With Thanksgiving and Black Friday behind us, the holiday season is officially in full force. For many, thoughts now turn not only to shopping and decorating, but to expanding waistlines.

It's easy to stand in front of a platter of cookies and tell yourself you'll start a program on Jan. 2 after all the egg nog and yuletide cheer has run its course. The fitness industry is counting on you to eat your way to a New Year's resolution to lose weight.

Which is why this is the perfect time to remind readers that good health — or for for the purposes of this editorial, a healthy BMI (or body mass index) — isn't quite so simple as "eat less, move more."

Conventional wisdom says that counting calories and getting regular exercise will keep us slim and healthy. But a growing body of scientific research suggests a more complicated picture.

If you ever looked at a photo album of baby boomers and Gen Xers in their younger days and thought everyone seemed to be in better shape in the '60s, '70s and '80s, you're not crazy.

In 2004, the National Center for Health Statistics at the Centers for Disease Control and Prevention (CDC) concluded both men and women in the United States are roughly an inch taller and 25 pounds heavier than they were in 1960.

Since then, researchers have tried to account for the change. The extra height doesn't explain the extra pounds.

A study conducted by York University Faculty of Health published in the medical journal Obesity Research & Clinical Practice shows Americans are actually more physically active than they were in the 1980s. So why the growing BMI numbers? Data sets show that Americans today are 10% heavier than they were in the 1960s, even if they follow the same diet and exercise plans.

The Atlantic interviewed one of the researchers who proffered the factors that might be making it harder for adults to stay thin.

People are exposed to more chemicals that might be weight-gain inducing. Pesticides, flame retardants, and the substances in food packaging might all be altering our hormonal processes. Many antidepressants have been linked to weight gain and their use has exploded since the 1980s.

But the study authors think "microbiomes" or the bacteria in our guts may have changed somehow between the 1980s and now, perhaps due to animal products treated with hormones and antibiotics or the proliferation of artificial sweetners. More research may yield better hypotheses.

All this to say there's an X factor that simply makes it harder to maintain a healthy BMI. Harder — not impossible.

Sentinel columnist Dr. Michael J. Pramenko often reminds readers that behavior is the leading factor in health-care costs. We often turn to medicine after our behavior has compromised our health.

Healthy living is a year-round commitment to moderation. It just becomes top of mind this time of year when our resolve is constantly tested by holiday gatherings featuring good food and drink.

The best advice is simple and rarely changes. Limit intake of processed foods, eat mostly fruit and vegetables and exercise regularly. Get more sleep. Manage stress levels (exercise helps, but try meditating, too) to reduce the release of the cortisol, which promotes fat storage.

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