Health Claims

People who do a moderate amount of exercise seem to have the most stable morning stress hormone levels, but the data isn't clear enough to say for sure that exercise level actually affects this stability.

Correlational

Exercise might help your body handle stress better, especially when studied in controlled trials, but the effect isn't huge and varies a lot between people.

Correlational

Working out more or less doesn’t seem to change how your body’s stress hormone, cortisol, spikes when you wake up — it’s about the same no matter how active you are.

Correlational

People who move more during the day tend to have healthier stress hormone patterns, with cortisol dropping more naturally as the day goes on — a sign their body handles stress better.

Correlational

In middle-aged people who aren't very active, the level of a brain protein called BDNF in their blood doesn't tell us whether their brain's ability to control muscles will change — no matter if they sit all day, take regular movement breaks, or do some exercise afterward.

Correlational

If you're a middle-aged person who doesn't move much, your body's stress hormone levels might help your brain become more responsive when you take regular movement breaks—but not if you just sit all day or go for one workout.

Correlational

If you're a middle-aged adult who doesn't move much, your stress hormone levels go up more after sitting all day and then doing a 25-minute workout, compared to just sitting or breaking up your sitting time with short walks every half hour.

Mechanistic

Exercise can change how your body handles stress by rewiring the system that controls cortisol, the stress hormone.

Mechanistic

Whether you have more 'slow-twitch' or 'fast-twitch' muscle fibers before training doesn’t seem to affect how much muscle you can build with lifting weights.

Correlational

People who build more muscle from lifting weights might be more sensitive to testosterone because their muscles have more 'testosterone receivers' after training — but scientists aren’t completely sure yet.

Mechanistic

Some people build more muscle from weight training than others, and it might be because of tiny molecules in their bodies that control muscle growth genes. One study found these molecules act differently in big vs. small muscle gainers, but we’re not totally sure yet.

Mechanistic

People whose muscles grow a lot with training might get that boost because of special cells that help muscle fibers expand, while those who don’t gain as much might not see the same cell activity — but scientists aren’t fully agreed on this yet.

Mechanistic

People who build more muscle from weight training tend to make more ribosomes—the tiny machines in cells that build proteins—compared to those who don’t gain as much muscle. One study found big gains in these protein-makers in strong responders, and lab studies back up that this process helps muscle grow.

Correlational

If you're new to working out, doing leg exercises for 10 weeks can make one of your big thigh muscles grow bigger, even if the individual muscle fibers don't get thicker.

Quantitative

Doing 10 weeks of leg exercises that strengthen your knee muscles boosts one specific hormone receptor in your muscles (ERα), but doesn’t change two others (AR and ERβ).

Causal

If you're new to strength training, having more tiny blood vessels around your fast-twitch muscle fibers might help those muscles grow more when you start working out.

Correlational

People new to weight training who have more of a certain protein in their muscles (called ERα) tend to gain more muscle size after 10 weeks of leg exercises — so this protein might help explain why some people build muscle faster than others.

Correlational

Doing full-body strength training three times a week for three months helps older adults, even those over 85, gain about 2% more muscle — and it works just as well for the very elderly as it does for the 'younger' elderly.

Quantitative

Older adults, even those over 85, can gain just as much leg strength as younger seniors from 12 weeks of regular weight training — and some even gain a bit more!

Quantitative

Doing full-body strength training three times a week for three months can boost thigh muscle size by about 10–11% in older adults, even for those over 85 — showing that very old age doesn’t stop muscles from growing with exercise.

Quantitative

Women after menopause who have higher levels of a protein called GDF-15 in their blood tend to be weaker in their legs and walk slower, especially if they're healthy and haven't had cancer.

Correlational

Doing strength training for 12 weeks may help postmenopausal women — both healthy ones and breast cancer survivors — move better, walk farther, and get up and go faster.

Correlational

Doing strength training for 12 weeks can boost muscle strength by about a quarter to nearly half in women after menopause — whether they're healthy or have survived breast cancer.

Quantitative

If postmenopausal women do strength training three times a week for three months, their thigh muscles get about 5 to 8% thicker — whether or not they’ve had breast cancer.

Quantitative