Our muscles, hormones, metabolism, and other body systems are in flux as we get older. (In other words, it’s complicated.) But there are five main factors that may be to blame if your jeans feel tighter these days.
1. You’re Experiencing Age-Related Muscle Loss
The amount of lean muscle we have begins to decline by 3 to 8 percent per decade after age 30, a process called sarcopenia. You may also lose muscle if you’re less active because of age-related health conditions, such as arthritis, or if you’ve been sidelined with an injury or surgery for several days, Primack says. “All of these [factors] individually do not cause a significant decline, but cumulatively they surely do,” he says.
Why does that loss of muscle matter? Because lean muscle burns more calories than fat, even at rest, according to the Mayo Clinic. Unless you’re regularly strength training with weights to maintain and build muscle, your body will need fewer calories each day. That makes weight gain likely if you continue to consume the same number of calories you did when you were younger.
“Most people will not adjust calories,” explains Marcio Griebeler, MD, an endocrinologist at Cleveland Clinic in Ohio. “They keep eating the same amount, but because they have less muscle mass to burn those calories and less activity, they end up gaining weight over time.”
2. You’re Undergoing Normal Hormonal Changes
According to data from the National Center for Health Statistics, both men and women undergo changes in hormone levels that help explain why middle age is prime time for putting on pounds.
For women, menopause — which tends to happen between ages 45 and 55 — causes a significant drop in estrogen that encourages extra pounds to settle around the belly, explains Dr. Griebeler. This shift in fat storage may make the weight gain more noticeable and increase the risk of high blood pressure, heart disease, high cholesterol, and type 2 diabetes.
In addition, Griebeler notes, fluctuations in estrogen levels during perimenopause, the years leading up to menopause, may cause fluctuations in mood that make it more difficult to stick to a healthy diet and exercise plan.
Women may gain an average of 1 to 1.5 pounds annually during the years around perimenopause and menopause, which can add up.
Men, meanwhile, experience a significant drop in testosterone as they age. It begins to decline gradually around age 40 at a rate of about 1 to 2 percent per year. Testosterone is responsible for, among other things, regulating fat distribution and muscle strength and mass. In other words, lower testosterone can make the body less effective at burning calories.
The pituitary gland’s production of growth hormone (GH) also slows from middle age onward, per Harvard Health. One of GH’s many functions is to build and maintain muscle mass. So with less GH, it’s harder for your body to make and maintain muscle, which, in turn, also impacts how many calories you burn.
“It’s a snowball effect,” Griebeler says. “You start accumulating more fat, have less lean body mass; you burn fewer calories, and that just keeps adding up over time.”
3. Your Metabolism Is Slower Than Before
That decrease in muscle mass is likely to slow your metabolism, a complex process that converts food calories into energy. Having more fat and less muscle reduces calorie burning. Many people also become less active with age for various reasons, which further slows the number of calories you burn.
The number of calories a body at rest uses is known as basal metabolic rate, and muscle mass is the primary factor in determining it. Age, body size, and sex also play a role. Men tend to have less body fat and more muscle than women of the same age and weight; and people who are larger or have more muscle burn more calories, even at rest, according to the Mayo Clinic.
Certain health conditions, such as hypothyroidism or Cushing’s syndrome, which become more prevalent with age, can also affect your metabolism.
4. You’re Busier With Work
By the time you’ve reached your forties and fifties, your career is likely in full swing — which is great, but brings its own weight loss challenges. For one thing, you may be moving less. You may commute an hour or so to and from work, sit at a desk for eight or more hours a day, and have so much on your plate that there’s no time to go for a walk or exercise during the workday.
You may also find yourself too busy to break for lunch, increasing the odds that you’ll scarf down something from the vending machine or order in calorie-dense takeout food, notes Rachel Lustgarten, RD, a nutritionist in New York City. As for work-related stress, that can militate against a healthy weight, too. The stress hormone cortisol increases the level of the hormone ghrelin, which causes you to eat more, according to research.
5. You’re Experiencing Major Lifestyle Changes
Some of the reasons for weight gain in middle age have nothing to do with what’s happening inside your body and everything to do with the way life changes as people enter their thirties and forties. One of the biggest changes comes when you start a family. Suddenly, the hour you spent at the gym after work is spent with your toddler at home. Later, your child’s after-school time is filled with playdates, homework, and other activities that require your attention. Similarly, around middle-age, you may also be caring for your parents or aging relatives. “You do not seem to have time anymore for yourself,” Primack says. As a result, your diet and exercise intentions might slip, causing a few pounds to creep on.
