Your Metabolism: An Owner’s Manual

If you feel like your metabolism is slowing, you're right. If you believe you're helpless to fix it, we've got good news for you.

If you’ve ever struggled to lose weight or keep hard-lost pounds from creeping back, you’ve probably wondered if part of your problem is a “slow metabolism.”

A more scientifically astute question, it turns out, is how well your metabolisms—plural—are working.

"The word 'metabolism' describes much more than just energy balance," explains obesity researcher Nik Dhurandhar, Ph.D., professor and chair of nutritional sciences at Texas Tech University. "It includes all the complex biochemical handling of nutrients, hormones, and enzymes that goes on within the body."

To be sure, weight researchers and dieters alike often use “metabolism” as shorthand for one narrow slice of these processes: calories in versus calories out and the impact this has on weight and body composition.

But even here, we're still talking multiple metabolisms. "Think of glucose metabolism, lipid metabolism, protein metabolism," says Dhurandhar. For each of these separate pathways, researchers have discovered hundreds of genes that influence how different individuals respond to what and how much we eat (too little or too much) and its impact on energy expenditure (including, but not limited to, conscious and unconscious physical activity).

Take, for instance, our R.Q., or respiratory quotient, an arcane measurement few people have ever heard of. It refers to the ever-shifting ratio of carbon dioxide production and oxygen intake each of us experiences over the course of 24 hours. In simplified terms, an R.Q. of 1.0 represents 100 percent pure carb burning. The closer R.Q. gets to 0.08, the greater the percentage of fats you're burning.

Ideally, your own R.Q. should be highly flexible-with carbohydrate use increasing nicely after meals and during exercise, and fat burning predominating the rest of the time. This is often the pattern observed in lean individuals. Unfortunately for those who struggle with their weight, the fat-burning mode is much harder to trigger. For reasons that are not fully understood, it's as if their bodies have decided to hold onto fat stores at all costs, surrendering these only during times of starvation.

The 3 Ways You Burn Calories

Researchers who study the metabolism of energy usage divide it into three broad categories: B.M.R., T.E.F., and P.A.

B.M.R. refers to your basal metabolic rate, or resting metabolism, which represents all the energy you expend when essentially doing nothing physical. Passivity here, of course, is an illusion. Inside our bodies, a host of metabolic processes—from body temperature regulation and blood circulation to immune system surveillance and tissue repair—carry on constantly, sight unseen. For most of us, B.M.R. accounts for 60 to 70 percent of all the calories we burn each day. But this can vary tremendously depending on different health conditions. An extreme example: burn victims, who can expend up to 8,000 calories a day while lying motionless on a hospital bed. Even though they look to be merely resting, their bodies are hard at work with repairs.

T.E.F., or the thermic effect of food, is the energy expenditure associated with freeing the energy content from what you eat. You may be happy to hear that if you eat a 700-calorie meal, your body only nets 595 to 630 of these. This is because it takes calories to digest, store, or make calories available for immediate use. This handling cost is not insignificant. T.E.F. accounts, on average, for 10 to 15 percent of all the calories most of us burn each day.

P.A., or physical activity, is the final element in your daily calorie expenditure. For many, it accounts for 15 to 30 percent of our energy metabolism, but such figures can vary wildly depending on how active or sedentary we are. P.A. comes in two main forms: E.A.T. and N.E.A.T.

E.A.T., or exercise activity thermogenesis, refers to the calories we burn through work, recreation, and intentional physical activity. Manual laborers, for example, have much higher E.A.T. than cubicle dwellers. Elite athletes, such as Michael Phelps or Tour de France cyclists, can burn through as many as 9,000 calories a day to fuel their training and performance.

N.E.A.T., or non-exercise activity thermogenesis, refers to all sorts of less conscious or unintentional physical activities: fidgeting, leg-bobbing, pacing, and a host of similar actions that come naturally to some and less so to others. In a 2005 study in Science, researchers at the Mayo Clinic showed that lean people, on average, expend 350 more calories per day via N.E.A.T. than overweight people.

The Impact of Age on Metabolism

"Most metabolisms become impaired or at least somewhat impaired with aging," says Dhurandhar. He adds that such changes lead to an oft-repeated refrain among his middle-aged and older patients: My diet has been exactly the same for decades, so why am I gaining weight?

"They may, in fact, be eating exactly the same as before," says Dhurandhar, "but they're not accounting for their energy requirement dropping." In other words, you may not need as many calories as you used to, so if you're still eating the same way, that can lead to weight gain.

There are a variety of reasons for metabolic slowdown, adds Leslie Bonci, M.P.H., R.D.N., a Pittsburgh-based nutrition consultant whose clients include the WNBA, Kansas City Chiefs, and Pittsburgh Ballet Theatre. "Part of it is due to hormonal shifts over time," she explains. "The pituitary gland's production of growth hormone, for instance, decreases with age. Growth hormone is used to build muscle and effectively utilize fat, so as levels decrease, body composition shifts to a higher fat-to-muscle ratio."

A related phenomenon is sarcopenia of aging: the progressive loss of muscle mass at about 1 to 5 percent a year and higher in some cases. The cause of sarcopenia remains unclear. Some experts believe behavioral changes are the main culprit: With adult responsibilities come fewer opportunities for, or inclination toward, the constant physical activity that comes naturally to us in youth. Others have pegged diminishing testosterone levels as a likelier cause.

Regardless, the switch to less muscle and more fat not only makes us weaker, it also slows our overall metabolic rate. Fat burns very few calories; muscle burns more calories even at rest. Researchers have found that the percentage of lean body mass may be responsible for two-thirds of B.M.R. variability between individuals. Just as the rich get richer, the strong get leaner.

As body composition shifts with age, this can further alter how different hormones function, leading to problems like insulin resistance. People at a healthy weight, of course, can also develop metabolic conditions. Regardless of which comes first-an unhealthy body composition or metabolic disorder-once established, the two tend to amplify each other.

Slow Metabolism? Start Here

If you suspect one or more of your metabolic processes have gone awry, see your doctor to rule out a few common but treatable problems. These include:

Hormone imbalances: Among women, the most likely one is hypothyroidism, which occurs when your thyroid gland doesn’t produce enough of its namesake hormone. In men, dropping levels of testosterone—a natural process of aging—can also slow down metabolism. Bonci also recommends talking to your doctor about your vitamin D levels and the best ways to get enough. “Vitamin D can play a role in muscle and fat metabolism,” she says.

Medication side effects: Some drugs like antidepressants and seizure meds are among the many drugs known to cause weight gain. If you’ve started packing on significant weight after beginning any new med, continue taking your medication as prescribed, but talk to your doctor about the possibility of switching to another pill.

Sleep disorders: From sleep apnea to insomnia, sleep disorders do more than just rob us of restorative slumber. Researchers have learned that even modest sleep deprivation can increase levels of the hormone ghrelin, which stimulates appetite, and decrease levels of leptin, which tempers it. The result, says Bonci, is akin to stepping on the accelerator while disabling the brakes.

Emotional states, particularly depression: Some people with depression want to do little but eat and sleep, while others lose their appetites entirely and become too agitated to even sit still. In either case, the condition is having a major and dangerous impact on a variety of metabolic processes. Treatment can help return you to your normal self.

Crash diets and skipping meals: “Do not undercut your calories too drastically,” advises Bonci. For one thing, extreme calorie restriction sends a message to your brain that you’re starving, which causes calorie burn to plummet in an effort to hang onto the fat stores you have left. Extreme diets boomerang in another way too. “If we don’t consume enough,” Bonci explains, “the brain and other organs still need to be fueled, and to generate this fuel, the body will break down protein.” This, in turn, further lowers your lean muscle mass.

Overreliance on climate control: Our bodies must work to maintain core body temperature. Sweating to discharge excess heat and shivering to warm up require calories. No, you shouldn’t subject yourself to dangerous heat and cold. But if the temps are milder or you can get by with air conditioning or heat at a moderate intensity, no need to blast it. You’ll burn more calories naturally and save money too. (And whether it’s summer or winter, don’t forget to stay hydrated.)

Gain Control with Healthy Eating and Movement

If none of the above applies to your situation, there are still proactive steps you can take to make a difference. For instance, combining exercise with healthy eating (no skipping meals!) may seem like a cliché, but it does help most, if not all, people gain some control.

On the dietary side, says Bonci, it's important to consume some protein at every meal. Aim for a daily total of 0.5 to 1.5 grams of protein for every pound of bodyweight. For someone who weighs 140 pounds, that's a minimum of 70 grams a day. "Protein intake," she explains, "is important to maintaining the muscle mass we have and to stimulating additional muscle formation."

As for the type of exercise, both cardio and strength training can have big impacts on metabolic health and weight control. As part of the National Runners' Health Study, Paul T. Williams, Ph.D., found that distance running can slow and sometimes even halt the creeping weight gain that afflicts almost everyone by middle age. Just don't expect it to be easy-the older we get, he says, the more work we need to do.

"We found that running distance needs to increase annually by 1.4 miles per week to compensate for the expected increase in waist circumference between ages 20 and 50," says Williams. "This means that runners who average 10 miles per week at age 30 should increase their weekly running distance to 24 miles by age 40 if they plan to still fit into the tuxedo they bought 10 years earlier."

Thankfully, ever-increasing endurance exercise isn’t the only option to maintain a more youthful metabolism. It may not, in fact, even be the best way. “Cardio is great for the heart, but strength training has a greater impact on muscle and metabolism,” says Bonci, who describes herself as a fit, fed, and fearless grandma who does regular resistance training.

And don't forget about N.E.A.T. To hardcore gym rats, N.E.A.T. may seem like an afterthought. But consider: Even those who exercise for an hour a day, five days a week, are spending less than 3 percent of their week working out. N.E.A.T., by contrast, consumes a much larger percentage.

One key to increasing N.E.A.T. is to look for every chance to choose an active life over a sedentary one. Take the stairs instead of an elevator or escalator. Don’t park in the nearest spot at the grocery store. Make sure you don’t sit longer than an hour at a time. Even better, take short walks throughout the day.

Finally, keep things in perspective. Weight loss is difficult for almost everyone; sustaining it over the long term even more so. There's no quick fix that will change your metabolisms in a meaningful, permanent way.

But the good news is that if you're overweight, shedding even 5 to 7 percent of your weight and keeping it off can make a big difference in your blood pressure, blood sugar, and cholesterol. For someone who is 200 pounds, that's 10 to 14 pounds. If that seems challenging, remember that avoiding future weight gain is a victory too.