Healthy and Unhealthy Saturated Fats: Process Contaminants May Explain Dietary Fat Responses

The healthfulness of saturated fats, primarily composed of saturated fatty acids, dates to the 1800s and remains deeply controversial since modern scientific methods began to be applied in the 1950s. That history suggests that underlying factors that are unrelated to the saturated fatty acid content are major determinants of the healthiness of any particular fat.

In contrast to the controversy over saturated fats, nutrition scientists are nearly unanimous in regarding partially hydrogenated vegetable oils (PVHO), usually called “trans fats”, as unhealthy. A 2017 advisory of the American Heart Association (Sacks, 2017) invoked four human studies (Oslo, London, Helsinki, Los Angeles) as the core evidence conducted in the 1960s and 70s that were said to compare control diets with no trans fatty acids to those with soybean oil. Careful examination of the diets reveals that the controls were rich in industrial trans fats and thus not relevant to saturated fats such as butter/ghee/dairy, tallow, lard, and other unprocessed saturated fats (Astrup, JACC 2020).

Chemical treatment of oils – processing – is increasingly recognized to result in process contaminants that are related to development of chronic disease.  Though process contaminants are better studied as carcinogens, some evidence suggests that they contribute to CVD. The most harshly processed oils of the 1970s were coconut and palm kernel oils, recognized at the time as causing hypercholesteremia. In contrast, the gently processed cocoa butter, also high in saturated fatty acids, is not associated with CVD.

Another potential explanation for the divergence of study outcomes are genotypic differences that render some individuals more susceptible to ill effects of specific saturated fatty acids or the unsaturated fatty acids that they replace in the diet, chiefly linoleic acid.  Emerging evidence suggests that a widely prevalent genetic polymorphism drives some cancers and may be related to imbalanced PUFA intake.