Hara hatchi bu, the Okinawan people’s habit of eating only till they are 80 percent full, is thought to be one of the secrets of their extraordinary health and longevity. In addition to one of the highest percentages of people in the world who live past 100, Okinawans appear to be less prone to heart disease, diabetes and obesity.

Indeed, ever since it was discovered in the 1930s that laboratory rats fed a caloric-restricted (CR) diet lived almost twice as long as their well fed counterparts, scientists have pursued caloric restriction research in the hopes of finding novel strategies for extending human life and preventing disease. Given the growing older population at risk for memory problems and the rising rates of obesity, the role of diet in maintaining peak brain performance has taken on added importance.

Although the links between caloric restriction and longevity are still not fully proven in humans, short-term human trials have clearly shown that CR can improve many vital surrogate health markers, such as body weight, blood pressure, blood sugar and insulin levels, blood cholesterol and triglyceride levels, and measures of inflammation. High insulin levels and inflammation have both been linked to cognitive problems. In mice, reducing calories also promotes neurogenesis and slows certain Alzheimer’s-related changes in parallel with reductions seen in blood insulin and inflammation. Thus, there is great interest in examining the effects of CR on brain health in humans and in comparing its effects with those of other diets (for example, diets rich in healthier unsaturated fats) that may also help memory.

Diet and Memory

A recent study on caloric restriction and memory led by the neurologist Agnes Floel and her colleagues at the University of Munster took the first step in examining this issue. They recruited 50 older (ages 50 to 80 years) adults with a normal memory. Subjects on average were slightly overweight. The researchers assigned the volunteers to three groups, based on their age, gender and weight. Group 1 got a diet with 30 percent reduced daily calories and normal levels of other essential nutrients; the minimal level was set at 1,200 calories daily to prevent malnourishment. Group 2’s diet had 20 percent increased unsaturated fatty acids with no increase in total fat—thus boosting the ratio of healthy (unsaturated) to unhealthy (saturated) fats. The control was Group 3)—who had a diet as usual. None of the participants were advised to change their exercise habits. The researchers gave subjects in the first two groups individualized dietary plans and monitored their diet via self-reports. All subjects underwent memory and blood tests before and after the three months in the trial.   
At the end of three months, the reduced-calorie diet group showed a small reduction in body weight (by 2.4 kilograms), whereas the other two diet groups showed a slight increase in weight (by about one kilogram). There was, however, a highly significant (about 20 percent above baseline) improvement in the CR group’s ability to recall words they had on a list (called delayed recall), and they also made fewer errors. Their memory improvement tended to be correlated with reductions in blood insulin and markers of inflammation (C-reactive protein and TNF-alpha). Memory did not change in the other two diet groups.

This study is commendable because it is the first prospectively planned trial in older adults to demonstrate memory benefits of a low-calorie diet. The replication in humans of some of the findings seen in earlier animal studies provides an important proof of concept step that will encourage and guide the design of larger future studies. Further, it demonstrated improvements in the type of memory (delayed recall) that is typically the first to fail in very early stages of Alzheimer’s disease.

Further Questions

As with any single center pilot study, this study also has some limitations (many of which the authors acknowledge), such as: small sample size, considerable differences in baseline characteristics of the three groups, unreliability of diet self-reports, the possibility of chance findings from multiple comparisons, greater social contact with subjects in diet groups, and highly variable adherence to diet as evidenced by the small weight loss in the CR group. For these reasons, the results should be considered preliminary, but promising. 

What next? There are several unanswered questions. Will memory benefits of CR continue once body weight has dropped below a certain level? Was it the caloric restriction per se or was it the type of diet that led to the effects observed? Might a low carbohydrate diet, without reducing calories, achieve a similar effect? Is a 30 percent caloric restriction even sustainable for most people in the face of constant bombardment with food products? Can the combination of diet and exercise lead to greater benefits? Could prolonged caloric restriction cause wasting or other harm in older people? If inflammation mediates the cognitive benefits of diet, why have prior trials of anti-inflammatory agents failed to improve memory? Is there an interaction between one’s genetic makeup and diet? Further dietary intervention studies of much larger samples with known risk factors (such as elevated C-reactive protein, insulin resistance, or mild cognitive impairment) would be needed to address these questions. 

But until such studies are completed, a good rule of thumb with regard to diet is “what is good for the heart is generally good for the brain.” Think Hara hatchi bu if you overeat, but never start a diet without consulting a doctor.

Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters editor Jonah Lehrer, the science writer behind the blog The Frontal Cortex and the book Proust Was a Neuroscientist. His latest book is How We Decide.