What do you REALLY know to be true?

There are certain lessons that I have had to learn multiple times through personal experience, about which I no longer have any doubt. Here are a few examples.

1) Low-carb diets lead to fatigue, bad breath, poor mood, poor athletic performance, and cravings.
2) Although there are many factors that influence metabolism, calories-in/calories-out is basically true.
3) Lactose intolerance is very real (but may not be permanent).
4) Exercise leads to gradual physical adaptations (in speed, stamina, or strength), and it has a powerful effect on mood.

There are a few more things that I’m certain about, but overall, there are far more things in the field of health and nutrition that I still puzzle over.

I’m sorry, maybe I misunderstand you. What would differing metabolisms have to do with whether our bodies adapt to exercise? Did you mean number 2?

I probably shouldn’t allow myself too many caveats, since this thread is supposed to be about what we know with 100% certainty, but I should back up #2 with a couple.

Caveat 1) Two individuals of the same weight and height may not have the same basal metabolic rate (BMR), mainly because of differences in body fat percentage, bone and muscle mass, and age. The research I have looked at suggests that more than 70% of variation between individuals can be explained by those factors. However, the rest of the variation might be explained by thyroid status, exercise, diet, genetics, or other unexplained factors.

Caveat 2) Not everyone digests everything they eat, and certain foods are harder to extract calories from.

Nevertheless, allowing for these caveats, I don’t believe there’s much an individual is able to do, in the short-term, to change his or her BMR. As far as I can tell, building muscle mass would probably be the most obvious way to increase it, and losing muscle mass would be the easiest way to decrease it. But it takes a long time to build muscle (a pound a month is doing well), and you’re not going to lose much muscle unless you’re starving yourself or dieting too much.

So I believe that under normal conditions, weight gain and weight loss can be controlled by monitoring food intake and activity levels. Psychological factors can make this process enormously difficult, but I don’t think the chemistry is all that complicated.

I know you’ve complained about not being able to gain weight despite eating a lot. But you also exercise a lot, and (if I remember correctly), you eat a lot of high fiber foods, like oatmeal, that can hinder calorie absorption. Or maybe your family has a naturally high BMR because of genetics, in which case you’d just need to eat more calories to achieve balance.

I’ve also read that, while obese individuals tend to underestimate the amount they eat, and overestimate their exercise, people with eating disorders do the opposite. They think they are eating more than they actually are, and they think they are exercising less than they are. This could also be a factor in your situation.

In some cases you might be able to gain faster, like if you’re a complete beginner or if you’re regaining muscle that you had before, but a pound a month isn’t so bad. If you could keep up that level of gains for two years, that would be 24 pounds of muscle, and that is the difference between a skinny guy and a stud.