Browse evidence-based analysis of health-related claims and assertions
IGF-1 helps build muscle by turning on growth signals inside cells, and the IGF-1 made in the muscle itself might matter more than the kind that comes from the liver.
Mechanistic
Lifting weights makes your body release more growth hormone, especially when you do medium-heavy sets with short breaks. This effect is weaker in older people.
Descriptive
When women work out during the part of their cycle when estrogen is high, they might build more muscle because estrogen helps protect muscles and speed up recovery.
In young men, how sensitive their muscles are to testosterone—measured by androgen receptor levels—matters more for muscle growth than how much testosterone is in their blood.
Correlational
When men lift weights, their testosterone helps their muscles grow by turning on certain signals in the muscle cells. This hormone response can last up to two days after a workout, making muscles more responsive to testosterone during that time.
Young men need normal testosterone levels to get the full muscle-building benefits from weight training — low testosterone blunts the body’s muscle-growth signals.
Young men with low testosterone don't get the same muscle-building boost from weight training because their bodies don't increase the machinery needed to make new proteins — and that seems to be because they're missing testosterone.
When young men have low testosterone on purpose for the study, their muscles don’t respond as well to weight training at the cellular level — testosterone seems to help muscles fully 'feel' and react to the workout.
Young guys need their natural testosterone to get the full muscle-building benefits from weightlifting — if their testosterone is blocked with drugs, their muscles don’t grow as much, even if they train just as hard.
If young guys are given a drug that lowers their testosterone, they don’t build much muscle from weight training—while others with normal testosterone gain about 1.5 kg of muscle in 6 weeks. This shows testosterone is key for getting the most out of lifting weights.
Causal
Rats that swam regularly didn't get blood sugar problems from stress like the ones that didn't exercise — their bodies handled sugar just as well as unstressed rats.
Quantitative
Stressing out male lab rats for a long time makes their bodies worse at handling sugar, even though their resting blood sugar stays the same.
Swimming helps lower stress hormone levels in stressed-out male rats, which might mean exercise can calm an overactive stress system.
Stressing out male rats every day for 12 weeks made their stress hormone levels more than double, showing the stress method really works.
If male lab rats swim for an hour, five days a week for three months, they can swim much longer by the end — up to 2.7 times longer — which means their stamina and heart-lung fitness really improved.
Rats that slowly get used to running on wheels can run much longer than rats that don’t, showing that practice helps them handle exercise better.
Just being around a running wheel—even if they don’t run—helps male rats handle exercise stress a little better, but not as much as if they were used to using the wheel regularly.
Rats that got used to running on a wheel before a tough workout had lower sugar and stress markers in their blood compared to rats who weren’t used to it — meaning getting used to exercise might help their bodies handle new workouts better.
Running on a wheel for 8 days doesn’t seem to keep stress genes turned on in the brains of male lab rats, which means their stress system probably isn’t stuck in overdrive after this training.
Rats that slowly get used to running on a wheel over 8 days don’t show signs of stress in their blood — their stress markers either stay the same or go down, suggesting the routine isn’t stressing them out.
Exercise and your daily stress hormone pattern are only slightly linked, but that link stays pretty much the same no matter the person’s age, sex, weight, or how researchers measure things.
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.
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.
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.