Hidden causes of weight gain

The reason for gaining weight isn’t always a mystery. For example, you might know you’ve been eating more and exercising less, a potent combo that often results in extra pounds. But sometimes the cause isn’t quite so obvious. And you might not be aware of many of the other factors that can contribute to weight gain.

Age-related causes

Getting older brings physiological changes that can affect weight. Chief among them is muscle loss. Starting in middle age, we lose about 1% of muscle mass per year, which affects strength and metabolism (how fast we burn calories). “Smaller muscles use fewer calories. If your diet doesn’t change, you’ll consume more calories than you need. The excess is stored as fat,” says Dr. Caroline Apovian, an obesity medicine specialist and co-director of the Center for Weight Management and Wellness at Harvard-affiliated Brigham and Women’s Hospital.

Here are other age-related changes that can affect weight.

Chronic stress. It’s harder to manage stress as we get older. And if you’re constantly under stress, you might have consistently high levels of the stress hormone cortisol. One job of cortisol is helping the body replenish energy stores. In some people, that might indirectly promote weight gain by increasing appetite (since the body thinks it needs energy) and increasing the storage of unused energy as fat. “But mostly, stress leads to compulsive behaviors, such as eating ‘comfort’ foods, which are often full of sugar, unhealthy fat, extra calories, and salt,” Dr. Apovian says.

Poor sleep. Age-related changes affect our ability to sleep well. “If you’re a chronic ‘short sleeper,’ getting six hours or fewer each night, it might affect hormones that regulate appetite. Short sleep is associated with higher levels of hormones that make us hungry,

Underlying conditions

Weight gain, especially if it’s new, can signal a number of health conditions. For example, someone with heart failure might experience weight gain from fluid retention—which might appear as swelling in the feet, ankles, legs, or belly. “This would likely be accompanied by symptoms such as fatigue or shortness of breath,” Dr. Apovian says.

Other underlying conditions associated with weight gain include

  • diabetes
  • certain kidney diseases
  • sleep apnea (pauses in breathing during sleep)
  • thyroid problems.

Medication side effects

Taking certain medications regularly can lead to weight gain. Some drugs, such as prednisone, can make you retain fluid and increase your weight.

And many medications affect brain chemicals that regulate appetite, which might make you hungrier than usual, so you might eat more and gain weight. Examples include

  • antidepressants such as paroxetine (Paxil) or phenelzine (Nardil)
  • antihistamines that contain the ingredient diphenhydramine (the active ingredient in Benadryl)
  • antipsychotics such as clozapine (Clozaril) or olanzapine (Zyprexa)
  • beta blockers such as the drug atenolol (Tenormin) or metoprolol (Lopressor)
  • sleep aids that contain the drug diphenhydramine, such as Sominex, Unisom SleepGels, or ZzzQuil.

Other potential causes

Some potential causes of weight gain aren’t yet well understood and are currently being studied.

One possibility is late-night eating. Some evidence, including a 2022 Harvard study, suggests that eating late at night might make us hungrier in the daytime, slow metabolism, and increase body fat.

Another suspected factor in weight gain is the population of microbes that live in your gut (their genes are called your microbiome). Considerable evidence suggests gut microbes might influence appetite, metabolism, blood sugar, and fat storage. The strongest support comes from studies of animals. In humans, the evidence is less clear.

“Studies have found that the gut microbes of people with obesity are different from those of people who are lean,” Dr. Apovian explains.

“But we don’t know if this causes people to develop obesity. It could be that people who are genetically programmed to gain weight have a certain microbiome,” she notes. “Or it could be that people with obesity are eating a different way than lean people are eating, which may change the microbiome. We need more research for better answers.”

What you should do

Any recent or excessive weight gain warrants a visit to your doctor, who can hunt for new underlying conditions and check if your medications might be influencing your weight. Getting those two aspects of health under control are essential.

It might also help to see a dietitian to determine the right calorie intake for your current needs.

Beyond that, Dr. Apovian says the best way to control weight as we age is clean living: eating a healthy diet rich in fruits, vegetables, legumes, limited amounts of whole grains and starchy vegetables, and ample amounts of lean protein to help build muscle; avoiding late-night eating; getting seven to nine hours of sleep per night; exercising briskly every day for at least 20 minutes; and strength training at least twice a week. “You can rebuild muscle,” Dr. Apovian says, “but it takes a combination of healthy lifestyle habits to control weight.”

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lower levels of hormones that tell us we’re full, and higher levels of cortisol,” Dr. Apovian says.

Sex hormone changes. Older men and women experience reductions in certain sex hormones. In women, low estrogen levels are associated with sleep problems and increased body fat. In men, reduced testosterone levels are linked to less muscle mass.

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