Many people believe that the modern American diet must be healthy because average life expectancy has risen more than 25 years in the past century and is higher now than ever before. However, the increase in how long people live, on average, in modern countries has much more to do with public health measures such as chlorinated drinking water that eliminated cholera, typhoid fever and other water-born illnesses that still plague third world countries and were still a major health problem in the United States 100 years ago.Better public health measures have drastically cut deaths from food-born illnesses as well. Vaccination against diseases such as smallpox, polio and others has dramatically reduced the risk of dying from viral illnesses. Pest control (mosquitoes and rodents) coupled with the widespread use of antibiotics has dramatically cut the death rate from bacterial diseases. Other medical advances such as the use of insulin for children with type-1 diabetes, has greatly extended the lifespan of many people who would have died soon after diagnosis 100 years ago.There is no evidence the aging process has been slowed at all by public health and medical advances. Americans are aging just as fast today as they did at the turn of the last century. They are living longer on average largely because public health and medical advancements have reduced or prevented disease-related mortality. It is clear that the average increase in life expectancy is not related to any slow down in the aging process.1Does Diet Have An Effect?In 1935, McCay was the first to demonstrate that maximal life expectancy in rats could be increased by restricting calorie intake.2 Since the 1930s, research has continued to accumulate to the point that it is now proven that calorie restriction retards biological aging and delays or prevents most age-associated disease processes in many different animal species.3 Perhaps the best measure of the rate of aging is the time required for the mortality rate to double (MRD). In calorie-restricted rats, the MRD is increased from 102 to 197 days.4 The anti-aging effect of calorie restriction is not due to changes in the amount of any specific nutrient but rather to the reduction in calories alone.5The MRD for humans is about 8 years. This means the risk of dying doubles every 8 years. If the aging process were slowed by calorie restriction as much in humans as rats, then life expectancy could increase to 150 years.There is no credible evidence that any other types of nutritional manipulation (e.g., increased antioxidants) can slow the aging process in animals. Nor is there any pharmaceutical agent or hormone (e.g., DHEA and HGH) that has been proven to slow the aging process itself.6 No one knows precisely how reducing energy intake by 20 to 40 percent in animals slows aging.One likely factor is the reduction in blood sugar. Blood sugar binds to other molecules (a process called glycation) and disrupts their normal function. In calorie-restricted rats, blood sugar levels stay about 15mg/dl lower throughout life than rats fed all they want.7 Of all sugars, glucose is the least likely to form glycation products that disrupt cellular function.8 Fructose is much more prone to form glycation products than glucose. In rats, a higher intake of fructose is associated with a decreased life expectancy.9 It might be that diets high in refined starches (which increase blood glucose) and sugar (which is half fructose) might increase glycation products and cellular deterioration more rapidly than more natural foods high in complex carbohydrates.Can Calorie Restriction Slow Aging in Humans?No one knows for sure. However, ongoing research in monkeys at the National Institute of Aging has shown they experience many of the same reductions in markers of biological aging seen in rodents.10 It seems likely that the slow down in the aging process associated with decreased calorie intake in animals would also occur in people. It should be noted that the degree of calorie restriction that maximally increases life expectancy is less in older than younger animals. Severe calorie restriction can actually increase overall mortality in both older animals and people. It should also be noted that the later in life that calories are restricted the less increase there is in life expectancy in animals so this would likely be the case in humans as well.The oldest documented human population lives on the Japanese island of Okinawa. Middle-aged and older Okinawans have an average BMI of about 21.6 whereas middle-aged and older Americans have an average BMI of 26.5. The prevalence of centenarians is 5-10 times as high in Okinawa as in the U.S. Life expectancy and disability-free years are six years higher on Okinawa as in the United States. In the United States, the MRD for young adults (in their third decade) with a BMI of 19 to 21.5 is 2-3 times greater than those who are obese (BMI > 30).11Most Americans are overweight and would be healthier and probably live longer if they cut their calorie intake and lost excess body weight. While not proven, it seems likely that restricting calorie intake might not only dramatically reduce the risk of diabetes and other diseases, but also slow the aging process.By James J. Kenney, PhD, RD, LD, FACN.References:1. Science. 1990;250:634-402. J Nutr 1935;10:63-793. Am J Clin Nutr 1992;551250S-2S4. Biological Reviews of the Cambridge Philosophical Society . 1986;61:329-685. J Gerontol Biol Sci 1988;43:B59-646. www.sciam.com/explorations7. J Am Geriatr Soc 1985;33:626-348. Natural History 1992;2:25-569. Am J Clin Nutr 1995;62(suppl):284S-93S10. J Clin Endocrinol Metab 2002;82:2093-611. J. Nutr Rev 2000;58:129-37