Were Paleolithic hunter-gatherers often in ketosis?

Ketosis is a metabolic state characterized by elevated levels of ketone bodies in the blood or urine. It can occur because of fasting or a high-fat, low-carbohydrate diet. The popularity of the ketogenic (ketones-producing) diet has raised the question of whether a state of ketosis is natural for humans.
Based on my study of Paleolithic hunter-gatherers’ diet and behavior, the answer is yes. A state of ketosis is natural and has probably been common during the Paleolithic. However, reconstruction of the Paleolithic diet can’t tell us if a prolonged period of ketosis, say more than three weeks, is natural for us.
In 2021 Herman Pontzer and his group published a paper in Science (Kraft et al., 2021) showing that humans adopted a High-risk – High-return subsistence strategy. In other words, Paleolithic humans did not attempt to reduce their energetic expenses. Instead, they dedicated their efforts to increasing their energetic income. Although they did not mention it in their paper, this is what hunting added to the human subsistence repertoire.
Studies of recent hunter-gatherers’ groups show that the net energetic return on hunting is 10 times that on gathering. Gathering plants returns, on average, 1400 calories, hunting 14,000 calories, and most probably even more for large animals. The increased risk came from the more erratic nature of hunting than gathering. Prey is a large package of energy that can be obtained relatively cheaply, but hunting is an odds game. On the other hand, the gathering is a low-return-low-cost strategy.
So how humans managed to survive the high-risk strategy? Naturally, for humans, by physiological and cultural adaptations. Humans carry relatively high-fat reserves. Male chimps have nearly zero fat reserves, while human males in the Hadza tribe of Tanzania have an average of 10% fat, allowing about three weeks of fasting. Also, food sharing, and in particular prey sharing, is a common behavior in hunter-gatherers’ groups. Humans can enter quickly into a state of ketosis that supplies ketones to the brain (Cahill Jr, 2006). We see that humans were uniquely adapted to weeks of fasting. Adaptations do not occur or are maintained if they are not useful or, in other words, used.
Was ketosis also prevalent after successful hunts? This is more difficult to tell. Protein (meat) was always more available than fat, so humans consumed quite a lot of protein, close to the limit of 35-40% of the calories in their diet. An elevated level of protein consumption is anti-ketogenic. They also ate plants at times. Are these conditions that can lead to a state of ketosis? Early contact circumpolar populations that survived on meat and fat were not found to be in ketosis (Heinbecker, 1928). However, the technology for measuring ketosis at the time was not great.
In summary, ketosis during fasting was common in societies that relied on hunting but could last up to three weeks while fasting. The occurrence of a high-fat, low-protein diet was not likely. However, we don’t know if a low protein is necessary for ketosis under long-term adaptation to bouts of ketosis.

Cahill Jr, G. F. (2006). Fuel metabolism in starvation. Annual Review of Nutrition, 26, 1-22.

Heinbecker, P. (1928). Studies on the metabolism of Eskimos. JUournal of Biological Chemistry, 80, 451-475.
Kraft, T. S., Venkataraman, V. V., Wallace, I. J., Crittenden, A. N., Holowka, N. B., Stieglitz, J., Harris, J., Raichlen, D. A., Wood, B., Gurven, M., & Pontzer, H. (2021). The energetics of uniquely human subsistence strategies. Science, 374(6575), eabf0130.

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