A high-carb diet may explain why Okinawans live so long

By David Robson 18 January 2019

The Economics of Change

The search for the “elixir of youth” has spanned centuries and continents – but recently, the hunt has centred on the Okinawa Islands, which stretch across the East China Sea. Not only do the older inhabitants enjoy the longest life expectancy of anyone on Earth, but the vast majority of those years are lived in remarkably good health too.

Of particular note is the number of people who reach 100 years of life. For every 100,000 inhabitants, Okinawa has 68 centenarians – more than three times the numbers found in US populations of the same size. Even by the standards of Japan, Okinawans are remarkable, with a 40% greater chance of living to 100 than other Japanese people.

Little wonder scientists have spent decades trying to uncover the secrets of the Okinawans’ longevity – in both their genes and their lifestyle. And one of the most exciting factors to have recently caught the scientists’ attention is the peculiarly high ratio of carbohydrates to protein in the Okinawan diet – with a particular abundance of sweet potato as the source of most of their calories.

“It is quite the opposite of current popular diets that advocate a high protein, low carb diet,” says Samantha Solon-Biet, who researches nutrition and ageing at the University of Sydney. Despite the popularity of the Atkins and Paleo diets, however, there is minimal evidence that high-protein diets really do bring about long-term benefits.

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So could the “Okinawan Ratio” – 10:1 carbohydrate to protein – instead be the secret to a long and healthy life? Although it would still be far too early to suggest any lifestyle changes based on these observations, the very latest evidence – from human longitudinal studies and animal trials – suggest the hypothesis is worth serious attention. According to these findings, a low protein, high carbohydrate diet sets off various physiological responses that protect us from various age-related illnesses – including cancer, cardiovascular disease and Alzheimer’s disease. And the Okinawan Ratio may achieve the optimal dietary balance to achieve those effects.

Much of this research comes from the Okinawa Centenarian Study (OCS), which has been investigating the health of the ageing population since 1975. The OCS examines inhabitants from across the Okinawa prefecture, which includes more than 150 islands. By 2016, the OCS had examined 1,000 centenarians from the region.

Rather than suffering a prolonged demise, the Okinawan centenarians appeared to have delayed many of the usual effects of ageing, with almost two thirds living independently until the age of 97. This remarkable “healthspan” was evident across many age-related diseases. The typical Okinawan centenarian appeared to be free of the typical signs of cardiovascular disease, without the build-up of the hard “calcified” plaques around the arteries that can lead to heart failure. Okinawa’s oldest residents also have far lower rates of cancer, diabetes and dementia than other ageing populations.

Genetic jackpot

Given these results, there is little doubt that Okinawa has an exceptional population. But what can explain that extraordinary longevity?

Genetic good fortune could be one important factor. Thanks to the geography of the islands, Okinawa’s populations have spent large chunks of their history in relative isolation, which may has given them a unique genetic profile. Preliminary studies suggest this may include a reduced prevalence of a gene variant – APOE4 – that appears to increase the risk of heart disease and Alzheimer’s. They may also be more likely to carry a protective variant of the FOXO3 gene involved in regulating metabolism and cell growth. This results in a shorter stature but also appears to reduce the risk of various age-related diseases, including cancer.

Even so, it seems unlikely that good genes would fully explain the Okinawans’ longevity, and lifestyle factors will also be important. The OCS has found that Okinawans are less likely to smoke than most populations, and since they worked predominantly in agriculture and fishing, they were also physically active. Their tight-knit communities also help the residents to maintain an active social life into old age. Social connection has also been shown to improve health and longevity by reducing the body’s stress responses to challenging events. (Loneliness, in contrast, has been shown to be as harmful as smoking 15 cigarettes a day.)

It is the Okinawans’ diet, however, that may have the most potential to change our views on healthy ageing. Unlike the rest of Asia, the Okinawan staple is not rice, but the sweet potato, first introduced in the early 17th Century through trade with the Netherlands. Okinawans also eat an abundance of green and yellow vegetables – such as the bitter melon – and various soy products. Although they do eat pork, fish and other meats, these are typically a small component of their overall consumption, which is mostly plant-based foods.

The traditional Okinawan diet is therefore dense in the essential vitamins and minerals - including anti-oxidants - but also low in calories. Particularly in the past, before fast food entered the islands, the average Okinawan ate around 11% fewer calories than the normal recommended consumption for a healthy adult.

For this reason, some scientists believe that Okinawans offer more evidence for the life-enhancing virtues of a “calorie restricted” diet. Since the 1930s, some doctors and scientists have argued that continuously limiting the amount of energy you consume could have many benefits above and beyond weight loss – including a deceleration of the ageing process.

In one of the most compelling experiments, a group of resus macaques eating 30% fewer calories than the average monkey showed a remarkable 63% reduction in deaths from age-related diseases over a 20-year period. They also looked younger – they had fewer wrinkles and their fur retained its youthful lustre rather than turning grey. Due to practical difficulties, long-term clinical trials in humans have yet to be completed to test the effects on longevity, but a recent two-year experiment, funded by the US National Institute on Aging, was highly suggestive: participants on a calorie restricted diet showed better cardiovascular health – including lower blood pressure and cholesterol.

It’s still not clear why a calorie restricted diet would be so beneficial, but there are many potential mechanisms. One possibility is that calorie restriction alters the cell’s energy signalling, so that the body devotes more resources to preservation and maintenance – such as DNA repair – rather than growth and reproduction, while limiting ‘oxidative stress’ caused by the toxic by-products of metabolism that can cause cellular damage.

The benefits of the Okinawan Diet may not end with its calorie restriction.

Solon-Biet has conducted a series of studies examining the influence of dietary composition (rather than sheer quantity) on ageing in animals, and her team has consistently found that a high-carb, low-protein diet extends the lifespan of various species, with her most recent study showing that it reduces some of the signs of ageing in the brain. Amazingly, they have found that the optimum ratio is 10 parts carb to one part protein – the same as the so-called Okinawan Ratio.

Although there aren’t yet any controlled clinical trials in humans, Solon-Biet cites epidemiological work across the world that all point to similar conclusions. “Other long-lived populations have also been shown to have dietary patterns that include relatively low amounts of protein,” she says. “These include the Kitavans, [who live on] a small island in Papua New Guinea, the South American Tsimane people and populations that consume the Mediterranean diet.”

Once again, the exact mechanisms are murky. Like calorie restriction, the low protein diets seem to promote the cell repair and maintenance. Karen Ryan, a nutritional biologist at the University of California, Davis, points out that the scarcity of amino acids can encourage cells to recycle old material (rather than synthesising new proteins).

“Together, these changes may prevent the ageing-associated accumulation of damaged proteins within cells,” she says. This build-up of damaged proteins may usually be responsible for many diseases, she says – but the regular clean up when we eat a low-protein diet could prevent it.

So should we all start adopting the Okinawan Diet? Not quite. Ryan points to some evidence that low protein intake may limit bodily damage up to the age of 65, but you may then benefit from increasing your protein intake after that point. “Optimal nutrition is expected to vary across the life history,” she says. And it’s also worth noting one study, which found that the relative merits of protein and carbohydrates may depend on the protein's source: a diet higher in plant-based protein appears to be better than a diet rich in meat or dairy, for instance. So the Okinawans may be living longer due to the fact that they are eating (mostly) fruit and vegetables, rather than its high carb, low protein content.

Ultimately, the Okinawans’ health is probably due to a lucky confluence of many factors, Ryan says. “And specific interactions among these factors will also be important.” And we may need many more years of research to understand the importance of each of those ingredients before we finally come up with a true recipe for the “elixir of youth”.

David Robson is a senior journalist at BBC Future. He is @d_a_robson on Twitter.

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