Re-evaluating The Risks Associated With Saturated Fats

The Dietary Guidelines for America (DGA) 2020–2025 state that saturated fat should be limited to less than 10% of total calories for all people older than two years.¹

Since 1980, the DGA has recommended that we reduce our consumption of saturated fats. The reasoning behind this advice is the diet-lipid-heart hypothesis, that excess intake of saturated fat leads to coronary heart disease. Forty years later, this hypothesis has been disproved. Armed with the most up-to-date knowledge about nutrition, fats and heart disease, scientists in 2024 say saturated fats do not cause heart disease.

The diet-lipid-heart hypothesis: the creation of a myth 

You have probably heard it countless times: ‘Avoid saturated fats because they cause heart disease!’ Why would a recommendation exist for decades almost unchanged if it were not true? Let’s take a look at its beginnings.

The story started with Ancel Keys, a biologist and physiologist, who pieced together several observations in the 1950s.^{2, 3, 4} He noticed that the number of heart attacks in the US was much higher than in Europe. He found the lowest rates of heart disease in the European countries that ate a diet low in saturated fat. Americans eat more fat than Europeans, and because eating fat raises blood cholesterol, he speculated that this could explain their higher amounts of heart disease.

Keys combined these observations into the diet-lipid-heart hypothesis: a diet high in saturated fat raises blood cholesterol. The cholesterol clogs the arteries, which increases the risk for heart disease. This captivating theory not only links saturated fats and heart disease but also proposes a mechanism for how fat works in the body.

Re-examing Research

Although Keys conducted a lot of experiments, stating that heart disease is a direct consequence of eating too much saturated fat is a stretch. Given the huge number of factors that can influence heart disease (for example smoking, a substantial risk factor for heart attacks, increased dramatically during this period), researchers today explain that Keys’ evidence was over interpreted. Still, in 1961 the American Heart Association adopted this idea and coined the longest lasting dietary recommendation of all time: limit saturated fats.

 The most convincing support for Keys hypothesis came ten years later from his seven-country study, in which he analysed heart disease and dietary patterns in Italy, Greece, Yugoslavia, Finland, the Netherlands, the United States and Japan.⁵ His conclusion: Saturated fat increases the risk for heart disease, while unsaturated fats (found more abundantly in plant-based oils) were considered beneficial for heart health.

The seven-country study by Ancel Keys cemented the idea that saturated fats are bad, however, Keys’ study had problems. Keys had handpicked the seven countries that fit his hypothesis, excluding others that didn’t. What’s more, the data from Greece were collected during lent. Lent is a religious holiday lasting 40 days wherein people eat a more restricted diet than normal, including consuming less meat. The study also focused solely on men, ignoring half the population. Later analyses in these countries found a stronger correlation between heart disease and sugar intake than with saturated fat.⁶

Where the Evidence Stands Today

Since then, countless studies have assessed this relationship. The largest investigation ever conducted – the PURE study – followed almost 140,000 people on five continents for ten years.⁷ A higher intake of saturated fats, it turned out, was not correlated with heart disease. The researchers noted instead, that heart disease actually correlated with a higher carbohydrate intake. Moreover, people who ate high amounts of saturated fat lived longer and had a lower risk for stroke. A 2010 review of 21 reports covering around 347,000 subjects confirmed that dietary saturated fats do not increase the risk of heart disease.⁸

Similarly, in 2017, another analysis of multiple studies confirmed that eating saturated fats do not increase the chances that a person would die from heart disease, with the authors advocating for an update of dietary guidelines.⁹ Astonishingly, 88% of the studies used by the 2020 DGA to support its advice to limit saturated fats, do not, in fact, support that conclusion.¹⁰

Even large-scale reviews promoting a link between saturated fat intake and heart disease, did not find any relation between eating saturated fats and a higher risk for heart attack or stroke or earlier death. They only found that saturated fat intake can make it more likely to have any issue related to the cardiovascular system, a conclusion based on what they refer to as ’moderate-quality data’.¹¹
 

 Rehabilitating Fat

Over the years, fat has gained a bad reputation, it’s an attractive idea that easily sticks. Fats are complicated molecules to understand and even more complicated to explain: saturated, polyunsaturated, trans, omega are enigmatic terms for most of us (see Infobox). This has made it easy for simple narratives like ‘bad’ animal fats and ’good’ vegetable oils to take hold. As a result, the US has made successful efforts to reduce the amount of animal-based fats in food over the last 60 years. For example, availability of butter, lard, and tallow has halved, while availability of plant-based fats, like vegetable shortening, margarine, vegetable oils has tripled.¹²

However, for decades the American food industry has offered ‘healthy’ vegetable oils that were partially hydrogenated to make them more solid and more suited for baking and frying. This proved to be a fatal error. As of 2021, the FDA has banned these partially hydrogenated vegetable oils because they are considered a serious health hazard.¹³

What Is a 'Saturated' Fat?

Fats consist of fatty acids. The graphic shows three fatty acids that all occur in soybean oil. Fatty acids are made up of long chains of carbon atoms (C). Each carbon atom can make four bonds. If each carbon atom is connected through a single bond to other carbon atoms and hydrogen atoms (H), the fat is fully saturated. No more atoms can be added. If a carbon atom establishes a double bond with another carbon atom (red arrow) instead of a single bond with hydrogen, the fatty acid is called unsaturated. Potentially, a hydrogen could still be added. A fatty acid is called monounsaturated, if it contains one double bond; polyunsaturated, if it has two or more double bonds.

Fat is an essential nutrient that is not only used for energy but also as a building material in the body. For example, we employ fats to build hormones or membranes of cells. Saturated fats are especially needed in the brain, to form the myelin that wraps around nerve cells to increase the speed with which nerve cells can communicate. We also need fat tissue in our bodies to keep our organs in place and to prevent them from sagging – this is why we have visceral fat. 

No Absolutes in Nature

Not all dietary fat is bad; some fats are even essential to our diet. In scientific terms, this means our bodies need them but cannot produce them on their own. We need to get these vital nutrients from food, and that’s where the difficulties lie. How much is too much?  
The demands of 21st century industrial food production place heavy emphasis on consistency, shelf life and conforming to nutritional labelling standards.

Man-made versions of fats (trans fats, hydrogenated fats) and carbohydrates (sugar substitutes such as aspartame or sucralose) provide an easy way for manufacturers to be transparent about fat and sugar content. In contrast, natural fats are never 100% one thing or another, after all we have more than one type of fat in our body–different fats have different purposes. As with most things in nature there are few absolutes. We need fats but for a balanced diet, natural fats are best suited to our bodies’ needs. Animal fats are not 100% saturated and plenty of plants are rich in saturated fats. Nature does not produce saturated animal fats and unsaturated plant oils. Beef fat is 54% unsaturated, chicken fat is 70% unsaturated while coconut oil is 90% saturated.¹⁴ 

Finding a Balance

In contrast, industrially processed fats can be created to exact composition of saturated or unsaturated fats. For example, 100% saturated palm oil is marketed as fully hydrogenated oil perfect for baking. Maintaining arbitrary cut-offs for saturated fats at 10% is not compatible with eating a natural and balanced diet. Instead it supports a synthetic diet, in which processed foods with mathematically precise ratios of ingredients promise the illusion of optimal nutrition. Looking at food-related diseases, the last 60 years have not been triumphant for the US. Obesity rates have doubled; rates of diabetes and metabolic syndrome have never been higher.¹⁵

Scientific studies from the last decades have given the green light for a higher fat consumption. We will see if the next DGA edition covering 2025 to 2030 changes its recommendations for fats or adheres to an outdated idea held up only by habit rather than current scientific evidence.

References

1. https://www.dietaryguidelines.gov/sites/default/files/2020-12/Dietary_Guidelines_for_Americans_2020-2025.pdf
2. Keys A. Atherosclerosis: a problem in newer public health. J Mt Sinai Hosp N Y. 1953 Jul-Aug;20(2):118-39. PMID: 13,085,148.
3. Keys A. Prediction and possible prevention of coronary disease. Am J Public Health Nations Health. 1953 Nov;43(11):1399–1407. doi: 10.2105/ajph.43.11.1399. PMID: 13,104,699; PMCID: PMC1620415.
4. Keys A, Vivanco F, Minon JL, Keys MH, Mendoza HC. Studies on the diet, body fatness and serum cholesterol in Madrid, Spain. Metabolism. 1954 May;3(3):195–212. PMID: 13,165,043.
5. Keys, A. (Ed.) (1970) Coronary Heart Disease in Seven Countries. Circulation, 41, 1–211.
6. Menotti A, Kromhout D, Blackburn H, et al. Food intake patterns and 25-year mortality from coronary heart disease: cross-cultural correlations in the Seven Countries Study. Eur J Epidemiol 1999; 15:507 – 515.
7. Dehghan M, Mente A, Zhang X, et al. Associations of fats and carbohydrate intake with cardiovascular disease and mortality in 18 countries from five continents (PURE): a prospective cohort study.Lancet2017;390:2050 – 2062.
8. Siri-Tarino PW, Sun Q, Hu FB, Krauss R M. Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease. Am J Clin Nutr. 2010 Mar;91(3):535-46. doi: 10.3945/ajcn.2009.27725.
9. Harcombe Z, Baker JS, Davies B. Evidence from prospective cohort studies does not support current dietary fat guidelines: a systematic review and meta-analysis. Br J Sports Med. 2017 Dec;51(24):1743–1749. doi: 10.1136/bjsports-2016-096550. Epub 2016 Oct 3. PMID: 27,697,938.
10. Astrup A, Teicholz N, Magkos F, et al. Dietary saturated fats and health: are the U.S. guidelines evidence-based? Nutrients 2021; 13:3305.
11. Hooper L, Martin N, Jimoh OF, Kirk C, Foster E & Abdelhamid AS. Reduction in saturated fat intake for cardiovascular disease. Cochrane Database Syst Rev. 2020;5:CD011737.
12. Lee JH, Duster M, Roberts T, Devinsky O. United States Dietary Trends Since 1800: Lack of Association Between Saturated Fatty Acid Consumption and Non-communicable Diseases. Front Nutr. 2022 Jan 13;8:748847. doi: 10.3389/fnut.2021.748847.
13. https://www.fda.gov/food/cfsan-constituent-updates/fda-completes-final-administrative-actions-partially-hydrogenated-oils-foods
14. Enig, M G. (2000) Know your fats: the complete primer for understanding the nutrition of fats, oils and cholesterol. Silver Spring, MD: Bethesda Press
15. Newport MT, Dayrit FM. The Lipid-Heart Hypothesis and the Keys Equation Defined the Dietary Guidelines but Ignored the Impact of Trans-Fat and High Linoleic Acid Consumption. Nutrients. 2024 May 11;16(10):1447. doi: 10.3390/nu16101447.