For millennia, man has dreamed of achieving immortality. Today we are no closer to that goal, but science is helping us to understand how dietary, lifestyle, environmental, and genetic factors modulate the exceptionally complex ageing process. A healthy lifestyle involves eating the right foods, cutting out high-fat and high-sugar products, exercising regularly, avoiding tobacco, and drinking alcohol in moderation. Although studies have found that sticking to plant-based diets slows ageing, delays the onset of serious illnesses, and can lead to a longer life, it is useful to review some of the evidence and investigate which components in a healthy diet promote longevity with an emphasis on nutrition in Europe and North America.
What is a healthy diet?
Among healthy diets are certain plant-rich diets, including the Mediterranean, Japanese, New Nordic, and various vegetarian diets. While they differ in detail, they have in common fruits, vegetables, whole grains (oats, wheat, rye), pulses (peas, beans, and lentils), and nuts. Conversely, these healthy diets exclude or reduce red and processed meats, pre-packaged meals, high-fat dairy, and high-sugar products.
Fruits and nuts are a recurrent theme in these healthy diets. Fruits such as blueberries, blackberries, and oranges are a good source of the essential vitamins C and E and are high in fibre. These and other fruits have abundant levels of certain flavonoids (polyphenols, anthocyanins) that exhibit potent antioxidant and anti-inflammatory activities. Nuts are a source of omega-3 unsaturated fatty acids together with good levels of B vitamins and vitamins C and E. Like the flavonoids, omega-3 fatty acids and vitamins C and E exhibit potent antioxidant and anti-inflammatory activities. Whole grains and pulses are rich in nutritious substances. Whole grains are an excellent source of fibre that helps to lower both blood pressure and ‘bad’ LDL (low-density lipoprotein) cholesterol. Pulses are also a rich source of fibre and are high in iron and low in saturated fats.
The pure Mediterranean diet is followed by people living in some of the countries bordering the Mediterranean Sea. It consists of mainly fresh seasonal plant-based foods with ample dietary fibre. They observe long-established traditions of growing, preparing, and cooking local foods with olive oil derived from olives grown locally. A typical Mediterranean diet includes olive oil, moderate amounts of fish, fresh fruits and vegetables, whole grains, pulses, and yogurt. Olive oil is rich in beneficial unsaturated omega-3 fatty acids which lower both blood pressure and ‘bad’ LDL cholesterol and displays antioxidant and anti-inflammatory properties. Surprisingly, the Mediterranean diet includes limited amounts of dairy and poultry, as well as small quantities of red meat and ultra-processed products such as sugar, bread, and pastries.
The ageing process
It is important to consider what we know about ageing because life expectancy is linked to the rate of ageing, which in turn is associated with the choice of diet. Ageing is an extremely complex multi-factorial process characterised by progressive deterioration in the functioning of organs, tissues, and cells, and the gradual shortening of telomeres (the unique repeated DNA sequences at the ends of chromosomes that are required for multiple cell divisions). Increasing numbers of dysfunctional cells accumulate with limited regenerative potential and altered energy metabolism. Consequently, the organism has a reduced ability to respond to external stresses and serious illnesses, which are the leading causes of death and disability.
A favoured mechanism to partially explain ageing in detail, states that defects in cellular DNA repair mechanisms cause damaging mutations in DNA coding for many cellular components. When these mutations impair the functioning of antioxidant enzymes, reactive oxygen species (ROS) accumulate, causing inflammation and other widespread damage to the organism that contributes to ageing and the development of chronic diseases like cancer, cardiovascular disease, diabetes, and dementias.
To what extent does a healthy diet increase life expectancy?
Certain communities living in Sardinia and other areas in the Mediterranean have some of the healthiest and oldest individuals in Europe. Research across various countries confirms that model Mediterranean diets contribute to reduced risks of chronic diseases, better health and extended life expectancy. A large survey of 19 studies was performed involving over half a million subjects who adhered to a traditional Mediterranean diet and were then followed up for periods averaging 12.4 years. There was a consistent reduction in the risk of dying ranging from 7% to 47%. In studies such as these, it is always difficult to control for differences in previous health problems, preexisting diets, and exercise. Nevertheless, the results are impressive.
Some of the world’s longest living people and highest number of centenarians live in the largely vegetarian community of Seventh-day Adventists in Loma Linda, California. Studies involving approximately 150,000 of these people were analysed to understand the relationships between their nutrition, healthy lifestyle, and life expectancy. One study showed that their lifespan was 1.5-2.4 years longer than other Californians. In another study, it was found that Seventh-day Adventists outlive other Americans by about 10 years. A combination of lifestyle choices – eating a strict vegetarian diet, taking sufficient exercise and rest, avoiding tobacco, limiting alcohol intake, and maintaining a good body weight – can account for the increases in lifespan.
A major survey of numerous studies of over 400,000 individuals examined the relationships between diets and life expectancy in different populations around the world. Together with a massive quantity of data from the international Global Burden of Disease study, the survey modelled the consequences of switching from a typical Western diet rich in meat, dairy, and ultra-processed products to various healthy diets including an ‘optimal’ plant-based healthy diet. Following the optimal diet for up to 10 years from age 20, increased life expectancy by over 10 years for both men and women in the USA, Europe, and China. Switching to the optimal diet at age 60 increased life expectancy by 8 years. Remarkably, switching at age 80 still resulted in over 3 years of additional life.
It was interesting that the earlier in life the optimal plant-based diet was started the longer was the increase in life expectancy. Given the challenge of drastically switching from a typical Western diet to a largely vegetarian diet and sticking to it, a ‘hybrid’ diet was formulated that combines elements of the optimal and typical Western diets. Even following this diet from age 20 resulted in increased life expectancy by up to 6-7 years.
A similar major study, carried out by the same authors, arrived at essentially the same conclusions. They utilised population-based data from the UK Biobank resource to show that sustained change from unhealthy diets to the UK Eatwell Guide dietary recommendations is associated with 8.6-8.9 years increase in life expectancy for 40-year-old females and males. In the same cohort, sustained change from similarly unhealthy diets to stricter longevity-associated diets resulted in impressive 10.4-10.8 years gain in life expectancy for both sexes. The largest increases in life expectancy were observed in individuals who consumed more nuts and whole grains and less sugary drinks and processed meats, implying that these dietary components alone might exert some influence on life expectancy.
There are several issues that may influence the accuracy of the data in these surveys. Reliable results depend on accurate reporting. Past health problems – which can affect lifespan – were not considered. Also, the surveys were not always performed beyond 10 years, which might have revealed even more beneficial effects. Overall, these surveys are considered robust and show that you do not necessarily have to follow a strict plant-based diet to live longer. Moreover, it is never too late to prolong your life through healthy nutrition.
Calorie restriction, ageing, and lifespan
Extensive research has shown that delays in ageing, reduction in serious age-related diseases, and increased life expectancy in diverse organisms is possible by simply limiting food intake by 20-40% without risking malnutrition. Calorie restriction leads to varying increases in lifespan in species from yeasts to non-human primates. An impressive 40% lifespan increase achieved in worms in one study; but other studies in different organisms showed no effect.
Although calorie restriction in some laboratory animals and humans leads to a reduction in age-associated diseases, there has been considerable uncertainty regarding lifespan extension in humans. While increasing life expectancy in humans has been claimed in a few studies, the experiments have sometimes not been conducted for long enough to draw any firm conclusions. Deleterious effects of calorie restriction have also been observed. They include hypotension, impaired regulation of body temperature, slower wound healing, and decrease in bone density. The effects of calorie restriction on prolonging life expectancy have been demonstrated in yeasts, fruit flies, worms, fish, rats, mice, dogs, and monkeys, suggesting that calorie restriction can act on common genetic pathways conserved across different species.
Many plausible theories have been proposed to explain how calorie restriction works, including the favoured oxidative stress theory, which states that calorie restriction reduces the generation of damaging ROS and increases the activity of enzymes that protect from ROS. However, it remains to be determined whether the reduction in oxidative stress by limiting food intake is sufficient to play a major role in increased life expectancy.
Concluding remarks
A recurring theme is that the antioxidant and anti-inflammatory activities of flavonoids, vitamins C and E, and omega-3 fatty acids present in plant-based diets are important factors in counteracting the damaging ROS generated during human ageing. Dietary fibre, lacking in unhealthy diets, is essential for overall health by helping to maintain a diverse constellation of ‘good’ gut microorganisms that digest complex macromolecules (the microbiome). A staggering 17 health benefits (notably lowering LDL cholesterol and blood pressure), have been found to be associated with a modest intake of omega-3 fatty acids.
Emerging evidence from many researchers suggests ultra-processed foods, abundant in typical Western diets are detrimental to health, and are associated with type 2 diabetes, some cancers, cardiovascular disease, and even mild mental health problems. A healthy diet has far less ultra-processed foods which might explain, at least in part, the longer lifespan observed with plant-rich diets. It is probable that the combination of these and many other elements interacting in an additive or synergistic manner explain how plant-based diets can lead to fewer chronic illnesses, slower ageing, and a longer healthier life in humans.
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