Food Reward Hypothesis
I think the food reward hypothesis is the best-supported current idea about why so many of us are overweight and obese these days. The basic idea of this is that we now have the ability to make (in many cases manufacture) foods that are both hyper-palatable (really delicious) and high reward (very tempting/craveable/"addictive" in a loose sense), and that eating such foods causes our bodies to try to keep us at a higher weight than is healthy.
This shouldn't be confused with the "thrifty gene hypothesis," which is the common-sense idea that we evolved to live under the threat of starvation and are going nuts now that food is so plentiful. According to the food reward hypothesis, if our food were still the sort of plain, comparatively unvaried food eaten by our ancestors, abundance itself wouldn't cause overeating.
Stephan Guyenet has a great series on the food reward hypothesis here: Part I, Part II. He talks about the thrifty gene hypothesis as well.
(Note: I will be linking Guyenet a lot in this post. He is not my only source of information here, but he has informed a lot of my thinking, and seems to have done his homework. In many cases he's the best concise link I can give.)
Conclusion: I should eat simple foods prepared at home.
The Role of Carbs
Books like Good Calories, Bad Calories by Gary Taubes make convincing arguments that overconsumption of carbohydrates, particularly sugar and refined flour, are responsible for the obesity epidemic. Certainly, obesity travels together with insulin resistance (a problem that is all about carbs, and is the fundamental cause of Type II diabetes), and insulin resistance is a big part of what causes heart disease in general.
Guyenet, the same guy as above, has a long critical post about the idea that carbs (or insulin resistance) cause obesity. He doesn't buy it at all.
I honestly don't know what to think about this topic. But what everyone (at least, everyone I'm currently reading) seems to agree on is that if you are insulin resistant, eating a ton of carbs is a bad idea. This is contrary to the official recommendations for diabetes patients (which stress a high-carb, low-fat diet), but I've read enough to convince me that that is bunk; I'll write more below about why.
I've been testing my blood sugar lately, and my fasting morning glucose levels are slightly elevated - not in the diabetic range, not quite in the "pre-diabetic" range, but not where they should be or where they have been in the past either. I have a strong family history of diabetes and I have other signs (like PCOS) of having trouble with insulin resistance.
Insulin resistance definitely leads to heart problems and a lot of other issues. Exercise and reduced carb intake definitely combat insulin resistance.
Conclusion: I should take care not to eat too many carbs.
The dominant hypothesis of the last half-century or so of medical advice has been that overconsumption of fat is what causes obesity and (separately) heart disease. This is why diabetics and overweight people (and, really, everyone) is encouraged to eat a low-fat diet.
It is a fact that you cannot eat a low-fat diet without eating a high-carb diet. Humans can't eat the bulk of their calories as protein (you will get sick if you try, but you'll also find it very hard to try, because you will start very very badly not wanting to eat any more lean meat).
The evidence that high fat intake causes obesity is based partly on common sense (fat has more calories per gram than carbs or protein) and partly on the observation that Americans got fatter and started (perhaps) having more heart trouble around the early part of the 20th century, when (it is believed) we may have started eating more fat.
We also started eating way more sugar and refined flour and packaged products and that sort of thing at the same time, so it's very hard to tease out the effects of fat vs. carbs vs. high-reward foods (vs. unknown other factors, of course). There seems to be very little evidence that eating a high fat diet causes weight gain. Many traditional cultures (Inuit, Masai) eat very high fat diets and are very lean (despite not facing any regular food shortages).
Conclusion: It's OK to eat a high-fat diet.
What about Saturated Fat?
As someone who has lived in our culture, it is very difficult not to believe that saturated fats, especially from meat and animal products, are dangerous. I feel as though I know in my very bones that these types of fats are unhealthy and cause heart disease. This is one of the very strongest messages about food from the medical establishment.
From what I can tell from my reading, it's just not true, or at least, many very large, long-lasting studies designed specifically to support this hypothesis have completely failed to. Now, my reading isn't (can't be) exhaustive and it's possible my sources are looking at the data in a really biased way, but it doesn't seem so to me.
Here's Guyenet on saturated fat and cholesterol, the diet heart hypothesis, coronary heart disease, and on findings about full-fat dairy. I also found Good Calories, Bad Calories pretty convincing on these topics. And, of course, when you look at recent studies comparing low-carb to low-fat plans for weight loss, everyone gets healthier on both sides.
An additional bit of common-sense thinking: for a very long time before agriculture, our human ancestors got most of their fat from animals. Either they ate a very high-carb diet (and probably some did), or they ate a lot of animal fats (definitely some did). It's likely we evolved to be able to handle eating these fats.
Conclusion: It is truly safe and healthy to eat saturated fats, including from animals.
The fundamental idea of the paleo and primal advocates is that we should eat the way that our hunter-gatherer ancestors did, rather than the way that our immediate agricultural ancestors did. In other words, rather than relying heavily on grains, legumes, and dairy for our food, we should eat more meat, vegetables, perhaps occasional tubers, berries, nuts, and things like that. We certainly spent a lot more time evolving to eat a hunter-gatherer diet than we have spent evolving to eat grains.
The only populations on the planet today who don't suffer from a lot of heart disease, obesity, diabetes, cancer, and other "diseases of civilization" (periodontal disease, appendicitis, gall bladder disease, etc.) are the ones people used to call uncivilized - small pockets of people living a traditional culture. It is true that they largely lack these health problems even when you adjust for age.
These groups are very different from each other. The Masai have a diet made up almost exclusively of milk, cow blood, and meat. Traditional-living Inuit have a diet that is something like 90% animal fat. On the other side, there are traditional cultures who live very healthy long lives (when they don't die in infancy) on diets composed almost exclusively of yams or sweet potatoes, and others who eat more balanced diets. But in general, my point is that there are very healthy cultures living on high-fat and on high-carb diets. It appears that healthy diets can have a wide range of macronutrient ratios.
What these cultures have in common is that they eat very little refined sugar, very little refined grains, and they don't eat "food that comes in boxes" - highly processed food manufactured to be as delicious and tempting as possible. They eat diets of mostly plain foods, with not a lot of variety by our standards.
It's hard to tease out what aspects of the western diet and/or lifestyle cause all of our problems. Is it the sugar? (There is some evidence that high-fructose foods like table sugar and corn syrup may have especially bad effects.) The comparatively recent introduction of grains to our diets? The refining of those grains and sugars? The even more recently introduced cottonseed, canola, corn, etc. oils (polyunsaturated fats)? Or the excessive variety and deliciousness of the manufactured food products that surround us?
At any rate, if the problem is dietary (as seems likely), then, while we can't say exactly what the problem is, we do know that simple diets of plain, pre-agricultural foods are healthy. (Of course, no food that I eat is actually pre-agricultural. I don't hunt or gather anything myself. But I think we can all agree that grass-fed beef or an apple is more "paleo" than a Big Mac.)
Conclusion: It may help to avoid modern agricultural products like sugar, refined grains (or any grains in large quantities), and seed oils.
Exercise decreases insulin resistance and has many other beneficial effects, but I seem currently incapable of incorporating much into my life. If I explained why, I would just be making excuses, so I'll just say it's not a big focus for me right now.
Based on what I've read lately, I do think that high-intensity interval training is way more effective than regular steady moderate-intensity cardio. If I were designing an ideal exercise program, I think I'd go with something like Mark Sisson's recommendations in The Primal Blueprint, which means a good base of low-intensity exercise (walking, slow swimming, jogging if you're much more fit than I am), small amounts of interval training (two or three times a week for 5 or 10 minutes), and regular natural, whole-body strength training (two or three times a week for 30 minutes or so).
Conclusion: I should exercise, but I don't.
I hope this helps to make sense of why I'm doing what I'm doing, or was at least interesting on its own.