Keto dieters who fuel up on bacon and butter are 'irresponsible' stewards of the planet, a Harvard nutrition expert says

• The keto diet limit's a person's carbohydrate intake to about 5% of daily calories. Dieters load up on anywhere from 70-90% fats.
• Keto dieters can gorge on all kinds of fat. Foods heavy in unsaturated fat include avocados, olive oil, and fish, while there are more saturated fats in butter, bacon, and cream.
• There's evidence that diets high in saturated fat (often from animal sources) are really bad for people, and for the planet, too.
• Long-term studies of the effects of keto diets on overall health are scarce.
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The world cannot run on bacon and butter.

Aside from the fact that there are not enough pigs and cows on the Earth to feed every person in such a high-fat way, this kind of meaty diet is dangerous for both human health and our planet's future.

That doesn't stop people from trying.

"Eating a keto diet that's especially high in red meat will be undermining the sustainability of the climate," Harvard nutrition professor Dr. Walter Willett told Business Insider. "It's bad for the person eating it, but also really bad for our children and our grandchildren, so that's something I think we should totally, strongly advise against. It's - in fact - irresponsible."

Keto diets run on all different kinds of fat

Trendy keto diets are designed to make people run on fat, strictly limiting a person's carbohydrates,so that the body switches into a fat-burning state called "ketosis." In practice, this usually means no sugar, no wheat, no beans, and very little alcohol. Sometimes, it also means a lot of red meat.

But there isn't a lot of evidence about whether fueling up on more saturated fats - bacon, butter, and cream are traditionally linked to health conditions like heart disease and higher cholesterol - might harm the long-term health of keto dieters. (That hasn't stopped the keto diet market from exploding: annual keto-friendly food sales are projected to skyrocket to a $12.35 billion market worldwide by 2024, and butter sales, especially of fancy versions like those from grass-fed cows, are on the rise too.)

Willett says he thinks people can be healthy on a keto diet, and that one of the best ways to go high fat may be to model the Greeks.

"It's pretty easy when you're in some place like Israel or Greece with so many good vegetables and healthy oils and fish to have a pretty healthy low carbohydrate diet," he said.

Beef consumption isn't good for the Earth

Both keto aficionados and Willett agree: there's something wrong with the way most people are eating. We could all stand to replace refined carbohydrates like white bread, snack cakes, and sugary drinks with more good fats and fresh produce.

But replacing carbs with more saturated fats doesn't always align with a planet-friendly way of eating.

Unlike olive oil, many fat sources that are solid at room temperature, like bacon fat, butter, and beef tallow, come from animals, not plants. To cultivate those fat sources, farmers need a fair amount of real estate for cows and pigs to roam.

A United Nations report released just last week suggests the world's beef-heavy consumptions patterns are taking a serious toll on the health of our planet: food systems are now responsible for 37% of greenhouse gas emissions, and cow manure is a major part of that equation, as it releases large amounts of climate-changing nitrous oxide and methane into the air.

"Diets that are rich in plant-based food emit lower greenhouse-gas emissions than diets that are very heavy in red meat consumption," UN report co-author Cynthia Rosenzweig said during a press conference.

There are things eaters can do to change that. A report Willett himself authored for the EAT Lancet commission earlier this year suggested that people around the globe should double their intake of "fruits, vegetables, nuts and legumes " while cutting red meat and sugar intake by at least 50% in order for the globe to remain healthy and well-fed.

Willett doesn't suggest that meat eaters must quit eating animals cold turkey. Even some of his own recipes, which include a mushroom meatloaf with beef and turkey, feature small amounts of red meats. But he does recommend people consume less farm-fresh cow.

"We're racing down a path that is going to lead to destruction of viable environments over the next hundred years or so, and we have to get off that path," Willett said. "That means limiting substantially - not totally eliminating - but greatly reducing our consumption of red meat and dairy foods."

Earlier this month, scientists calculated that if everyone in the US stopped eating beef completely, the carbon footprint benefit would be equivalent to taking 60,000 cars off the road.

"Today, with the state of the earth in dangerous shape, we do have look at virtually everything we do from an environmental health lens, as well as a direct human health lens," Willett said. "Because humans can't be healthy if we don't have a healthy environment."