J B:
I also see a lot of vegetarians in my work, and they tend to exhibit a lot of the same problems, which I won't delineate for the sake of avoiding controversy. I think it's due to their reliance on starch more than the avoidance of meat.
I agree with this point, to an extent. If you take the average American diet and try to turn it into a vegetarian diet by substituting cheese or fake meats in place of animal meat, you're looking at a recipe for disaster. Fake meats are supposed to imitate the taste and texture of meat, not make you feel good or healthy. Refined sugars and flour are bad, no matter how you slice it. Simple, whole foods are what you should eat regardless of your ethical views, just to become happier and healthier. Some people will thrive on bananas, oranges, pineapple, blueberries, sesame seeds, cucumbers, and figs; others will eat brown rice with carrots and raisins a lot. It takes some experimentation to find something that truly optimizes your health while being simple and affordable. Don't expect to thrive on staples that contain refined flour, white rice, "sugar" (in the list of ingredients), high fructose corn syrup, or large amounts of fat/oil. Unfortunately this is what most of the vegetarians I have known have tried, and it is why they have failed. The standard propaganda is half-correct: We do need iron, calcium, various vitamins, and we really should avoid refined carbohydrates. The incorrect half is that any of the good nutrients are only or "best" found in meat or dairy and that
all carbohydrates are unhealthy.
Regardless of the argument of optimal vs. slightly less than optimal nutrition, there are some examples of very successful long-term vegans, such as durianrider on youtube or a lot of the old-timers on rawfoodsupport.com; and plenty of examples of vegetarians and vegans—like myself—who may not be high performance athletes, but live happily without problems like weakness, tiredness, heartburn, headaches, nausea, shortness of breath, the need for happy pills, and all the other seemingly ubiquitous problems among people who eat junk, be it vegetarian or full of animal products. Personally, in the last three years, I had about three minor headaches, no major ones; I was sick to my stomach once; had a mild cold or flu (tiredness and/or runny nose and/or sore throat) maybe 5 times; and I had heartburn once or twice. No other major health problems that I can remember off the top of my head. I think that's pretty average or even better than average. That's because my intake of junk food is average or below average, but not nil. I make no secret of that.
Avoiding animal products to me is like avoiding the killing of insects: I don't need to kill insects, and I don't need to eat animal products, so I don't do either.
Back to a more literal dharma context: If you follow the 5 precepts (and sometimes the 8 precepts) and practice meditation, you should be fine. The person who taught me the most in the area of meditation eats meat to this day, and I have no reason to doubt that he is safe from a rebirth in the lower realms. Karma is about intentional action. If you intend to provide your body with the best possible nourishment, that is a wholesome intention. You're not eating meat because you think the animals deserve to die or you want them to suffer or die. I'm assuming you don't kill them yourself and you don't waste food, so ethically you are about on the same level as someone who eats no animal products, but drives a car, which crushes little animals, poisons the environment, etc. I don't have a huge monster truck, and I don't drive around just for the sake of creating noise and pollution, so I'm not overly worried about it. I think the same principle applies to anyone who eats a reasonable amount of meat out of conviction that it is necessary or beneficial for one's health.