Debates over the benefits and pitfalls of different diets have been around as long as, well, the diets themselves. Is the ketogenic diet a good way to lose weight, or a carb-free trip to bad health? Are vegetarians missing out on vital vitamins? What, exactly, is the omnivore’s dilemma? Can vegans eat sugar? And do paleo adherents actually know what our ancient ancestors ate?
A study published this week in The American Journal of Clinical Nutrition can at least put one diet debate to rest: the climate impact of our choices. Our food system is responsible for a third of global emissions—animal agriculture alone makes up 14%—and our diets could have a significant impact on what those emissions look like in the future.
Using data collected from the U.S Centers for Disease Control and Prevention’s (CDC) National Health and Nutrition Examination Survey of nearly 17,000 American adults, researchers at Tulane University identified six popular American diets—vegan, vegetarian, pescatarian, paleo, keto, and omnivore—and compared them on the basis of environmental impact and nutritional quality. The keto diet, which eschews carbs in favor of fats, was the most carbon-intensive, generating approximately 3 kg of carbon dioxide per every 1,000 calories consumed. The paleo diet, which avoids grains, dairy, and legumes, came in second at 2.6 kg of CO2, while the omnivore diet kicked in at 2.2 kg of CO2.
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The diets with the lowest carbon footprint—vegan, at 0.7 kg of CO2, and vegetarian at 1.2 kg—were also generally healthier than their carbon intense, meat heavy alternatives, says lead author Diego Rose, the nutrition program director at Tulane University’s School of Public Health and Tropical Medicine, calling it “a win-win for the environment and nutrition.” There was one notable exception: the pescatarian diet, at 1.6 kg of CO2 per 1,000 calories, scored highest in terms of nutrition, based on the USDA’s Healthy Eating Index. (The index is scored out of 100; the closer to 100, the healthier the diet.)
The report went on to note that were a third of the nation’s omnivores, who accounted for 86% of survey respondents, to give up meat on any given day, it would be equivalent to eliminating 340 million passenger vehicle miles-worth of carbon emissions. If a third gave up meat for a year, it would amount to 4.9% of total U.S. emissions reduction goals under the Paris climate accord, while substantially improving nutrition quality.
“Giving up meat entirely is the easiest thing to do to improve your [carbon] footprint and your health,” says Rose. “But almost everyone can do better by just eating less of it.” Even climate conscious ketovores and paleos have options, he adds. “Beef has eight to 10 times the climate impact of chicken by weight. So even those on a keto diet could do better if they moved to chicken and eggs for their protein.”
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