I can think of plenty of reasons why we're not naturally suited to a vegan lifestyle.

The latest can be found in the pages of the July edition of the American Journal of Nutrition, where a new study finds that vegans have a bone density that is about 5 percent lower than that of their meat-eating counterparts.

The good news from the study is that vegans don't seem to suffer from more fractures despite this difference in bone density. But I'm concerned because there are other problems associated with decreased bone density, including a higher risk for osteoporosis.

Plants and vegetables are crucial to our well-being, so vegans do have one part of it right. I can't think of many people who wouldn't do well to eat more greens, especially farm-fresh organic vegetables.

But I don't think a vegan diet is a particularly good idea for some people. Organic sources of eggs, meat, fish and animal products are not bad for you unless they come from a can, bag or drive-through window. There are so many vital nutrients, fats and minerals that come from good, fresh animal sources.

Vegans don't get any of these benefits, and it can be very difficult to try to make up for it using only plant-based foods. Many vegans rely heavily on supplements and protein powders, but they're still missing out on plenty of crucial fats and nutrients. Soy is not a very good substitute.

So while the latest study shows that vegans are more likely to have lower bone density, I wouldn't call that the biggest reason to avoid their lifestyle. The biggest reason is nutrition.

Take, for example, the amino acid tryptophan, which is absolutely vital to our mental well-being. Tryptophan is found in high concentrations in eggs, dairy products and turkey. Vegans eat none of these, of course, so it's harder for them to get it in proper amounts. A lack of tryptophan in the diet can lead to moodiness and even depression.

What makes it even more of a challenge is that misguided FDA policies have prevented supplements from containing tryptophan in effective amounts.

Vitamin B12, which helps keep our nerve cells and red blood cells functioning right, is hard to get in a strict vegan diet. It's found largely in – you guessed it – eggs, meat, and fish.

There's one other reason to eat meat: Nothing on earth can match the taste of a free-range organic Montana steak grilled just right. I wouldn't trade that for all the tofu in the world.