As I contemplate my 2010 New Year's resolutions, I'm considering returning to strictly vegetarian or perhaps even a vegan diet. Most of my adult life, I've either been vegetarian or close to it, and my diet has already been revamped quite a bit during the last year to reduce my weight. (I've lost 25 pounds thus far, going from the obese to the overweight category). The scientific establishment has made countless discoveries since the healthfood crazes of the '60s, but one fact that impresses me is that a person would have remained almost perfectly in line with all current medical advice if they simply ate a vegetarian diet (basically, a slightly de-Japanized version of the macrobiotic diet). The recent findings that lowering consumption of methionine (an essential protein) seems to correlate with longer lifespan puts one additional nail in the coffin of the meat and fish diet. All research that I've seen clearly shows significant health and longevity benefits (a good review can be found here in QJM). The concluding points of this 1997 article in Nutritional Reviews titled "Effects of vegetarian diets on aging and longevity," notes many of the key benefits found across studies:
1 ) From what we know, vegetarian diets result in a lower risk for many diseases such as obesity, cardiovascular disease, some types of cancer, constipation, hypertension, and type II diabetes. If we take the observed lower mortality rates as a parameter, longevity is higher.
Basically, these are the key "modern" diseases.
2) People on a vegan diet have an increased risk of iron, vitamin B12, vitamin D3, and calcium deficiency; eventually also zinc, n-3 fatty acids, and protein intake may not fulfill the basic requirements. Except for vitamin B12, it appears that a vegan diet can fulfill the requirements, but it takes quite a bit of knowledge and even expertise to choose the fruits and vegetables that contain all the necessary nutrients.
B12 is easy to get since it's put in pretty much everything (such as breakfast cereals) as a supplement, and vitamin D can be had through sunshine. The latest research even suggests that less protein might be a good thing.
3) Based on our present knowledge that high contents of vegetables, fruits, and complex carbohydrates and low amounts of saturated fatty acids are correlated with a reduced risk for the same diseases mentioned under point 1, it is obvious that total abstinence from eating meat is not a major factor for the beneficial effects of vegetarian diets.
I guess it isn't necessary to be extreme. I might still eat an occasional fish that I catch on a hiking trip.
4) The vegetarian-type diet with lots of vegetables and fruits and complex carbohydrates can be considered a prudent diet in the sense of today's guidelines (see, e.g., Dietary Guidelines for Americans 1990).48
5) History has shown that vegetarians were right when they claimed more than 100 years ago that the vegetarian diet including fruits, vegetables, fibers, and complex carbohydrates is a healthy one.
People's intuition about food seems to be pretty good. The calorie-reduction diet (CRON) that's now been found to increase longevity significantly has been around in East Asia (it's called soshik in Korean and shoshoku in Japanese).
6) The inclusion of some low-fat meat and fish does not seem to be harmful; it could actually be beneficial in lowering the risk of deficiencies in some extreme cases of vegetarianism.
7) Finally, we should realize that about 40% of today's world grain production is used for meat-producing livestock. This conversion of cereal grains and other food concentrates to animal products involves large losses in energy; 1 kg of American beef requires 5 kg of grain. Therefore, if meat consumption could be lowered, more cereal grains and other valuable food components could be used to improve the world's nutrition.
This last point brings out the economic impact. My guess is that if we took the 1400 calories per day that each American (woman, man, and child) tosses into the trashcan each day and then added in the food saved from eating grain instead of animals and from eating corn instead of creating ethanol, we could probably feed the whole of Africa (and cut our current healthcare costs by more than half in the process).