Too much sugar is a primary cause of obesity, diabetes, and tooth decay, and new guidelines from the World Health Organization call for a sharp reduction in the amount of added, or "free," sugars in the typical diet. Sugar accounts for 15 percent of the average American's daily calorie intake, and the WHO recommends that number be reduced to no more than 5 percent, or roughly 25 grams — six teaspoons — per day. That's less than what's found in a single can of soda. Free sugars are found in white and brown sugar, honey, maple syrup, and additives like high fructose corn syrup. "The key point is that we are consuming way too much added sugars for good health," says Rachel Johnson of the American Heart Association.

Artificial sweeteners may be just as bad. A new study indicates the additives can alter the body's metabolism to make weight gain more likely. Researchers at Israel's Weizmann Institute of Science found that three sweeteners widely used in low-calorie snacks and drinks — saccharin, sucralose, and aspartame — raised blood sugar levels in mice, increasing their risk of glucose intolerance, a condition linked to obesity and diabetes. Sweeteners, the report found, "may have directly contributed to enhancing the exact epidemic that they themselves were intended to fight."

Pot smoking can damage developing brains. Today's more potent strains, in fact, can cause structural changes in a key region of the brains of people younger than 26, especially if they smoke daily. Levels of THC, marijuana's psychoactive compound, have increased in weed from an average of 3.75 percent in 1995 to 13 percent in 2013. Scientists believe that the increased potency leads to abnormalities in the shape, density, and volume of the nucleus accumbens, the walnut-shaped area of the brain that's associated with pleasure and pain. The nucleus accumbens "is the core of motivation," says study co-author Hans Breiter. "This is a part of the brain you do not want to mess around with."

Pop stardom leads to an early grave, an Australian study has revealed. Sydney University professor Dianna Kenny looked at more than 12,000 popular musicians who died from 1950 to 2014 and found that their life spans were roughly 25 years shorter than the average American's. Although substance abuse contributed to the early deaths, musicians were also much more likely to die in accidents, commit suicide, or be murdered. Those who avoided such pitfalls still tended to die young, in their 50s and 60s rather than their 70s and 80s like most nonperformers. The culprit, says Kenny, is a culture that glorifies outrageous behavior by emotionally immature artists. "The pop scene is toxic and needs rehabilitation," says Kenny.

Playing football damages brains, even in players who don't suffer concussions. Studies have shown that at least one-third of NFL players will develop some form of cognitive disorder, and new research indicates the negative effects can already be seen in the brains of high school players. Scientists at Wake Forest University tracked a high school team for one season and found measurable abnormalities similar to the effects of mild traumatic brain injuries in all 24 players they studied. "For every one NFL player, there [are] 2,000 high school players," says study author Christopher Whitlow. "It's a really understudied population."

Low vitamin D levels can increase the risk for Alzheimer's disease and other forms of dementia. U.K. researchers studied more than 1,600 adults and found that those with vitamin D levels below 50 nanomoles per liter — the minimum healthy level — were up to two times more likely to suffer from dementia than those with higher levels. "We can't say anything about whether people should be supplementing, because that's beyond the scope of what we looked at," says study co-author Iain Lang, "but these are exciting and suggestive results."

An unhappy marriage can break your heart — literally. A new study from sociologists at Michigan State University indicates that people in contentious marriages are much more likely to develop heart disease or suffer a heart attack or stroke than their more happily wed contemporaries. Previous research has shown that married people tend to be healthier overall, but this study found that the stress of an unhappy union outweighs the usual benefits of wedlock. "It's not that every marriage is better than none," says study author Hui Liu. "The quality of marriage is really important."