Yes, fortunately, there is one. The answer to our diet problems is remarkably simple, and it's the common thread that explains why so many wildly different diets work. Let's rake a look at a few popular diets to see whether you can identify what that common element is.

The Paleo diet us is based on the same evolutionary arguments we've talked about here. The apparent goal of the Paleo diet is to replicate what we ate during most of our evolution, but given that certain Paleo foods are extinct or significantly different than they were during most of our evolutionary history, some 

sacrifices have to be made. (For example, selective agriculture has made many of the fruits we eat now much juicier and less fibrous than they were previously.) Paleo focuses on high-protein and low-carbohydrate foods. Approved foods include meat from wild animals, fruits, vegetables, nuts, and tubers. No grains or dairy are allowed because those came about relatively late in our existence. 

And it works for athletes, at least in the short run, as evidenced by the popularity of Paleo among competitive athletes, most notably the CrossFit crowd. 

 Then there's raw foodism. Here, the idea is that cooking our food is a recent enough technological advance that our bodies haven't yet had a chance to adapt to the change. Therefore, our bodies are designed to eat foods in their natural, raw state. Certain enzymes that help with digestion, along with other nutrients, are denatured during the cooking process, rendering them ineffective at their jobs. Some raw food-ists include raw dairy and meat products in their diet. Raw's cousin, fruitarianism (similar to the 30 Bananas a Day or 80/10/10 diets, which center on eating primarily simple carbohydrates and low amounts of protein and fat), focuses more on fruit than vegetables. The diet is composed of about 80 percent carbohydrates, includes no animal products whatsoever, and is entirely raw in its purest form. And Michael Arnstein, the most visible leader of the movement, has won the Vermont 100-miler and placed highly in the Leadville 100, one of the most famous ultramarathons in the world. 

 And of course, there's veganism, which I usually call "plant-based,"  But even within the realm of veganism, there are differing versions. Ultramarathon great Scott Jurek eats a traditionally balanced vegan diet (even if he consumes many more calories than the average vegan to fuel his 100-plus mile races). Then there's Brendan Brazier, Thrice author and former pro Ironman triathlete, who also eats plant-based foods, but focuses more on raw foods. The diets of both of these athletes include a relatively large amount of calories from fat, probably in the range of 25 percent. But there's another version of the vegan diet that has gained a ton of traction, thanks largely to the 2011 runaway hit documentary Forks Over Knives. The docu-mentary is based on the work of T. Colin Campbell, Ph.D., author of The China Study, and Caldwell Esselstyn, M.D., which links consumption of meat and dairy products to cancer and heart disease and advocates a "plant-based, whole foods diet". The diet is exactly what it sounds like—no animal products and no processed foods. Campbell and Esselstyn do not consider oils to be whole foods, for example, because in nature they only exist as parts of other foods, and so they advocate cooking with vegetable broth instead of even the olive oil that so many of us have considered healthy for so long. 

 Clearly, the diets we've looked at are strikingly different on the surface, especially when you consider the wide variation in the ratios of protein, carbohydrate, and fat to which the different diets adhere. But have you noticed the fundamental element that they all share? What they have in common is that each one of them focuses on whole foods, while avoiding processed foods and dairy. If you took those few steps and made no other changes to your current diet, you'd almost certainly experience major im-provements in your health, as long as your food sources are varied to ensure you get a good mix of vitamins and nutrients. Yes, making your diet healthy is really that simple. 

Visit any supermarket today and you'll see shelves lined with hundreds of items that just a few decades ago would have scarcely been recognized as food. 

• Yogurt in a tube

Food used to be simple. Tens of thousands of years ago, before the development of agriculture, our ancestors hunted and gathered. Nuts, legumes, roots, fruits, veg-etables, meat when it was available, and little else. There were no artificial preserva-tives, and most ways of preserving food had not been discovered. We ace what we acquired quickly, before it could rot or be stolen by another human being or animal, because the next meal was rarely a sure thing. We didn't know what protein, fats, or carbohydrates were, much less antioxidants and free radicals. But with all these seeming disadvantages compared to what we have at our disposal today, there was one huge factor our ancestors had going for them that we no longer have: Back then, ifa food tasted good, it was almost certainly good for you. In fact, that's precisely why it tasted good. If you've never thought much about evolution, it's worth taking a second to understand how beautifully elegant the process is.