“Our study provides some first evidence on the impact of caloric restriction on memory in the elderly,” principal investigator Agnes Flöel of the University of Münster in Germany wrote in an e-mail to ARF. With first author A. Veronica Witte and colleagues at the University, Flöel put 19 elderly volunteers on a diet containing 30 percent fewer calories than their usual fare for three months. The study participants were healthy but on the heavy side, with a mean body mass index of 28 (above 25 is considered overweight). Before and after the program, they were tested on how many words they could remember out of a list of 15. After eating a calorie-restricted diet, memory scores went up by an average of 20 percent; there was no change in a control group.

One of the limitations of this study is that the researchers relied on self-reporting to monitor people’s compliance with the diet, but they strengthened this notoriously weak link by also showing that caloric-restricted people lost weight and had lower blood insulin levels, suggesting they adhered to the study’s instructions. Those who followed the diet most closely saw the greatest memory improvement. Even with only a couple of dozen participants, this study “is one of the better powered studies that have been published,” said Corby Martin of the Pennington Biomedical Research Center in Baton Rouge, Louisiana, who was not involved with the paper. “This research is pretty expensive.”

The mechanism by which fewer calories might boost the brain is unclear (for review, see Martin et al., 2006). Lower insulin levels suggest that the body is regulating glucose more effectively, which may be neuroprotective (see ARF related news story). Another hypothesis is that caloric restriction reduces inflammation, which could be good for neuronal function. A third possibility is that the stress of under-eating activates neurotrophic factors, such as brain-derived neurotrophic factor.

If future studies support these results, it would imply that caloric restriction could be beneficial as a preventative measure for healthy older people, and perhaps those with mild cognitive impairment, Flöel wrote. However, she doubts such a diet would be appropriate for people with Alzheimer disease. It might be too late for people with AD to reap the cognitive rewards. Moreover, since people with AD already tend to lose weight—indeed, rapid weight loss in the years before AD diagnosis is considered a risk factor—caloric restriction might even be dangerous for elderly people who are already thin. Flöel also suspects that the diet would have little impact on younger people, whose memory performance, glucose metabolism, and inflammation levels are closer to ideal.

The ultimate effects of caloric restriction in humans remain unclear. That said, a caloric restriction diet is certainly a healthy one: lots of salads, lean protein, and whole grains. “If an individual is overweight, of course dieting will improve their health,” Martin said. “That will, presumably, help them live a longer life.”—Amber Dance.

Reference:
Witte AV, Fobker M, Gellner R, Knecht S, Flöel A. Caloric restriction improves memory in elderly humans. PNAS January 27 2009 106(4):1255-1260. Abstract