But some people get on these diets and can't lose weight, or they even gain it! What's the deal with that?
Well, there may be more than one reason. But one very common reason is that people are eating a low-carb diet without changing their high-carb eating habits. When you're eating a lot of bread, chips, and pasta, you get hungry a lot. You might be in the habit of eating every couple of hours (like me) or of having seconds or thirds after dinner. You know how much food you need, and the amount you need is a lot.
However, once you cut out grains and starches, the amount you need may be much less, volume-wise. Fat has 9 calories per gram compared to carbs and proteins, which each have 4. A high-fat meal can be much smaller and still be satisfying. Even protein, though it has the same number of calories as carbohydrates do, is digested more slowly and will leave you feeling satisfied for longer. So you may have to re-learn how much food it takes to satisfy you.
For example, when I went grain-free for a week, I ate some full-fat yogurt for breakfast. I felt satisfied after a serving of that so I went to work as usual. Then when I got home, I found myself rummaging in the fridge, also as usual. I am usually starving after work (at 10 a.m.) and eat a midmorning snack. But as I regretfully passed up the bread, I remembered to ask myself whether I was really hungry. I wasn't -- not a bit!
I didn't eat till lunchtime, and listening to my body led to eating a pretty small lunch, too (radish greens with a small amount of bacon and parmesan cheese). And then I didn't need to eat till dinner, though out of habit I ate a small piece of cheese. Normally I have at least one afternoon snack, sometimes two.
I'm not eating grain-free anymore, and I never did go low-carb, but I still have to remember to keep in tune with what I actually need. My homemade ice cream is way more satisfying than the storebought stuff -- but I have to remember to scoop a smaller serving of it, or I'll just eat what I used to eat.
It's a little sad needing less food, and thus requiring fewer delicious meals. But meals with a healthy amount of fat in them are so delicious and enjoyable that it's not that much of a sacrifice. It is important, though, to actually enjoy the food instead of eating while doing something else. That's a great way to feel psychologically deprived, even if you're eating enough to sustain your body.
As for our family, we are half grain-free (as in, John still avoids grains except rice, and the baby eats no grains) but not at all low-carb. We all have very high calorie needs, and even primal guru Mark Sisson agrees that some people who have no weight problems can eat starches. Our frequent potatoes and white rice don't seem to be doing us any harm!