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Q. To lose weight, should I focus more on my calorie count or the amount of time I spend exercising?

A. It is very, very hard to lose weight without both watching your calories and regularly exercising. But it's not that simple. For years, doctors (including me) told patients: "You've got to burn off more calories through exercise than the calories that you eat. If you do that, you'll maintain a healthy weight." We even implied that not being able to maintain a healthy weight, since it was so simple, reflected a weakness of character.

It's true that calories in, calories burned, and self-discipline are important. However, in my opinion, it's always been clear that some people really do have more trouble than others in achieving a healthy weight. You had only to observe your friends and family. The folks who always have seconds, yet remain thin. The folks who pick at their food, yet stay heavy. Now, we're learning why.

In the past 20 years, medical science has discovered a lot of the body chemistry that controls our appetite and metabolism. Some animals are born to be more hungry: the natural hormones that drive appetite overpower those that suppress appetite. The same probably is true of humans. Also, some of us are born to burn calories more readily. And in the past 10 years, we've discovered a new reality that may profoundly affect our ability to achieve a healthy weight: the microbiome.

We've known for over a century that many microorganisms (for example, bacteria and viruses) live on us and within us, like in our gut. All of their genes, collectively, are called our microbiome. And, boy, are there a lot of them: our microbiome has 250 to 800 times more genes than there are human genes. Why that matters is that these microbial genes make natural chemicals that can enter our bloodstream and affect our body chemistry.

As just one example of how the microbiome likely affects our weight, we have long assumed that the calories we swallow each day all get absorbed into our blood and distributed around our body, including into our fat cells. That isn't entirely true. Without getting too graphic about it, some of what we eat leaves our body as waste, taking some calories with it. There are people whose gut microbiome appears to cause them to absorb a lot more of the calories that they consume before the calories leave as waste.

Finally, don't forget that regular exercise produces health benefits even for those people who haven't achieved a healthy weight. Weight loss is only one reason for regular exercise.