Evolutionary Association, Chapter 1: The Composition of Body Fat
Changing the Diet
When dietary guidelines were published (first in the USA in 1977(1), later in many other countries too), they also came with recommendations on fat consumption - and roughly since then we “know"(2) that Fatty acids with no reactive molecular component — normally solid at room temperature. Typically animal fats. → more (SFA) are bad, and Fatty acids with multiple double bonds in the molecule — chemically more reactive than saturated fatty acids. Typically (but not exclusively) plant oils. Omega-3 and omega-6 fall into this category. → more (PUFA) are good. Somewhere in between, and mostly ignored, sit the Fatty acids with one double bond in the molecule — chemically more reactive than saturated fatty acids. Found in both plant oils (olive oil) and animal fats. → more (MUFA)
On top of that, the late 19th century saw the first industrially produced seed oils arrive in larger quantities. At first these were technical oils, used for lubrication and sometimes as fuel, for example in lamps. As electricity arrived, burning lamps became obsolete, and new uses for the oils were sought. They were found in the emerging food industry. The invention of hydrogenation, which allowed the oils to be solidified, opened up further applications - now they could even replace butter.(3)
The Phenomenon
Let me take a moment to first tell the story that motivated me to write this article, and in turn the whole series: a social-media discussion about the “knowledge” mentioned above - that saturated fats are bad.
I wanted to reply: “I hope you never want to lose weight - because then your body has to burn the saturated fats it stored itself.” So evolution had apparently brought our body to the point of storing its energy reserves in harmful substances. I looked into it, and found the following composition of human body fat:
| Fatty acid | Type | Typical share |
|---|---|---|
| Oleic acid | MUFA | 45 – 50 % |
| Palmitic acid | SFA | 20 – 24 % |
| Linoleic acid | PUFA / Omega-6 | 12 – 15 % |
| Palmitoleic acid | MUFA | 5 – 7 % |
| Stearic acid | SFA | 3 – 6 % |
| Others (e.g. myristic acid, Omega-3) | Mixed | Remainder |
That actually gave me the result I was looking for: 23-30% saturated fat in body fat is considerably more than the maximum 10% recommended by current dietary guidelines. So the body evidently does store its reserves for hard times as saturated fat - which means it should not only be able to burn them again, but we can assume this is “the optimal” composition.
And once you also know that the body itself converts some SFA into MUFA(5), we have another data point. Looking at the composition, human body fat resembles that of olive oil.(6)
The PUFA share (12-15%), however, struck me as remarkably high, so I dug further, until I hit a time dependence: this composition holds on average for people living today - who have been told since 1977 that seed oils - PUFA - are healthy, and that saturated fats, for example from butter and meat, should be avoided.
It turns out there are older data sets too: in 2015, Dr Stephan Guyenet examined the composition of body fat in the USA over the previous 50 years(7).
And we learn: the linoleic acid (PUFA) share in body fat rose by a whopping 136% in just 50 years.
This growth in omega-6 also shifts the ratio of omega-6 to omega-3 - a shift whose consequences reach far beyond fat storage, all the way into behaviour and the psyche. More on that in Chapter 3 (Mental Crises).
The Biochemical Association
Several associations stand out to me:
The parallel rise of metabolic diseases
In parallel with the arrival of seed oils - per-capita soybean oil consumption in the USA rose more than a thousandfold between 1909 and 1999(8) - metabolic diseases such as diabetes and obesity also rose sharply. Such a temporal parallel is not proof, however: many things changed over the same period (sugar, calorie intake, lack of exercise), and correlation is not causation. So it remains an association - which is exactly what this series is about.
Smoking and the once-common trans fats are often cited as competing explanations. But neither works well as the sole cause: the share of adult smokers in the USA fell from 42.4% (1965) to 11.6% (2022)(9) without metabolic diseases declining accordingly; and artificial trans fats have long been known to be harmful and are now banned (Denmark as early as 2003, EU-wide since 2021)(10) - while the seed oils themselves remain untouched.
Sun protection
The mass introduction of PUFAs also coincided with the age of sunscreen. Evolutionarily, humans did not live in houses, behind glass, in front of screens, but were exposed to the sun for large parts of the day - without constantly suffering sunburn.
Since many people living ketogenically, who avoid PUFA, report the same experience, and since there is research in this area, I’ll point to my own experience with the sun: after some time avoiding seed oils, my sun tolerance changed massively (studies are linked in that article). The bad news: reproducing this experiment takes a while - fatty acids in human tissue have a half-life of about 600 days, meaning it takes a few years for significant amounts of PUFA to be broken down once you stop eating them.
My experiences with the sun
Sunburn and UV
A surprising side effect of switching to a ketogenic diet: a new built-in sun protection factor
Read more →The Mechanism
Let’s look at a bit of biochemistry:
The liver produces fats via de-novo lipogenesis. The base product is palmitic acid, with a melting point of 63°C. Since that melting point is a problem in a body at 37°C, further fats are made from it - the end result is a mix of palmitic acid, stearic acid (both SFA), plus palmitoleic acid and oleic acid (both MUFA). Mixed like this and packed into triglycerides, the result is a fat with a consistency the body can process, move and distribute.
What the liver can never make is PUFA. So when it shows up in body fat, it must come from the diet. Look at traditional peoples who still eat in their original way, and their body fat has a strikingly low PUFA share: among the Kitava islanders (Papua New Guinea, studied by Staffan Lindeberg), for instance, linoleic acid intake is only around 1-3% of energy, and their tissue contains correspondingly little PUFA - compared with the 12-21% we saw in modern body fat above(11).
It shows up especially strikingly in breast milk: its linoleic acid content has roughly doubled in Western countries over the last century - and about 70% of it comes from the mother’s fat stores(12). So the food diary in the body fat is passed on directly: to the child.
How fast this diary writes is shown by a 1959 experiment: on an extremely seed-oil-rich diet, the linoleic acid share of breast milk rose from 8-10% to 42% within just 2-3 days(13). 🤯
Another important aspect is the properties of PUFA: “polyunsaturated” means there are several reactive sites in the molecule - and that’s where the molecule can oxidise. For example through heat, light, or other substances it comes into contact with. We know oxidised oil as “rancid”. Omega-6 fats are Essential nutrients are those that the body cannot synthesise itself and must obtain through food – vitamins, essential amino acids, and fatty acids. fats, but in the Western diet they are consumed in far greater amounts than the body needs. Omega-6 fats are pro-inflammatory (important for the immune system), omega-3 fats are anti-inflammatory. In today’s typical diet we are massively oversupplied with omega-6 fats - especially in relation to omega-3 fats.
This pro-inflammatory property is one of the reasons the oversupply can plausibly be regarded as at least “suspect” in this association.
In any case, since turning away from the current dietary guidelines - and the associated avoidance of PUFA from seed oils - I have had no bad experiences. My personal well-being and the markers measured by my doctor all point in the right direction. If anyone holds apocalyptic expectations in this case, I’d be curious in what time frame they typically materialise (14 years don’t seem to be enough), and how they show up.
