Might eating less help us live longer? What evidence is there, and are there any health risks if we eat less?
In this article, we look at some of the evidence for the benefits and the risks of eating less (‘caloric restriction’).
- What can we learn from animal experiments?
- Might caloric restriction work in humans?
- The risks of eating too little.
- Intermittent fasting.
- Life span or health span?
- Our conclusions.
What can we learn from animal experiments?
A Science Daily article in 2014 suggested that a very low-calorie diet could be the key to a long and healthy life.
For many years now there has been evidence from a number of different species that reducing the food – so called ‘caloric restriction’ – results in their living longer. However, this has usually been found in studies with mice or fruit flies in the controlled conditions of a laboratory.
The most widely accepted theory for this effect is that reduced food intake improves survival [in the wild] during times of famine.
Science Daily, 2014
The metabolism of human beings differs from that of mice and fruit flies in a number of important respects, and our everyday lives are affected by a wider range of factors than animals in a laboratory. So, can eating less also help humans to live longer?
Studies in monkeys are probably a more valuable source of research as monkeys live longer than laboratory animals and are biologically closer to humans. One study found that rhesus monkeys lived longer if they ate moderately less. It concluded that a low-nutrient diet delays the onset of age-associated pathologies and slows the ageing process in a primate species. However, another study found that eating less made no difference to the monkeys’ longevity.
What both studies agree on is that eating less resulted in healthier monkeys. The first study showed lower levels of cancer, cardiovascular disease, diabetes, and brain deterioration. The second found a statistically significant reduction in cancer and a less significant delay in the onset of diabetes, arthritis, and cardiovascular problems.
When the two studies were reviewed in Nature in 2017, the authors concluded that the benefits of calorie restriction extended to diseases that are among the most prevalent in human clinical care including cancer, cardiovascular disease, and parameters associated with diabetes.
Might caloric restriction work in humans?
Longevity and eating less
Some human-population studies suggest a similar picture as that for laboratory animals. For example, in Japan, people living on the island of Okinawa who followed a traditional lifestyle had some of the longest expected life spans in the world. Part of this traditional lifestyle involves eating only until you are 80% full – a tradition known as Hari Hachi Bu.
However, eating less is only one of a number of factors that potentially affect longevity. Exercise, the quality of their largely plant based diet, and their social lives are other factors. However, their longevity suggests a possible association between eating less and living longer. On the other hand, as Okinawans gradually adopted a Western lifestyle (including larger portions) over the last two decades, their life expectancy has plummeted.
Why caloric restriction might work is still being researched and fiercely debated. For example, biological, evolutionary and genetic factors have all been suggested.
However, what we do know is that caloric restriction typically extends the lifespan of some species researched so far and seems to provide some protection against age-related disease in monkeys (a species closer to humans). Population studies (in particular in Okinawa) and small-scale studies in humans suggest that eating less may help increase human longevity as part of a healthy lifestyle.
The latest research findings
However, a review of the evidence to date, published in 2022, found that there still isn’t a straight answer as whether caloric restriction prolongs human life. For example, the review asks whether it is simply reducing calories that makes the difference, or the type of calories that are consumed. Both the traditional Okinawan diet and the Mediterranean diet seem to reduce the risk of the main age-related diseases such as cardiovascular disease, cancer, cognitive decline, diabetes, hypertension, and Alzheimer's Disease.
For those people who are already eating a balanced, reasonable diet, extra caloric restriction may yield very small rewards.
The risks of eating too little
Eating less may help us live longer, but it is important to ensure we get both the right quality and the right quantity of food.
People who have anorexia illustrate what can happen if we eat too little. Anorexics are at increased risk of cardiovascular problems, coronary heart disease, damaged bones, anaemia, kidney failure and liver damage.
People who have anorexia often have a distorted image of their bodies, thinking that they're fat even when they're underweight.
Simply being seriously underweight (often linked to eating too little) can limit longevity, as identified in a study of over two million people published in The Lancet in 2018.
However, this could be due, at least in part, to an increased association between a very low body mass index (BMI) and mental health conditions. Arguably, this can sometimes result in poorer self-care and less access to physical health services.
Intermittent fasting – not eating for a period of time each day or each week – is a ‘half way house’, for instance, restricting how much you eat for one or two days a week, or fasting on alternate days. It has been suggested that this is more beneficial to ageing as it carries less of the health risks associated with chronic caloric restriction.
A review of the limited evidence available, published in 2015, found that there are potential health benefits, but it argued that significantly more research was needed before intermittent fasting could be recommended.
The Mayo Clinic agrees that there may be health benefits, but it also advises against:
- fasting and exercising at the same time (to prevent dizziness)
- fasting while taking diabetic medication (as this may lead to serious health issues)
- skipping breakfast (as this has been associated with an increased risk of obesity).
More recently, a 2019 article in the prestigious New England Journal of Medicine, concluded, ‘Preclinical studies and clinical trials have shown that intermittent fasting has broad-spectrum benefits for many health conditions, such as obesity, diabetes mellitus, cardiovascular disease, cancers, and neurologic disorders.’
However, the journal also noted, ‘It remains to be determined whether people can maintain intermittent fasting for years and potentially accrue the benefits seen in animal models.’
Types of intermittent fasting
A 2020 study notes there are at least five types of intermittent fasting:
other, less clearly defined intermittent fasting (for instance, a ‘fast mimicking diet’ and ‘juice fasting’).
The study also notes that intermittent fasting is not recommended for pregnant or lactating women, children or adolescents as they grow up and mature, the elderly or underweight people, and individuals who might be vulnerable to eating disorders.
Life span or health span?
How long we live isn’t the only issue. How long we stay healthy (rather than spending years burdened with chronic illness) is also an important issue.
Whether eating less adds years to our life, most evidence suggests that it is a healthy option, and that it reduces the risk of a range of chronic illnesses.
- How much we eat and our foods’ nutritional value are two of the factors that potentially affect our longevity.
- Other factors include how much exercise we take, our genes, whether we smoke or drink a lot of alcohol, our occupations and interests, our personality, and our social life.
- However, eating too much or too little is bad for our health and makes us more susceptible to chronic illness, and can even knock years off our lives.
- If we are aiming to eat less, we need to ensure that we eat a healthy, balanced diet so that we take in enough vitamins, minerals and nutrients.
Christiane Hahne, November 2022. Next review date October 2026.