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How Getting and Staying Fit Changes in Your 20s, 30s and 40s

April 09, 2015 4 min read

Your body—and fitness—changes a lot sooner than you thought. Read on to learn when you burn the most calories, if your muscle is already waning, and how you can become fitter, stronger, and faster with every decade.

In Your 20s...

Your Body's Built for Exercise The 20s are prime time for fitness, says Janet Hamilton, a certified strength and conditioning specialist and exercise physiologist at Running Strong in Atlanta. (Okay, maybe you’ve gained a few pounds since you ran high school track, but some of that weight is probably from muscle.) That’s because, during your 20s, estrogen, progesterone, testosterone, human growth hormone, and thyroid hormone are all working together to keep you in tip-top baby-making shape. As an added bonus, they improve your body’s muscle-building power, she says. Meanwhile, during your 20s, you are at your peak cardiorespiratory capability, says board-certified internist Sue Decotiis, M.D., a medical weight-loss and bioidenticial hormone-replacement therapy expert in New York City. Bring on the endurance races. But Your Metabolism Is Already Slowing Womp, womp. After 20, the average basal metabolic rate (BMR), the number of calories you’d burn if you stayed in bed all day, drops by one to two percent per decade, according to the American Council on Exercise. Most of that dip may be due to the unfortunate fact that when most women enter the “real world,” they sit at a desk more and walk around less, says ACE-certified personal trainer Kathy Kaehler, a health and wellness expert for USANA Health Sciences. At any age, increasing your activity levels and muscle mass can help keep your BMR high. Muscle makes up a large part of your body’s “engine,” so the more muscle you have, the more fuel you’ll burn, whether you are hanging out with friends or powering through a workout, says Hamilton.

In Your 30s...

DHEA Declines Levels of this guy, which is a precursor to both estrogen and testosterone, peak in your 20s and start tapering off once you hit the big 3-0. While it’s not clear if DHEA supplementation can have any effect on anti-aging, according to the National Institute on Aging, the loss of DHEA as you age may slow your exercise recovery time and increase your body’s muscle-to-fat ratio, says Decottis. To help combat fat gain, focus on strength training. In a new study from Harvard University, men who lifted weights each day for 20 minutes put on less belly fat as they aged compared to guys who spent the same amount of time doing cardio. Babies Block Your Workout With your doctor’s permission, you can keep up your workout routine with a bun in the oven. But pregnancy is not the time to start a high-intensity training program. Your body is going through enough changes as it is. During pregnancy, your hormones shift, and your body focuses the bulk of its energy on the pregnancy, not your muscles, says Kaehler. After the baby comes, breastfeeding can burn crazy calories, which helps some women lose the pregnancy weight. But still, it’s not the right time to push your workout to the max, says Hamilton. “Trying to train at a high level immediately after giving birth is risky because hormones are not balanced back to ‘normal’ as long as you’re breastfeeding," she says. "I’ve seen an increased incidence of stress fractures in women who push to train at a high level during the postpartum period. I recommend training be moderated until after breastfeeding has ceased and normal menstruation has returned, which to me signals that the woman’s hormonal balance is back.” Once your baby has permanently detached from your nipples, though, you can crank up your workout’s intensity.

In Your 40s...

Sarcopenia Sets In A fancy word for muscle loss, sarcopenia is a natural part of the aging process—but it still sucks. While it doesn’t hit full force until around 75, The Journal of Nutrition, Health & Aging suggests that it can start as early as 40. Researchers believe most muscle loss comes from your fast-twitch muscle fibers, the ones that are responsible for powering high-intensity, largely anaerobic exercise. “In order to minimize the effects of sarcopenia, it is helpful to do strength training, which recruits different muscle fibers [your fast-twitch ones], compared to endurance training activities like biking, swimming, and running," as those train your slow-twitch, endurance-focused ones, says Hamilton. Increasing your protein intake may also help. Eating twice the current RDA of protein (1.5 grams instead of 0.8 grams of protein per kilogram of body weight) increases the rates of muscle growth and lessens muscle breakdown due to aging, according to research from the University of Arkansas for Medical Sciences. You Enter Perimenopause Menopause isn’t an on-off phenomenon. It happens gradually, typically in the 40s (or sometimes even earlier) with perimenopause. Common symptoms, like irregular periods, are due to fluctuations in estrogen and progesterone, which can also cause weight gain and a lower metabolism, says Decottis. What’s more, during perimenopause, insomnia and night sweats can make finding the energy to work out more challenging, says Hamilton. Still, continuing to exercise will not only help you feel younger—after all, exercise can ease your hot flashes and sleep troubles—it may help you look and perform even better than you did at 20. “Some women don’t even start training until later in life,” says Hamilton. “Whatever your age, you can improve your fitness.” Source: Women's Health

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