For most of my adulthood, having fat in the diet was considered not a good thing. Due to the hypothesized link of dietary saturated fat and cholesterol to diabetes, atherosclerosis, and heart disease (Astrup 2021, Ludwig 2018). “Low fat” diets were considered healthy, but to replace the fats in foods, sugars were added. Because of the consequences of high carbohydrate (especially refined carbs such as sucrose and fructose) on appetite and glucose regulation, this has been associated with increases in obesity, disease, and cardiometabolic disease.
But now the pendulum has swung away from carbs and toward fats and protein. High-fat diets, such as the “keto,” “carnivore,” and “lion diets,” advocate for reduced or no dietary carbohydrates, and thus a high fat and protein intake. But how safe is a high fat diet?
And what are we even talking about when we are talking about fats? First of all: not all fats are created equal (Ludwig 2018, Deen 2021). To understand the benefits and risks of fats, it is important to understand the differences.
Here is some background to understand. There are three main kinds of differences:
- Saturated or unsaturated: Fats are chains of carbons and hydrogens. Saturated fats have as many hydrogens as the carbon chain can take (for fuller explanation of this, you can refer to my book) and are very stable. They store a lot of energy, which is why they are the main fat in adipose tissue. Unsaturated fats are missing one or more hydrogens, so carbon atoms have to share. This makes them less stable than saturated fats, but more flexible. In the body they are mostly found in cell membranes. So basically, the structure of the fats determines their functions.
- Chain length: short, medium, or long: Chain length refers to how many carbons make up the chain. Short chain fats have fewer than 6 carbons, and the main ones (such as butyrate) are made by our gut microbes when fermenting fiber (Du 2025). Medium chain fats, such as coconut oil and many dairy fats, have between 6 and 12 carbons. Long chain fats are more than 12 carbons long and are found in “red meat” (beef, pork, etc.), some dairy fats, and palm oil. Although there is still a need for good research, what we do know points to differing effects of fats of different chain lengths. Short chain fats are necessary for the gut and help control inflammation (Wang 2024). They, like medium chain fats (Dunn 2023, Perna 2022), seem to be either neutral or protective for cardiometabolic disease, aging, and brain-related disorders. In contrast, long chain saturated fats may increase cardiometabolic disease risk.
- Oxidized fats and industrial “trans” unsaturated fats: These are the only truly “bad fats” (Folahan 2023, Miller 2017, Oteng 2020). Oxidized fats are ones that are damaged, usually by having a hydrogen “stolen” during inflammation, or by being heated, especially in the presence of oxygen. This process happens sooner for unsaturated fats (vegetable oils), but saturated fats (lard, tallow, butter, coconut, palm) will also be oxidized (Li 2027); it just takes a little longer. The damaging of the fats is a problem because when damaged the fats cannot do their jobs, and they can become pro-inflammatory.
Trans fats are polyunsaturated fats that are mostly produced through chemical processing. They have a lone hydrogen on the opposite side of “native” cis unsaturated fats (Oteng 2020), so their structure is subtly different than native unsaturated fats. They originally were produced as an alternative to saturated fats, in an effort to be more “heart healthy.” But it turns out that they aren’t as “heart healthy” as native unsaturated fats (Oteng 2020), and there have been suggestions that trans fats can promote fat storage in the liver (fatty liver) and induce inflammation. Trans fats can be naturally produced by microbes, and it is reported that dairy products and meat from cows can contain natural trans fats (Oteng 2020). However, the naturally produced trans fats do not seem to have the same deleterious effects as the industrial ones (Oteng 2020), but no one knows why.
What do we know now that can help us decide which fats to eat or cook with?
Overall, our bodies need more unsaturated fats than saturated fats, so it’s best to consume saturated fats sparingly. We can make most fats ourselves, but we must get “essential” fats, omega 3 and 6 polyunsaturated fats, from our diets. Both are precursors for important molecules in our bodies, and we can get them from fish, nuts, and vegetable oils. Green vegetables, such as chard, kale, and lettuce, also have essential omega 3 & 6 fats (Meyer 2003), which may be one reason diets with green leafy vegetables seem to be protective against a wide variety of disease conditions (Wallace 2020), including cancer, immune disorders, and brain-related conditions including cognitive impairment and depression.
Short chain fats, such as butyrate, are not yet classified as “essential” even though we need them for gut health and immune regulation, but we do not make them ourselves. We get butyrate from some of the microbes in our guts (Morrison 2016), and a small amount is found in dairy fats. Probiotic fermented foods can contain the microbes that make short chain fats, and we can eat plenty of fiber to support them. This is probably the main reason why fiber is so important in the diet.
There are often recommendations for cooking only with saturated fats and using vegetable oils only for cold food, but I still prefer my extra virgin olive oil for roasting and sautéing. I’m not heating the oil for very long, and looking at the data it is clear that although a little oil is getting oxidized by 15–30 minutes (Li 2017, Song 2027), it increases more as heating continues. So I keep my cooking times short, and on medium rather than high heat.
Industrial trans fats, such as in margarine, should probably be avoided, but natural trans fats (such as the small amount that can be found in dairy) seem to be okay (Oteng 2020), so we needn’t worry about them.