Lessons learned from the monkey diet
With the current obesity epidemic in the United States, diet regimens are popular topics for healthcare discussion. Many yearn for the slim waistline they once had, but repeatedly fail in attempts to regain former slimness. The results are now in on a long-term prospective study that began in 1987.
One of our closest primate relatives, rhesus monkeys, were placed on a calorie restricted diet for their entire life. Researchers affiliated with the National Institutes of Health published their findings online on August 29 in the journal Nature.
The researchers noted that calorie restriction, a reduction of 10–40% in intake of a nutritious diet, is often reported as the most robust non-genetic mechanism to extend lifespan and healthspan. They explained that calorie restriction is frequently used as a tool to understand mechanisms behind ageing and age-associated diseases. In addition to and independently of increasing lifespan, calorie restriction has been reported to delay or prevent the occurrence of many chronic diseases in a variety of animals. Beneficial effects of calorie restriction on outcomes such as immune function, motor coordination, and resistance to sarcopenia in rhesus monkeys have recently been reported. (Sarcopenia is the degenerative loss of skeletal muscle mass and strength associated with aging.)
For 25 years, the monkeys were kept on a calorie-restricted diet; thus, they were semi-starved, lean, and hungry. The males’ weights were so low that they were the equivalent of a 6-foot-tall man who weighed only 120 to 133 pounds. The investigators theorized that if the monkeys lived longer, healthier lives by eating a lot less, then maybe humans, their evolutionary cousins, would, too.
Many enthusiasts of calorie restriction looked forward to results to support the concept that calorie restriction would result in a longer, healthier life. Unfortunately, that was not the case. The super-skinny monkeys did not live any longer than those kept at more normal weights. Some laboratory test results improved; however, that occurred only in monkeys put on the diet when they were old. The causes of death for cancer and heart disease were the same in both the starved and the normally fed monkeys.
Another interesting finding was a gender discrepancy. Lab test values for cholesterol and blood sugar were lower in the male monkeys that started eating 30% fewer calories in old age; however, not in the females. Both males and females that were placed on the diet when they were old had lower levels of triglycerides, which are linked to heart disease risk. Monkeys put on the diet when they were young or middle-aged did not derive the same benefits; however, the incidence of cancer was less. All things considered, the calorie-restricted monkeys did not live any longer than those that ate normally.
Despite the recent publication of their findings, the researchers plan to continue the study until the youngest monkeys are 22 years old. They note that, although their findings essentially rule out the concept that the low-calorie diet will increase average life spans, there still is a possibility that the study might find that the diet increases the animals’ maximum life span.
Take home message:
The results of this study should not encourage one to go out and order a supersize meal at McDonald’s. A sensible diet and exercise plan is still an appropriate way to increase the chances of a longer, healthier life. These monkeys were starved! Thus, this study notes that moderation not extremism in diet is the way to go.