A large fraction of "agriculture" goes to feeding livestock, inefficiently. Some land is used for animal feed that is not usable for crops people would eat, but the conversion ratio for a lot of it would be better if we grew food for ourselves directly.

Feed is an efficient way of transferring water from wet places to dry places.

The UN report [1] says that if the world population switches to a fully plant-based diet by 2050, accounting for the expected growth in population, we would actually use less land and less water as we do now. So we could in terms of land and water use. Or do you mean some other reason why this would be impossible?

[1] https://www.ipcc.ch/site/assets/uploads/2019/08/4.-SPM_Appro...

I'm also sceptical that the recently proposed diets of fruits, vegetables and nuts actually work out. AFAICT it's an outstanding problem in nutrition science concerning how many of the calories in nuts that humans are able to digest. And yet it seems like the proposed diets just assume "all calories are equal". I've seen reports from journalists who have tried following those diets saying they were constantly hungry.

> I've seen reports from journalists who have tried following those diets saying they were constantly hungry.

This is the inherent problem of journalism - they need sensational results, otherwise the majority of folks wouldn't be bothered to read their journalism.

My 2 cents on this topic: I'm 10+ years vegan and everything is good, it's just no clickbait-worthy story...

Oh, I'm not saying you can't live as a vegan. But the criticism of these diets is they prescribe X grams of this, Y grams of that, and use those numbers to compute a global environmental impact. If the X and Y are both underestimates compared to what an adult needs, then so is the impact.

We tend to count calories in foods via combustion of their components then adding the average caloric content of each gram of fat (9kcal/g), protein (4), alcohol (7), and non-fiber carbohydrate (4) in the average serving size of that food. Keep in mind that US food calories on labels are equivalent to a kilocalorie.

Last I checked, combustion was not one of the stages of digestion in the human digestive tract. I'm not a nutrition researcher, but it's always seemed intuitive to me that different compounds behave differently in a complex, gradual chemical system even if they behave similarly in a simple, rapid high-energy reaction. This is already acknowledged in the fact that dietary fiber is known to be mostly excreted and so isn't counted along with other carbohydrates.

This story is much about a small change by proportion changing several aspects of how digestion works in a mammalian digestive tract, including how efficiently the energy is taken up from other parts of the diet. Now in vitro is not in vivo and cattle aren't humans. But I think we're still far from settled on all calories being the same, and more evidence is building for the opposite conclusion. There are even multiple cases of fecal transplants leading to weight gain or weight loss, with the recipient's digestion suddenly resembling that of the donor causing body weight changes.

https://www.scientificamerican.com/article/how-do-food-manuf...

https://clinicaltrials.gov/ct2/show/NCT03789461

Can you sustainably feed all of humanity by feeding cattle from all the world's grasses? I don't understand your position. What's the better alternative to a more plant-focused diet?

Why not rabbit? It's a much leaner meat, takes less space, and they breed much faster.

1. No. 2. Insects.

Also 3. Less people.