In theory, resistance training is supposed to strengthen your bones. However, if you’re coupling your strength sessions with strenuous endurance exercise—or if you walk out of the weight room just as sweaty as if you’d spent the past hour sprinting hills—your intensity could be making your bones weaker due to the large amounts of calcium that escape via your sweat.
This phenomenon—in which calcium is freed up from the bones in order to replenish amounts of the mineral that get pumped out as you perspire—has been observed in competitive road cyclists, runners, and basketball players, but it also affects exercisers who sweat profusely during extended training sessions.
The good news: Preliminary research presented at the 2013 meeting of The Endocrine Society found that athletes may be able to offset some of this bone loss by taking calcium supplements—as long as they do it at the right time.
In the study, men between the ages of 18 and 45 took calcium and vitamin D supplements either 30 minutes before or one hour after a simulated 35-kilometer cycling time trial. All of them experienced a drop in their blood calcium levels, but the men who took the calcium supplement before exercising experienced less of a decrease.
So how much of this mineral do you need to offset what’s lost in your sweat? Adult men can take 500–1,000mg of calcium per day, before exercise if possible. And in order to help your body absorb the calcium, you’ll also need a steady supply of vitamin D. Pair your calcium with 400–2,000 IU of vitamin D3 (cholecalciferol) daily—look for supplements that pack both in one pill.
But just be careful not to overdo it. A 2013 study in JAMA Internal Medicine found that men taking more than 1,000mg of supplemental calcium each day may have an increased risk of heart disease, although the study didn’t focus on competitive (and sweaty) athletes.
You can also boost levels of the mineral naturally by bulking up on a variety of calcium-rich foods such as dark greens, like collard greens and kale; canned salmon (with bones); and calcium-fortified soy products.
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