beta-carotene. Carnivores cannot utilize beta-carotene as a precursor of vitamin A. They have no need to; they have always had a plentiful supply of preformed vitamin A (Retinol) found in the meat. Carnivores also do not have the ability to synthesize Niacin, which is plentiful in meat. Human males have seminal vesicles – no other meat-eating animal has these important collecting-pouches as part of their reproductive anatomy. The seminal vesicles are paired sacculated pouches connected to the prostate, located at the base of the bladder. They collect fluids made by the prostate that nourish and transport the sperm. Ejaculation occurs when the seminal vesicles and prostate empty into the urethra of the penis. In many ways ejaculation is the ultimate act of male performance – seminal vesicles are essential organs for proper male function and therefore, they should tell us much about his true nature. Eating meat diminishes sexual performance and masculinity. The male hormone testosterone that determines sexual development and interest has been found to be 13 % higher in vegans (a strict plant diet – no animal products of any kind) than in meat-eaters. Reproduced with permission by Dr. McDougall. For complete article with references see http://tinyurl.com/5pjkm

This question relates to life-span, everyone will ask it sometime in life or at least the though will flash by for a moment. In the animal world survival is the agenda for the day, animals will try to eat what is edible but there form (physiology) allows it to eat some of the many edible foods. The first step is getting the food from outside to inside. After the size of the mouth, teeth will reveal a great deal about diet. Three general types of teeth are incisors (rabits, rats, great of carrots and such), the canines (pointed teeth, cats, dogs, great for tearing at meat) and finally molars (cows, good for grinding stuff like grain, cereals). If the animal has one of these groups, they probably eat mostly the food best suited for the teeth. If an animal wants to tap into more food sources, they need the associated teeth for getting the food to a manageable package for the stomach. Animals who did not specialize in a particular food source (maybe because it was already dominated by another species) where forced to eat difficult foods and developed all three sets of teeth in the mouth. Generally scavengers have all types of teeth. scavengers don,t pick or choose food but can eat leftovers from others which adds up to a meal or more. The human mouth is designed to eat that way and intelligence has resulted in windfall. Humans can live on a diverse diet where no animal can come close. One only has to pick up a few menus worldwide to see the tens of thousands of natural ingredients that make our food (you can add man made foods for a higher fiqure. Our physiology craves fats and sugars. Combine both and that invents an irresistible food, ice-cream. Our physiology might last longer with some foods over the other since be have science to see beyond taste. But it is not something to be worried about. The most you might get from devoting your intelligence to the subject is 30 more years. Only then if you are lucky to have genes that don,t cause some food groups to be harmful. Some food dangers to some individuals are to expensive or subtle to test for and show up when you are dead. One diet that works across the board with any physiology and whether vegetarian or not, animal or not, is the low cal diet, it does not really matter what you eat as long as you get enough nutrition. you can even pick any popular diet that meets your needs. Whether you combine a Big Mac or supersize for that matter, and get the other nutrients from vegetables or maggots,etc- that works the same. The key is how much you eat (specifically the number of calories). Since the turn of the century, the low calorie diet has been documented and repeated all over the world by many scientist with different views. All their data concludes with the fact that animals on a low calorie diet (you can tell when you leave the table no longer hungry but not satisfied) lived longer, averaging a third longer, that is close to living twice as long. You won,t see much on the subject because there is no money to made in something that is already free. Our physiology supports almost everything edible but it will last much longer if it gets less than what you want.

Given that bowel cancer,obesity and heart disease rates are so much higher in the US than anywhere else, and the US consumes more meat per head than any other country worldwide is that evidence enough?

Vegetarians live five years longer than omnivores on average. That alone should be a clue to even the biggest meat proponent. Pretty tough to argue with extended life, isn't it? Ever hear of a vegan with arteriosclerosis? How often do you hear of a vegan with intestinal cancer? Is it a hint when meat can't even be consumed (digested) when you have certain diseases?

When the Atkins diet was really popular, there was an influx of sick people on it being admitted to the hospital where my sister works. Maybe a hint as to what our bodies think of primarily being a carnivore?

Yes, we have teeth in our mouths that are suited for tearing meat, thus we should include meat in our diets!