Get full access to Outside Learn, our online education hub featuring in-depth nutrition, fitness and adventure courses, and more than 2,000 instructional videos when you sign up for Outside+..
A study of more than 5,500 girls between the ages of 9 and 14, led by Harvard Medical School, found that drinking sugar-sweetened sodas lowered the age when menstrual periods begin, regardless of other lifestyle factors. The study, published in Human Reproduction, found that drinking more than 1.5 servings of sugary soda daily lowered the age when periods start by an average of 2.7 months. When menstrual periods start one year earlier, risk for breast cancer increases by about 5 percent, so this soda effect would only slightly increase such risk, but some girls drink larger amounts of sugary soda. Diet soda and fruit juice did not have the same effect.