1. The excessive protein intake that attends a meat-heavy diet produces excessive acidity in the body (a low pH is required to digest protein, and protein is amino acids which are themselves acidic). The body therefore must buffer (neutralize) the excess acids. It does so by leaching the bones of calcium. The result is to WEAKEN THE BONES and cause osteoporosis and possible bone fractures down the road.
2. The kidneys then must excrete the acid in the form of hydrogen ions, and the calcium the body has taken from the bones. This poses an undo amount of stress on the renal system. It is analogous to the inveterate drinker who calls on the liver to detoxify the body of alcohol, in doing so placing the liver under stress and leading to alcoholic hepatitis, cirrhosis, liver failure. In the case of the meat eater, the overworked kidneys can incur damage and in the long run fail, setting up the need for dialysis in extreme cases. Well, you may say, we all must die of something, and for most it is one of the major organs which fail (heart, liver, brain especially - and the kidneys). If it were up to me, I would choose NOT to die of kidney failure, as it is a slow and potentially terrible way to go. But that's just me.
3. Animal foods also lack fiber, and diets which rely heavily on them fail to achieve the requisite 35 grams or more of fiber per day (I advocate 50 to 100 grams). Our gastrointestinal tract, being long and convoluted, is - unlike the carnivore's short, straight, chimney shoot set-up - dependent on high quantities of roughage to move food through and to the anus. Without fiber food festers in the tract and (in the case of animal foods) carcinogenic metabolites can find their way back into the bloodstream and set up health problems. To achieve enough fiber on a meat-centric diet would require about 35 cups of spinach ( assuming that's the preferred vegetable of the day). That's a lot of spinach! Yes, fruit is allowed on the Paleolithic diets, but sparingly, and not enough to meet the hefty fiber requirement. And yes, nuts do contain fiber, but for every 1 gram of fiber, nuts have nearly four times the amount of fat. Which brings us to our next point:
4. Fat content. On a high meat diet, it is unlikely that one's fat intake won't be quite high. Even low fat animal products (skinless chicken breast, canned tuna, lean beef) are not without high levels of fat compared with beans (which meat-centric diets shun). Consider that lean roast beef contains 15 grams of fat per 370 calories. That many calories are found in a can of your favorite bean - kidney, legume, garbanzo. The amount of fat in beans? Zero. And unlike protein, which your body can excrete if you eat too much, excess fat gets deposited on the hips, waist, thighs, and buttocks pretty immediately. Which is why the judicious eater should enjoy fat sparingly.
5. Food contamination. We won't go into it in too much depth, but food poisoning is a real risk for those that eat out with any regularity and aren't fastidious in food handling and preparations. And most of us aren't perfect in that regard, raising the risk that some of the chicken and fish will carry unhealthful levels of pathogenic bacteria leading to intestinal damage and the inconvenient stomach cramps, flatulence, vomiting and diarrhea, and in extreme cases intestinal bleeding and death. Cases of food poisoning are on the rise, and because many cases don't have GI symptoms (you just feel fatigued, feverish, and achy) often you have gotten bit without even knowing it. Sources of food contamination are invariably animal products, since that's what bacteria cling to. Bacteria help the environment absorb waste, and dead flesh is exactly that. (Be reminded that cases of spinach or fruit contamination are from manure run-off, so even if it is vegetable matter that is infected the ultimate source is flesh and fecal matter).
Yes, the meat-centric diet has a short-term benefit of weight loss, but it helps you accomplish this at a risk of serious health problems and merely by excluding food groups that lead to weight gain (grains and dairy especially). As such it is an improvement on the standard American diet which is made up largely of flour and cheese. But why not take another and the ultimate step in the service of your health and now that you are eating a lot of fruits and vegetables substitute the meat in your diet for beans and some of those nuts for seeds. By doing so you will have extracted the full benefit from your food and eliminated potential health hazards.