Cutting calories from the diets of many animals dramatically increases their life span, leading some people to try to extend their own lives in the same way.
If you could live to be 130 years old, what would you willingly give up?
How about 30 to 50 percent of the calories you eatâ€¦ for the rest of your life?
Proponents of the dietary practice known as calorie restriction (CR) happily make this trade-off every day in hopes of having life spans that leave todayâ€™s centenarians in the dust.
This may sound like a fad diet, but thereâ€™s quite a bit of research to back up the use of calorie restriction for longevityâ€¦ although most of it has been done in animals other than humans.
So how likely is it that eating child-size portions for every meal will gain you an extra few decades of life?
Mouse, fish, and yeast everlasting
Studies have shown that calorie restriction can extend the life span â€” and reduce age-related chronic diseases â€” of many species, including mice, fish, worms, and yeast.
But these creatures are not people.
Which is why scientists turn to primates like rhesus monkeys, which age similarly to humans, as well as develop cancer, diabetes, and some traits of Alzheimerâ€™s disease.
Six of the 20 monkeys on a calorie restricted diet have lived beyond 40 years. The average lifespan for monkeys in captivity is around 26 years. One male is currently 43 years old, a record for the species.
Researchers also found that calorie restriction benefitted older monkeys, but not younger ones. This is in contrast to other studies in mice that showed that starting calorie restriction at a young age gives the best results.
The sex of the monkeys and what they ate â€” not just the number of calories â€” also affected how much monkeys benefitted from calories restriction.
While the results of animal studies are promising, scientists know less about how calorie restriction affects people, especially long-term.
Given that Americans live on average around 78 years, researchers would have to wait decades to see if calorie restriction extended human life span.
To compensate for this, Duke University researchers instead looked at measures of biological age.
The calorie restriction group aimed to cut their caloric intake by 25 percent â€” although by the end of the two-year study they had only achieved a 12 percent reduction.
After each one-year period, the biological age of people in the calorie restriction group increased by 0.11 years, compared with 0.71 years for people who stuck with their usual diets.
Researchers calculated biological age using chronological age and biomarkers for things such as cardiovascular and immune system function, total cholesterol, and hemoglobin levels.
However, researchers only followed people for two years. Whether these benefits continue after this point, and at what level, is unknown.
How does calorie restriction increase life span?
No one is certain why calorie restriction increases the life span of so many organisms.
Some scientists think it may have to do with free radicals â€” atoms with an unpaired electron â€” that are released when the body turns food into energy.
Free radicals can damage important parts of the cell, like DNA and the cellâ€™s membrane. So cutting back on the food you eat may decrease the number of free radicals circulating in the body.
Insulin could also play a role. As we age, our bodies can become resistant to this hormone, leading to excess glucose in the blood that can damage organs, blood vessels, and nerves.
Some researchers, though, think calorie restriction increases longevity by rejuvenating the bodyâ€™s biological clock.
This â€œclockâ€ is actually a set of genes that change activity in order to sync with the cycle of day and night.
However, when researchers cut the calorie intake for older mice by 30 percent for six months, the energy processing in the cells resembled that of young mice.
Mimicking fasting offers benefits
If signing up for a lifetime of hunger to gain a few extra years of life doesnâ€™t sound appealing, you may have other options for breaking the 100-year mark â€” or at least living healthier.
A team led by gerontologist Valter Longo, PhD, director of the University of Southern California Longevity Institute, tested the effects of a â€œfasting- mimicking dietâ€ â€” an alternative to only drinking water â€” on the risk of developing major diseases.
People on the fasting-mimicking diet ate about 750 to 1,100 calories per day, for five days per month, over three months.
Adult women usually eat 1,600 to 2,400 calories per day, and adult men generally eat 2,000 to 3,000 calories per day.
Food used in the study contained exact proportions of proteins, fats, and carbohydrates.
People on the fasting-mimicking diet saw a drop in their blood pressure, blood glucose, and markers of inflammation, compared with people eating a regular diet.
People who â€œfastedâ€ also lost weight over the three months, but not muscle mass, which is a concern with a calorie-restricted diet.
As with other calorie restriction studies in people, this one doesnâ€™t show that cutting back on calories increases lifespan, only that it may reduce certain risk factors for disease.
Dietary lifestyleâ€¦ or eating disorder
The CR Society International, an organization that offers resources for people wanting to live longer by cutting out calories, lists some of the potential unwanted effects of long-term calorie restriction.
These include loss of bone mass, sensitivity to cold, and decreased sex drive.
Some experts are also concerned that calorie restriction could cross the line into an eating disorder, such as anorexia nervosa.
Dr. Ovidio Bermudez, chief clinical officer and medical director of child and adolescent services at the Eating Recovery Center, said if someone walked into his office saying that they were going to cut their caloric intake by 30 or 50 percent for the rest of their life, â€œI would raise serious concerns about that.â€
â€œYou may be awakening a monster that you donâ€™t want to deal with,â€ said Bermudez.
He emphasized, though, that not everyone who does calorie restriction will develop anorexia.
Like other eating disorders, anorexia has a strong genetic component that puts some people at risk more than others, although scientists donâ€™t fully understand the genetics.
However, genetics alone is not enough to trigger the disease.
â€œThe genetic predisposition [to eating disorders] is insufficient and needs to interact with some other influences,â€ said Bermudez. â€œIt seems that, in this day and age, those other influences are, to a great extent, environmental.â€
There is no single environmental trigger for anorexia.
Some teenage girls or boys may take a health class in high school and decide to eat less and exercise more. Or a young adult may look around and try to conform with the â€œfast-paced, thin-ideal culture that we live in,â€ said Bermudez.
Or someone wanting to live longer may restrict their calories.
Not everyone in these situations will develop an eating disorder. But the danger is that someone who drastically cuts down on eating will cross a threshold leading to a â€œneurobiological change that seems to both trigger and cement the illness process,â€ said Bermudez.
People doing calorie restriction who end up with pre-anorexia or anorexia may not even realize they are in trouble.
â€œThere is a subset that will probably cross the threshold and lose perspective,â€ said Bermudez, â€œand those are the people who are unlikely to have a keen awareness of whatâ€™s really happening with them.â€
Bermudez said that even though eating disorders are serious illnesses â€” â€œwith the highest mortality rate of any mental illnessâ€ â€” there is hope, whether the trigger was extreme veganism or calorie restriction.
Treatment, though, works best with early diagnosis and effective intervention.
Since many people who do calorie restriction see a doctor regularly to make sure they arenâ€™t slipping into malnutrition, these visits could also be a good time to check their mental health.
Asked whether calorie restriction makes sense in people, Bermudez pointed to the lack of long-term studies in humans.
â€œIf I owned a rat or a worm, and I wanted them to live for a long time, I would do calorie restriction for them,â€ said Bermudez. â€œBut I wouldnâ€™t do it for my kids or my family because the data is simply lacking.â€