Cutting calories has been shown to increase the life span of some animals and protect them from signs of aging and disease. Although some humans have been eager to adopt a low-calorie diet to see similar results, so far, there is relatively little evidence that calorie restriction has the same benefit in people. A new study from researchers at the University of Münster, in Germany, adds new evidence in favor of cutting calories: older adults who reduced calories for three months fared better in memory tests. The results, published in Proceedings of the National Academy of Sciences, offer the first evidence that calorie restriction could prevent age-related mental decline in humans.
The study’s subjects ranged from normal weight to overweight, so cutting back calories did not necessarily translate into severe weight loss; instead, it allowed many subjects to reach a healthier weight. Mark Mattson, a neuroscientist at the National Institutes of Aging, who was not involved in the study, says that it “adds to considerable evidence from animal and human studies that high calorie intake is not only bad for your heart, but it’s bad for your brain.”
Agnes Flöel, lead author of the study, says that most evidence for the benefits of a low-calorie diet in humans comes from long-term epidemiological studies, such as one on an aging population in Okinawa, Japan. But ongoing trials testing the effects of calorie restriction in humans have not yet produced definitive findings. These include the U.S.-government-funded CALERIE study, which follows adults ages 25 to 50 on a calorie-restricted diet. “Animal experiments suggest that both calorie restriction and modified fat intake could be beneficial for the brain,” Flöel says.
The new trial tested reducing total calorie intake, as well as boosting the ratio of unsaturated fat over saturated fat, which is thought to help brain function. A group of 50 healthy older adults with an average age of 60 were divided into three groups: one group was counseled to follow a calorie-restricted diet; another increased the proportion of unsaturated fat over saturated fat in their diets; and the third group had no dietary changes. Flöel says that the subjects in both interventions received dietary counseling and an individualized plan for modifying their diets. Those in the calorie-restricted group were advised to reduce their food intake by about 30 percent without changing the proportions of nutrients in their diet. Subjects reported lowering their typical intake by anywhere from 200 to 1,000 calories per day. Not every person in the calorie-restriction group was able to cut calories by 30 percent, but overall, the subjects in the group lost weight, supporting their own reports that they were eating less.
The participants also underwent memory tests before and after going on the diet. At the end of three months, the calorie-restricted group increased its scores by about 20 percent, while the performance in the other groups did not change. Flöel says that it’s not clear why eating healthier fats did not seem to benefit the brain in this trial, but her team is now investigating whether boosting just omega-3 fatty acids, rather than all unsaturated fats, could have a stronger effect.
Mattson says that, in addition to showing a boost in memory, the study also suggests that the same underlying mechanisms uncovered in animals could be at work in humans too. The researchers found that people who cut calories had improvements in several indicators of metabolic health, such as blood levels of insulin and C-reactive protein, a marker of inflammation. In fact, the rise in cognitive test scores correlated with lower insulin levels. In animal studies, high insulin levels and low-grade inflammation–products of being overweight and of high calorie intake–have been shown to hamper cognitive function. Mattson says that limiting calories in mice boosts a molecule in the brain called BDNF, which has a key role in memory. Regular exercise, along with calorie restriction, also boosts the growth of new brain cells in mice. Flöel says that the current results “suggest that those pathways from animal studies might also work in humans.”
However, Mattson cautions that older adults are at a higher risk of nutrient deficiencies, and could experience health problems that outweigh the potential benefits if they cut back on calories too severely. “Not everyone is going to benefit from calorie restriction,” he says. But for those who are already consuming more than they need and have a little extra weight to spare, there’s yet another health reason to cut back.