Consider the almond.
Almonds and other nuts are often touted as healthy snacks, because they can help you maintain a healthy weight and are linked to a lower risk of heart disease and Type 2 diabetes.
But almonds are grown in drought-stricken California, and the amount of water required to produce them has come under intense scrutiny in recent years. So if you're an environmentally minded eater who also wants to embrace a healthy diet, are almonds a responsibly green snack?
Relatively speaking, yes, says ecologist David Tilman of the University of Minnesota.
In a vast new analysis published Monday in the Proceedings of the National Academy of Sciences, Tilman and his co-authors looked at the health and environmental impacts of 15 different food groups, including nuts, fruits, vegetables, red meat, dairy, eggs, fish, olive oil, legumes and sugar-sweetened beverages.
The foods were ranked relative to one another based on how they influence the risk of disease and the toll they take on the planet in terms of water and land use, greenhouse gas emissions and how they impact pollution of water and soil.
Most of the time, the researchers found that foods that promote good health also tended to be better for the planet — and vice versa. While nuts require lots of water to produce, Tilman says, water was just one factor that affected their environmental ranking.
"If water is going to be used to irrigate crops, it would seem better for it to be used to grow healthy crops," he says. Producing a serving of nuts has about five times the negative effects on the environment compared with producing a serving of vegetables, according to the study.
That may sound like a lot, until you compare that to red meat; both processed and unprocessed, it's "uniformly bad," Tilman says. Producing a serving of processed red meat, the researchers found, has about 40 times the negative environmental impact of producing a serving of vegetables – and eating an extra daily serving raises the relative risk of overall mortality by 40 percent.
"That doesn't mean you're going to die with a 40 percent chance in a given year," Tilman notes. "It just means whatever your chance was of dying that year for your age, [the relative risk is] about 40 percent larger."
However, just because a food is bad for us doesn't always mean it's bad for the planet. Sugary beverages, for instance, have been linked to an increased risk of Type 2 diabetes, coronary heart disease and stroke, but the study found their environmental impact isn't much more than that of growing vegetables. On the flip side, fish consumption is associated with a lower risk of several diseases, but it's not as great for the planet as a plant-based diet.
That said, Tilman notes that how a fish is caught or grown matters a lot. Fish caught by trawlers in the open ocean have a much higher environmental impact because these boats use "lots of diesel fuel for not a lot of fish," he says in an email. "Fish such as tuna and salmon caught on lines or with seine nets near the surface, and aquaculture fish such as salmon, steelhead, catfish and tilapia grown in ponds, lochs, fjords and ocean cages have moderate greenhouse gas emissions per serving that are about 6 times those of the typical plant-based foods."
To reach their conclusions on diet and health, the researchers looked at 19 previous meta-analyses that followed millions of people over time, mostly in Western nations. They used that data to calculate how eating an extra serving of a given food each day affected the relative risk of colorectal cancer, coronary heart disease, stroke and Type 2 diabetes, as well as overall mortality. Data on the environmental impact of food was derived from life cycle analyses, which looked at the land, equipment and other resources required to grow or raise a food.
"This is a useful study because it aims to compare, using similar and consistent methods, how different foods influence the joint health of humans and the planet," says Dariush Mozaffarian, the dean of the Friedman School of Nutrition at Tufts University.
Despite recent controversy over the science on red meat and health, these findings are in line with the overall body of evidence that suggests that cutting back on processed red meat is a healthy choice, he says. And if you do eat red meat, he says, how the cattle was raised matters.
Jessica Fanzo, a professor of global food and agriculture policy at Johns Hopkins University, who was not involved in the current research, says that the big takeaway message for consumers is this. "If you want to care about the environment and your own health, eating less red and processed meat is key." And, she says, if you substitute something like fish, "think a little bit more about how those are sourced and how they're raised."