A man sunbathes in Malmo, Sweden, in July. Humans (and other animals) make vitamin D when exposed to sunlight. Johan Nilsson/AFP via Getty Images hide caption

itoggle caption Johan Nilsson/AFP via Getty Images

Cycling has at least tripled over the past two decades in several big cities across the U.S., including Minneapolis, Chicago and San Francisco. Jonathan Steinberg hide caption

itoggle caption Jonathan Steinberg

Eat Up: Eating a large meal at the holidays won't have a big impact on your weight, says one physiologist. That's because your brain keeps a close watch on food intake and can tolerate the occasional big meal. It's slow, steady weight gain that's more problematic. iStockphoto.com hide caption

itoggle caption iStockphoto.com

An AquAdvantage genetically engineered salmon behind a non-transgenic Atlantic salmon sibling of the same age. Courtesy of AquaBounty Technologies hide caption

itoggle caption Courtesy of AquaBounty Technologies

Experts recommend that parents consult with a nutritionist to make sure vegetarian children get what they need. Kim Gunkel/iStockphoto.com hide caption

itoggle caption Kim Gunkel/iStockphoto.com