In order to answer the question - What is a healthy diet? - I think that it is important to first take a look at what our body is designed for. Basically, is the human body designed to eat plant foods, animal foods, or both? Respectively, are we herbivores, carnivores, or omnivores? The answer is quite straightforward. While the human gastrointestinal tract is capable of digesting both animal and plant foods, there are indications that it can accommodate plant foods much easier than the harder to digest animal foods. Specifically, our teeth are composed of twenty molars which are perfect for crushing and grinding plant foods along with eight front incisors which are well-suited for biting into fruits and vegetables. Only our front four canine teeth are designed for meat eating. Our jaws swing both vertically to tear and laterally to crush, while carnivores' jaws only swings vertically. Additional evidence to support the body's preference for plant foods is the long length of the human intestinal tract. Carnivores typically have a short bowel while herbivores have a bowel length proportionally comparable to humans. Thus, the human bowel length favors plant foods.
A look at our closest wild relatives
To answer the question what humans should eat, many researchers look to other primates, such as chimpanzees, monkeys, and gorillas. Non-human wild primates are also omnivores - or as often described herbivores and opportunistic carnivores. They eat mainly fruits and vegetables but may also eat small animals, lizards, and eggs if given the opportunity. The gorilla and the orangutan eat only 1 and 2%, respectively, animal foods as a percentage of total calories. The remainder of their diet is from plant foods. Since humans are between the weight of the gorilla and orangutan, it has been suggested that humans are designed to eat around 1.5% of their diet as animal foods. Most Americans derive well over 50% of their calories from animal foods.
While most primates eat a considerable amount of fruit in their diet, it is critical to point out that the cultivated fruit in American supermarkets is far different than the highly nutritious wild fruits these animals rely on. Wild fruits have a slightly higher protein content and a higher content of certain essential vitamins and minerals while cultivated fruits tend to be higher in sugars. Cultivated fruits are therefore very tasty to humans, but because they have a higher sugar composition and also lack the fibrous pulp and multiple seeds found in wild fruit that slow down sugar digestion and absorption of sugars, the cultivated fruits raise blood sugar levels much more quickly than their wild counterpart. 
Wild primates not only fill up on fruit, but also other highly nutritious plant foods. As a result, wild primates weighing 1/10 the amount of a typical human ingest nearly 10 times the level of vitamin C and much higher amounts of many other vitamins and minerals. There are other differences in the wild primate diet that are also important to point out such as a higher ratio of alpha-linolenic acid - the essential omega-3 fatty acid - compared to linoleic acid - the essential omega-6 fatty acid.
Estimated mineral intakes of wild monkeys and humans
|Mineral||Total daily intake|
7 kg adult monkey
|RDA for adult male|
Determining what humans are best suited for may not be as simple as looking at the diet of wild primates. Humans have some structural and physiological differences compared to apes. The key difference may be a larger, more metabolically active brain. In fact, it has been theorized that a shift in dietary intake to more animal foods may have produced the stimulus for brain growth. The shift itself was probably the result of limited food availability forcing early humans to hunt grazing mammals such as antelope and gazelle. Archeological data supports this association - brains of humans started to grow and be more developed at about the same time as there is more evidence of animal bones being butchered with stone tools at early villages.
While improved dietary quality alone cannot fully explain why human brains grew, it definitely appears to have played a critical role. With bigger brain, early humans were able to engage in more complex social behavior, which led to improved foraging and hunting tactics, which in turn led to even higher quality food intake fostering additional brain evolution.
Data from anthropologists looking at evidence from hunter-gatherer cultures are providing much insight as to what humans are designed to eat.  However, it is very important to point out that these groups were not entirely free to determine their diets.  Instead their diets were molded as a result of what was available to them. For example, the diet of the Inuit Eskimos is far different from the Australian aborigines. It may not be appropriate to answer the question "What should humans eat?" simply by looking at these studies. Nonetheless, here is something important to point out: whether a hunter-gatherer community relied on animal or plant foods the rate of diseases of civilization such as heart disease and cancer is extremely low.
It should also be pointed out that the meat that our ancestors consumed was much different than the meat we find in the supermarkets today. Domesticated animals have always had higher fat levels than their wild counterpart, but the desire for tender meat has led to the breeding of cattle which produce meat with a fat content of 25-30% or higher compared to a fat content of lower than 4% for free-living animals or wild game. In addition, the type of fat is considerably different. Domestic beef contains primarily saturated fats and virtually undetectable amounts of omega-3 fatty acids. In contrast, the fat of wild animals contains over 5 times more polyunsaturated fat per gram and has good amounts of beneficial omega-3 fatty acids as well.
What does all this mean?
Basically it means that humans appear to be better suited to a diet composed primarily of plant foods. That does not mean that everyone should become a vegetarian, but rather we should stress plant-based foods such as vegetables, fruits, grains, legumes, nuts, and seeds over animal foods in the diet. This contention is supported by the tremendous amount of evidence showing that deviating from a predominantly plant-based diet is a major factor in the development of heart disease, cancer, strokes, arthritis, and many other chronic degenerative disease. It is now the recommendation of many health and medical organizations that the human diet should focus primarily on etc.
The Government and Nutrition Education
Throughout the years various governmental organizations have published dietary guidelines, but it has been the recommendations of the United States Department of Agriculture (USDA) which have become the most widely known. In 1956, the USDA published "Food for Fitness - A Daily Food Guide." This became popularly known as the Basic Four Food Groups. The Basic Four was composed of:
- The Milk Group - milk, cheese, ice cream, and other milk-based foods.
- The Meat Group - meat, fish, poultry, eggs, with dried legumes and nuts as alternatives.
- The Fruit and Vegetable Group.
- The Breads and Cereals Group.
One of the major problems with the Basic Four Food Groups model is that it graphically suggests that the food groups are equal in health value. The result - over consumption of animal products, dietary fat, refined carbohydrates, and insufficient consumption of fiber-rich foods like fruits, vegetables, and legumes. This in turn has resulted in diet being responsible for many premature deaths, chronic diseases, and increased health care costs.As the Basic Four Food Groups became outdated, various other governmental, as well as medical, organizations developed guidelines of their own designed to either reduce a specific chronic degenerative disease like cancer and heart disease or reduce the risk for all chronic diseases.
In an attempt to create a new model in nutrition education, the United States Department of Agriculture first published the "Eating Right Pyramid" in 1992. Since that time it has received harsh criticisms from numerous experts and other organizations. One big question consumers may want to ask' "Is it appropriate to have the USDA making these recommendations?" After all, the USDA serves two somewhat conflicting roles: (1) it represents the food industry and (2) it is in charge of educating consumers about nutrition. Many people believe that the pyramid was more weighted towards dairy products, red meat, and breads due to influence of the dairy, beef, and grain farming and processing industries. In other words, the pyramid was not designed as a way to improve the health of Americans but rather promote the USDA agenda of supporting multinational agri-foods giants.
One of the main criticisms of the Eating Right Pyramid is that is does not stress strongly enough the importance of quality food choices. For example, the bottom of the pyramid represents the foods that the USDA thinks should make up the bulk of your diet: the Bread, Cereal, Rice, and Pasta Group. At 6-11 servings a day from this group and you are supposedly on your way to a healthier life. What the pyramid doesn't tell you, though, is that you are setting yourself up for insulin resistance, obesity, and adult onset diabetes if you consistently make poor choices in this important category. The Eating Right Pyramid does not take into consideration the glycemic index of foods. The glycemic index tells us how quickly blood sugar levels will rise after eating a certain type of food. If we take a quick look at the glycemic indices of some of the foods that the pyramid is directing Americans to eat more of it is easy to see the problem.
A New Food Pyramid
It is quite now that the USDA Food Pyramid is wrong. In fact, some believe that it has been proven a dangerous and misleading dietary guide that has contributed greatly to the growing problems of obesity and type 2 diabetes. Different medical organizations are offering their own version, so I would like to do the same. With the help of Michael Lyon, M.D., I have created "The Optimal Health Food Pyramid."
If you compare this pyramid to the USDA's you will notice some clear differences. Our version incorporates the best from two of the most healthful diets ever studied - the traditional Mediterranean diet and the traditional Asian diet. These diets have also been shown to be protective against heart disease and cancer. Our pyramid also provides additional recommendations for foundational supplement and lifestyle components. It graphically illustrates "What is a healthy diet?" and represents the current evidence on what humans are designed to eat for optimal health.
- Milton K. Nutritional characteristics of wild primate foods: do the diets of our closest living relatives have lessons for us? Nutrition 1999;15:488-98.
- Cordain L, Miller JB, Eaton SB, et al. Plant-animal subsistence ratios and macronutrient energy estimations in worldwide hunter-gatherer diets. Am J Clin Nutr 2000;71:682-92.
- Milton K. Hunter-gatherer diets-a different perspective. Am J Clin Nutr 2000;71:665-7.