I am a male standing 5 feet 10 inches, now 60 years of age. Almost three years ago, in January 2010, I stepped on the scale and saw 215 lbs; I decided to take action to begin losing weight. I have lost a bit over 35 lbs so far. I call what I have done a "Lazy Man's Diet" because it has been effortless; I have never gone hungry, never done exercise beyond what I was doing before, never suffered cravings or felt deprived. I have never counted calories, never weighed or measured my food beyond the simplest rules of thumb, never kept a journal or confessed before a crowd. I have read a lot of books about diet, but never followed any single book strictly. It could be said that I have slowly focused in on one coherent approach, and gotten gradually stricter.
I had gained my weight slowly, over decades; I was content to lose it slowly. Good thing, because I hit several "plateaus" lasting months. I resumed losing weight only after making some changes, and it seems this pattern will stay with me.
The approach I am using, the biochemical logic of it, the research backing it, is best presented in the book WHY WE GET FAT and what to do about it, by Gary Taubes.
It is often said that weight gain and loss is a matter of simple physics; calories in from food must balance calories expended, in metabolism and activity. Taubes points out that a 165 lb man who maintains a steady weight for 20 years (and there are some who do) has matched calories in to calories out to a precision of better than one twentieth of one percent. If you overeat by 20 calories a day (one bite of some foods) you would gain two pounds a year, 50 lbs in 25 years. Those who maintain a steady weight long-term clearly do not do this consciously. They do not count calories in their food to see if they should eat two more grapes, or three fewer potato chips. They do not calculate that to balance their lunch they should walk exactly 1,637 yards. Their body does the balancing for them, automatically, by the balance of hormones in their bloodstream. They feel hungry or satisfied, restless and energetic or lethargic and tired, as needed to achieve this balance.
Using the "calories in, calories out" model, people have tried to lose weight by "eating less and exercising more". But eating less leaves you hungry, and exercising more makes you hungrier. Few people are willing to suffer hunger for a long time. By this approach, many people can lose some weight, but sooner or later they go off the diet and gain the weight back.
Taubes argues that people get fat, not because of gluttony or sloth or lack of willpower; they get fat because the hormones in their bloodstream have been thrown off balance. What hormones cause the body to store fat, to create more fat cells and fill them? Primarily insulin, with cortisol a distant second, and many distant also-rans. They get fat because their glands secrete too much insulin, over a long period of time.
The cells of the body need fuel, for metabolism and activity. They can burn three fuels: glucose (from digestion of carbohydrates, i.e. sugars and starches), fatty acids (from digestion of fats) and ketones (from digestion of protein, and conversion from fat.) The liver and the fat cells can convert glucose to fatty acids, and do when insulin levels are high. The cells burn glucose first, in preference to the others, because if the level of glucose in the blood gets too high it becomes toxic. The Pancreas secretes insulin in direct response to glucose levels; it secretes whenever the tongue or stomach tastes anything sweet, in anticipation of glucose soon to come; it even secretes insulin when the brain THINKS about eating sweets. Insulin does many things, has secondary effects on other hormones, but the net result is that when insulin levels in the blood are high, your body stores calories as fat; when they are low, your body releases fat to be used as fuel.
Eating fats or protein does not generate this kind of insulin response; your fat cells store the fat you eat, and release it for fuel as needed. Eating fat does immediately make you fatter, BUT it also satisfies your hunger quickly, so it does not prompt you to overeat. A diet low in carbohydrates, moderate in protein, and high in fat will leave your body burning fat for fuel most of the time, both the fat you eat and the fat you have stored. You can eat until you are satisfied, and your stored fat will stay stable or decline, depending on how low you keep your carbohydrates.
What relation does this have to the Paleo diet? For at least two million years our ancestors lived by hunting and gathering. The food they ate was the kind of food you can get BY hunting and gathering. This is the sort of diet our bodies are adapted to live on. What they ate, mostly, was meat of all sorts (fish, shellfish, birds, eggs, and whatever animals they could catch or trap), leafy greens and vegetables, fresh fruit in season, and tree nuts. What they ate very little of, on rare occasions, were tubers and honey. What they didn't eat were grains, beans, dairy, refined sugar, and white potatoes (white potatoes are native to North America, which was colonized by humans much more recently).
In 2000, researchers published a study of the diets of 229 hunter-gatherer populations that had been studied by anthropologists in the 20th century. Whenever possible, they ate lots of animal food; one in five of the 229 ate almost entirely by hunting and fishing. Only one in seven got more than half of their calories from plant foods. None were exclusively vegetarian. Averaged all together, these 229 populations of hunter-gatherers got about two-thirds of their calories from animal foods and one-third from plants. They typically averaged 19 to 35 percent of their calories from protein, 28 to 58 percent from fats, and 22 to 40 percent from carbohydrates. They ate the fattest animals they could hunt, ate all the fattest parts of the animal (organs, tongue, bone marrow), and ate essentially all of the fat on whatever animals they caught. The carbohydrates they ate were the sort with a low "glycemic index", i.e. they digested slowly, with lots of fiber, so they provoked only a small and gradual insulin response.
The paleo diet is a naturally low-carb diet. One Paleo writer, Mark Sisson (THE PRIMAL BLUEPRINT) makes the connection explicit. He recommends, for maintaining a stable weight, roughly 150 grams of "net carbs" per day (total grams of carbohydrates minus grams of fiber). For slow, effortless weight loss, keep your net carbs below 100 grams per day; for faster weight loss, below 50. He recommends a little booklet, DR. ATKINS' CARBOHYDRATE GRAM COUNTER, which I found available at Barnes and Noble. Other paleo writers don't bother with counting grams; if you don't eat grains, beans, potatoes, or sugar, only leafy greens, non-starchy vegetables, and fresh fruit, it is very easy to stay in the neighborhood of 150 grams net carbs per day. An example would be Robb Wolf, THE PALEO SOLUTION.
Gary Taubes in WHY WE GET FAT concludes with an appendix giving an example diet for treating obesity, derived from the Atkins diet, with a detailed list of foods that can be eaten "as much as you want", a list that can be eaten "in limited amounts", and a list of foods to be avoided. Basically, a diet of meat, salad, and non-starchy vegetables. But he says, there have been many variations on this theme published over the years, and every doctor who has discovered this approach has slightly different lists. We do not have the research to know in fine detail which foods should go on which lists. He says, summing up: eat as much as you like of meat, fish, fowl, and leafy green vegetables. Avoid starches, grains, sugars, and anything made from them, including bread, pasta, sweets, juices, sodas. Learn for yourself how much fruit and non-starchy vegetables your body can tolerate.