Fat Monkeys and chubby babies: Part 2
The second part of my celebration of fat is going to take a slightly different approach to part 1 and it will answer the question that’s been on everyone’s mind. Why are human babies so chubby? Maybe that wasn’t on your mind but think about it a second. Picture the new-born babies of other animals. Super cute but not so chubby. In fact, human babies are even fatter than baby seals.
But first, let’s bring it back to evolution. In part 1 I told a story of human evolution and why I think it happened to go that way. This was based on the fields of archeology and paleoanthropology. The other way to think about evolution, however, is by looking at the present.
For example, Daniel Lieberman writes a lot about how humans are ‘born to run’. He mainly talks about features of the human body that make us exceptional long-distance runners and walkers. Namely, our Achilles tendons, big glutes, and ability to stabilise our head in motion among many others. If Lieberman is right then it means at some point in the Homo Sapiens lineage those traits were selected for. It doesn’t necessarily tell us why, but the point is by studying the human body we can learn about the past and about how humans evolved.
Another example that is often talked about is the human gut. As I mentioned in part 1, there are marked differences between the gut of a chimpanzee and our own. They have large hindguts, which are full of fibre-digesting bacteria and microbes as well as a caecum, another fermentative vat. Humans don’t really have a caecum anymore, the large intestine is much smaller, and the small intestine is relatively speaking much bigger. This is where, fats, proteins and sugars are mainly absorbed.
This difference strongly suggests that humans evolved towards a more animal-based diet in the 6 million years since we split from our common ancestor with chimps.
What else might we find in the modern world that suggest that fat was central to making us human?
I think the therapeutic power of ketogenic diets strongly suggests that high-fat diets were a feature of our evolutionary past. Ketogenic diets are any diet that involve eating enough fat and little enough carbohydrates and protein such that the body enters a state of ketosis. This is where the body is primarily burning ketone bodies such as beta-hydroxybutyrate for fuel rather than glucose. This is usually what people are trying to achieve when they fast, however, by eating mostly fat and limiting carbs and protein, humans can remain in ketosis for prolonged periods of time without being in a calorie deficit.
Georgia Ede is a fascinating example of a physician who uses ketogenic diets to heal her patients. She is a psychiatrist and was frustrated by how difficult it was to successfully treat her patients, until she started treating them with diet. She now uses ketogenic diets to help her patients recover from serious mental health conditions, something that was essentially believed to be impossible. There is more and more research coming out proving the efficacy of low-carbohydrate diets as a treatment for conditions as diverse as type-2 diabetes, epilepsy, and Alzheimer's disease.
Could this mean that Ketogenic diets aren’t just a new fad but are instead a more central part of being human? Personally, I think it makes more sense to think of ketosis as our default state, the state we’ve evolved to be in the majority of the time. The fact that most of us usually aren’t is a big contributor to chronic disease in the modern world.
I’ll let you decide if you think this is relevant to our evolutionary past while we move on to talk about why babies are so fat.
Adult humans are also very fat compared with other land-dwelling animals. An extremely lean man (men tend to have lower body fat percentages than women) might reach a body fat percentage of under 10% but this is still way more than a chimpanzee that has a body fat percentage of less than 1%.
Human babies, however, hold considerably more chub than adults and it’s because of the rate of brain growth at this stage in development. Just like fat was the source of nutrition that allowed our brain to grow in during our evolution as a species, it’s also the fuel that supercharges brain growth in infants. Breast milk is of course, full of fats and being born fat means babies have a buffer to make sure our growing brain will have what it needs, even in leaner times.
Children also have a special relationship with ketosis. Babies are in a state of ketosis almost all the time and subsequently, the speed at which we enter ketosis decreases with age. That means that if a 4 year-old, a 10 year-old and an adult all ate a meal that takes them out of ketosis, the 4 year- old will return to ketosis first, then the 10 year-old and much later the adult. That suggests to me that growing brains are particularly hungry for ketones and are well-adapted to getting them. Even as adults we have a special relationship with ketosis compared with other animals. No other animals enter this state regularly or outside of times of starvation. Humans are potentially the only animals that can be well-fed and in ketosis. Dogs, for example, can get into ketosis but only with very severe restriction of protein.
So humans seem to have a special relationship with this state and I think it must have been crucial during our evolution.
If you haven’t tried it already I would urge everyone to try a low-carb or ketogenic diet for a while. It has changed the lives of so many people around the world and is a powerful tonic against the trials and tribulations of modern life.
References:
Bramble, D., Lieberman, D. Endurance running and the evolution of Homo. Nature 432, 345–352 (2004). https://doi.org/10.1038/nature03052
Danan A, Westman EC, Saslow LR and Ede G (2022) The Ketogenic Diet for Refractory Mental Illness: A Retrospective Analysis of 31 Inpatients. Front. Psychiatry 13:951376. doi: 10.3389/fpsyt.2022.951376
https://www.diagnosisdiet.com/full-article/ketogenic-diet
L. Amber O'Hearn - 'The Lipivore: What is Fat for?' https://www.youtube.com/watch?v=xAWReEm4l0w
Paoli A et al 2013. Beyond weight loss: a review of the therapeutic uses of very-low-carbohydate (ketogenic) diets. Eur J Clin Nutr. 67:789-796.
Sourbron, J., Klinkenberg, S., van Kuijk, S.M.J. et al. Ketogenic diet for the treatment of pediatric epilepsy: review and meta-analysis. Childs Nerv Syst 36, 1099–1109 (2020). https://doi.org/10.1007/s00381-020-04578-7