The Simplest Diet of All is Eating Less: Caloric Restriction and Longevity

 Diets are a critical part of modern health. One only has to look at any magazine stand to see a whole galaxy of different diets – Atkins, keto, paleo, vegetarian, raw, and on and on – each promising long lists of benefits in many different aspects of one's health. Each generally can cite good scientific evidence for at least some of what they claim. But there is an important finding dating back centuries that is much simpler than all of these: long life is strongly linked to eating less, or to phrase it more precisely, to “caloric restriction.”

Advice from doctors suggesting that eating less leads to a longer life stretches back to the sixteenth century, when an Italian nobleman named Luigi Cornaro wrote Discorsi della vita sobria, in which he shared his physician’s suggestion to eat “as little as possible” to live a long life - and he died at age 102. As far back as 1917, scientific studies suggested that feeding rats less food made them live longer. By the 1930s, studies in mice confirmed that restricting the amount of food they ate made them live longer, and that finding has been repeated again and again in many organisms, from yeast to dogs and in recent years to humans. Scientists have consistently found that lower calorie intake leads to a longer life. In the decades since, there has been much research to figure out exactly what is going on that explains these results.

Oxygen's wear and tear

But before talking about that research, it makes sense to briefly talk about what aging actually is. Though it may be tempting to make generic appeals to entropy and the ravages of time, the concept is more complex than that. There are many interrelated issues in the broad topic of aging, but the part of aging specifically relevant to caloric restriction is the idea of reduction-oxidation (“redox”) stress.

“Redox” broadly refers to the movement of electrons in chemical reactions, and organisms capture and use the energy from such reactions to live. And the same properties of oxygen that make it so useful for so-called aerobic life, such that animals like humans need to breathe it to live – its ability to participate in redox reactions, releasing energy for metabolism – also make it damaging to delicate machinery in the cell. This is something like how gasoline is full of energy for your car, but dousing it everywhere causes a risk of fire. The very process of metabolizing food creates byproducts like oxygen free radicals, called reactive oxygen species (ROS), that can damage and imbalance the extremely complex web of chemical reactions that makes up metabolism.

Identifying the mechanisms

With that in mind, we turn to some of the questions raised by research into caloric restriction, and what is actually happening. It is easy to say that eating less is good, but why would that be the case? What specific part of the paradigm causes the result of longer life that we care about?

There are many hypotheses as to what explains the results, which have had varying levels of popularity over the years. For example, up until the 1980s, it was thought that the primary way lower caloric intake lengthens lifespan was by slowing growth. Claims have been made that the cause is due to downstream effects on specific proteins in the body like insulin and insulin-like growth factor-1, reductions in body fat, reduced metabolic rate, smaller amounts of particular nutrients having better effects than larger amounts in an effect called hormesis, and many others. The most popular is that caloric restriction causes a decrease in the rate of redox stress accumulation, as discussed earlier, which is probably the chief cause. But all these hypotheses have at least some evidence supporting them, and since aging is a complex process in the complexity of a whole organism, the true answer is probably a combination of many different factors to different degrees.

Avoiding the pitfalls

It is very easy to mistake the benefits of caloric restriction for the dangers of obesity, which are not precisely the same thing. Generally a scientific study has an experimental group and a control group: one group of subjects, mice or what have you, to which the thing being tested is applied, and one group to which nothing is done, to have a baseline measurement of what the results should look like. The difference between the two groups can then be analyzed to make conclusions about the experiment. But in many studies about caloric restriction, the control group is fed “ad libitum” (they eat as much food as they want). Understandably, animals like mice will overeat if allowed to do so, just like how those Oreos in your pantry seem to call out to you. So if the study is not carefully designed, the differences in lifespan between restricted and control groups may be partly or entirely due to the negative effects of overeating and obesity, not any possible benefits of caloric restriction.

It is also important to realize that caloric restriction is distinct from malnutrition. The body does not just need calories, but specific nutrients for a wide variety of purposes. A diet based on caloric restriction must be careful to include all those nutrients in sufficient quantities to meet the recommended daily requirements. There are also concerns that despite its apparent benefits, caloric restriction may lead to issues such as decreased bone density and muscle mass, and may be particularly risky for groups such as the elderly.

Extending studies to humans

In recent years, these results in studies of animals have been supported by corresponding results in studies of humans. There have been retrospective studies of people with restricted diets, due to wartime restrictions or similar conditions, but the most prominent in modern times has been the Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE) study. It was a two-part study of the effects of a targeted 25% caloric restriction from a participant's normal diet over 6-month and 2-year periods, phase 2 completed in 2013, with analysis of the data collected that is still ongoing.

Those ongoing analyses of the data from CALERIE and many other studies strongly demonstrate a wide variety of positive effects. Participants experienced benefits like less DNA damage, lower oxygen consumption, improved energy production efficiency in their mitochondria, and lower oxidative stress, and showed decreases in various risk factors associated with cardiovascular diseases and inflammation. These mimic findings that caloric restriction improves the health of organs including the heart, the liver, skin, lungs, kidneys, and brain.

Recently, scientists have also become interested in the effects of timing as well as caloric restriction, as in paradigms such as intermittent fasting – not eating for 16 to 20 hours a day, limiting eating only to proscribed periods. In recent years evidence has been found that the benefits of caloric restriction are mirrored and enhanced by such a program. The body is more able to withstand stresses, and disease processes such as cardiovascular disease, arthritis, stroke, and cancer show better outcomes. 

There is still ongoing work investigating the benefits of caloric restriction and the finer details of the concept, such as intermittent fasting. But the overall result for the layperson is clear. So long as one avoids malnutrition and takes proper precautions, caloric restriction – temperance and moderation when eating – is very strongly connected to a longer, healthier life. 

Written by Robert Hubbard

Edited by Bianca Skinner

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