Will a diet that's good for the planet also help you live longer? Here's the evidence

Every time you scoop up a spoonful of overnight oats or sink your teeth into a cheeseburger, you're eating for two — for the sake of your own health and the health of the planet.

Researchers estimate that about 30% of greenhouse gas emissions, 40% of land use and 70% of freshwater use is tied to the production of food. The strain will only grow as Earth's population climbs toward the 10 billion mark by 2050.

Will it be possible to provide all those people with a nutritious diet in a way that's environmentally sustainable?

That question prompted an international group of scientists to create a "planetary health diet" that's heavy on plants — including vegetables, fruits, whole grains, nuts, legumes and unsaturated oils from sources like olives and canola — along with modest amounts of dairy, poultry, fish and other foods derived from animals. It also allows for a little bit of red meat, refined grains and sugar. (You can even have a burger about once a week.)

Read more: Your sustainable diet for the year 2050: More nuts, less sugar and red meat

If the whole world were to embrace a diet like this — along with adopting better agricultural practices and reducing food waste — greenhouse gas emissions would be cut roughly in half, the scientists calculated when they introduced their eating plan in 2019. They also projected that the number of premature deaths around the world would fall by up to 24%.

"That amounts to about 11 million deaths per year" that wouldn't happen, said Dr. Walter Willett, a co-chair of the group known as the EAT-Lancet Commission.

Now Willett and his colleagues at Harvard University have checked their work against real-world data.

The Harvard team created a Planetary Health Diet Index, which quantifies the degree to which a person's diet adheres to the goals put forth by the commission. There are 15 food groups, and people were scored on a 5-point or 10-point scale for each one. The maximum possible score was 140, which would signify perfect alignment with the ideal eating plan.

The researchers assigned PDHI scores to more than 200,000 people enrolled in the Nurses' Health Study, the Nurses' Health Study II and the Health Professionals Follow-up Study. All of the participants gave detailed information about their diets when they joined the studies in the 1970s and 80s, and they updated that information at least once every four years for more than two decades.

The women in the two Nurses' Health Studies improved their diets over time: The average index score for participants in NHS1 increased from 75.7 in 1986 to 84.5 in 2010, while the average for women in NHS2 jumped from 70.4 in 1990 to 85.9 in 2015. However, the average score for men in HPFS held steady at around 78.

By the time the tracking periods came to an end in 2019, 54,536 people in the three studies had died.

The researchers hypothesized that the higher a person's PDHI score, the lower their risk of being among the deceased. And after accounting for demographic factors such as age, race and neighborhood income as well as health issues like a family history of cardiovascular disease or cancer, that's exactly what they found.

“We did see a very strong, very clear inverse relationship,” said Willett, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health. “Right down the line, everything we looked at was lower for people who adhered most closely to the planetary health diet.”

Compared to the 20% of people with the lowest index scores, the 20% with the highest scores were 23% less likely to die for any reason during the study period. They were also 14% less likely to die of cardiovascular disease, 10% less likely to die of cancer, 47% less likely to die of a respiratory ailment, 28% less likely to die of a neurodegenerative disease such as Alzheimer's, and 22% less likely to die of an infectious disease.

Among all the men and women, eating more whole grains, fruit, poultry, nuts, soy and unsaturated fats were each associated with a lower risk of death. On the other hand, eating more starchy vegetables like potatoes, red or processed meats, eggs, saturated fats, added sugar or sugar from fruit juices were each associated with a higher risk of death.

Read more: Earth is 'really quite sick now' and in danger zone in nearly all ecological ways, study says

Willett and his collaborators also consulted a database that tallied the environmental impacts of various foods to see whether healthier diets were better for the planet. Compared to the diets of people with the lowest PDHI scores, the diets of those with the highest scores required 21% less fertilizer, 51% less cropland and 13% less water and produced 29% fewer greenhouse gas emissions.

Willett said he was "surprised by the strength of some of these findings," adding that the relationship goes both ways. For instance, when fewer acres are farmed, there's less particulate matter in the air, and when fewer animals are raised in close quarters, the risk of antibiotic resistance declines.

"There are lots of very important indirect effects on health that are mediated by a better environment," he said.

The results were published Monday in the American Journal of Clinical Nutrition.

This isn't the first study to link planetary health diets to a reduced risk of premature death — researchers have seen the connection in the United Kingdom and in Sweden. But the new work is the first to apply a more precise diet index to a large sample of Americans and use it to assess their risk of death.

That is an "important" advance, said Zach Conrad, a professor at William & Mary who specializes in nutritional epidemiology and food systems.

However, he said more work is needed to show that planetary health diets are as good for the Earth as they are for Earthlings.

“It has yet to be demonstrated that healthy diets are also more environmentally sustainable,” said Conrad, who was not involved in the new study. “It is important that we move away from inferring a link between diet quality and sustainability, and instead move toward measuring it.”

This story originally appeared in Los Angeles Times.