Understanding what happens with weight as your body ages will help you to control it. Beginning around age 25, total body fat starts to increase, while muscle mass and body water decrease. As a result, you may weigh more as you age or lose some of your youthful muscle tone.
Metabolism refers to the processes that the body needs to function. Basal metabolic rate is the amount of energy expressed in calories that a person needs to keep the body functioning at rest. Some of those processes are breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, and contraction of muscles. Basal metabolic rate (BMR) affects the rate that a person burns calories and ultimately whether that individual maintains, gains, or loses weight. The basal metabolic rate accounts for about 60 to 75% of the daily calorie expenditure by individuals. It is influenced by several factors. BMR typically declines by 1–2% per decade after age 20, mostly due to loss of fat-free mass, although the variability between individuals is high.
Metabolism is the complex biological process our bodies perform to turn the calories we eat and drink into energy.
Even when we’re just sitting around doing nothing, our bodies need energy for basic things like breathing, adjusting hormones, and repairing cells. The amount of calories we burn at rest is called our basal metabolic rate. You can use an (online calculator) [http://www.myfitnesspal.com/tools/bmr-calculator] to find yours, or get it measured in a doctor’s office. While the calories you burn each day can vary drastically depending on how active you are, your BMR stays pretty consistent. It’s regulated by hormones. Everyone’s is different, depending on things like genetics, age, gender, and body composition. As we age, “there are actual real hormonal changes that take place in our body that then affect the way we store fat and lose fat,” Gradney explains. “Our metabolic rate actually decreases because of these differences in hormones.”
Thirty gets tossed around as the magic number, but realistically, you can expect the biggest changes to happen closer to menopause.
The epic slowdown usually happens later than we think. Gradney says, “Menopause is more the indicator of when it happens, which is around 50 on average.” While multiple hormones are important for regulating metabolism, the decrease in estrogen around menopause makes a big impact.
We also lose muscle mass as we age, which alters how much energy our bodies burn.
The pituitary gland’s production of growth hormone also slows more noticeably as we age, according to Harvard Health. Growth hormone stimulates cell growth, and is especially important as we’re young and, yes, growing. But throughout our entire lives, the hormone is used to build muscle mass, boost protein production, and effectively utilize fat. As growth hormone decreases, your body can’t make or maintain muscle as well, and it can impact how efficiently your body breaks down calories, Jackie Baumrind, M.S., C.D.N., a dietician at Selvera Wellness.
Shifts in other hormones and other age-related changes like cell damage and inflammation, can further lead to sarcopenia, or age-related muscle loss. Muscle fibers may break down faster and be built back up more slowly. “Muscle mass is more metabolically active than fat mass,” Baumrind says, which means that it demands more energy from our bodies to maintain itself. Less muscle mass means our bodies will burn fewer calories at rest.
Weight gain in your 30s can be due partly to a changing metabolism, but it’s likely some other things are to blame.
Gradney says your metabolism may start to decline very slowly in your 30s and 40s, but lifestyle changes during this time (that you may not even know you’re making) are usually more responsible for weight gain. “Most people at 20 are a lot more active than when at 30,” says Gradney. “Evaluate what your level of physical activity has been over time and maintain that,” she suggests. If your lifestyle has changed—maybe you just had a baby (which comes with its own set of hormonal changes) or got a big promotion and are working more hours—that may mean getting creative and sneaky about fitting in activity. Same thing goes for healthy eating.
The good news: Being physically active and taking care of your body can keep your metabolism working faster for longer.
Your genetics matter, of course, but the way you take care of yourself also makes a difference. “If you have good genes but don’t exercise or eat right, there’s still the risk that you could see that decline earlier,” says Gradney. “The best thing to do is to remain physically active, maintain muscle mass, and have a good diet. If you do those things, that progressive decline will be slower.” If genetics are on your side and you make the effort to maintain healthy habits over the years, she says you can stave off big metabolism changes until you approach your 60s or for some, even early 70s.