Nutrition: Dietary fat — why the low-fat era was wrong, and what healthy fats actually do

For thirty years, the dietary advice was consistent and confident: eat less fat. It turned out to be one of the most consequential errors in the history of public health.

Why it matters

How fat became the villain

In the 1950s, the American physiologist Ancel Keys proposed that dietary saturated fat raised blood cholesterol and drove heart disease. His Seven Countries Study appeared to support the link.¹ The research had significant methodological flaws — Keys selectively chose countries whose data fit his hypothesis, ignoring others where it did not — but the hypothesis was compelling, well-timed, and politically convenient.

What came next was one of the most consequential — and, in hindsight, most illuminating — episodes in nutrition science. Internal documents later obtained through legal action revealed that in 1965, the Sugar Research Foundation (now the Sugar Association) paid Harvard researchers to publish a review that downplayed sugar's role in heart disease and redirected attention to fat and cholesterol.² The review was published in the New England Journal of Medicine without disclosure of the industry funding. One of the authors, D. Mark Hegsted, went on to help shape the 1977 US Dietary Goals — the government guidelines that told Americans to cut fat and increase carbohydrate intake. The food industry replaced fat in products with refined carbohydrates and added sugar, and labelled the results "low-fat" and "healthy."

This is not a conspiracy theory. It is documented history, published in JAMA Internal Medicine in 2016, based on internal Sugar Research Foundation records.² The lesson is not that all nutrition research is corrupted — it is that funding sources matter, that consensus can form around incomplete evidence, and that large-scale dietary advice affects millions of people before the errors are corrected.

What the evidence actually shows

Fat is not a single nutrient. The category includes saturated fats, monounsaturated fats, polyunsaturated fats (including omega-3 and omega-6 fatty acids), and trans fats — and these behave very differently in the body.

Trans fats (artificially produced by hydrogenating vegetable oils, found in many processed foods prior to regulatory action) are genuinely harmful: they raise LDL cholesterol, lower HDL cholesterol, and are associated with a significant increase in cardiovascular disease risk.³ Most countries have now banned or heavily restricted them.

Saturated fats (found in red meat, butter, dairy, coconut oil) raise LDL cholesterol — but the relationship with cardiovascular outcomes is more complex than originally believed. A 2020 systematic review of 35 randomised trials found that replacing saturated fat with refined carbohydrates did not reduce cardiovascular events, while replacing it with polyunsaturated fats modestly reduced risk.⁴ The source of the replacement matters enormously.

Monounsaturated fats (olive oil, avocados, nuts) are consistently associated with better cardiovascular outcomes and reduced inflammation. The Mediterranean diet — which is relatively high in fat, principally from olive oil and fish — is among the best-evidenced dietary patterns for longevity.⁵

Omega-3 polyunsaturated fats (oily fish, walnuts, flaxseed) are anti-inflammatory, associated with reduced cardiovascular risk, and linked to better cognitive function in later life. The omega-3 to omega-6 ratio matters — modern Western diets tend to be high in omega-6 (seed oils, processed foods) and low in omega-3, which promotes inflammation.

Dietary fat and satiety — fat slows gastric emptying and is highly satiating. Removing it from foods tends to increase consumption of refined carbohydrates and added sugar, which drive insulin spikes, fat storage, and hunger cycles in a way that whole food fat sources do not.

The conclusion from the contemporary evidence is clear: dietary fat is not the problem. The quality of the fat — and critically, what replaces it when it is removed — is what matters.

How to improve it

• Prioritise olive oil as your primary cooking fat — extra-virgin olive oil is rich in monounsaturated fat and polyphenols, and its health benefits are among the most robustly evidenced in nutrition research. Use it for cooking at low-to-medium heat and as a dressing.
• Eat oily fish two to three times per week — salmon, mackerel, sardines, and anchovies are the richest dietary sources of omega-3 fatty acids. If fish is not part of your diet, consider an algae-based omega-3 supplement (the source from which fish derive their omega-3s).
• Include nuts and avocado regularly — both are calorie-dense but deliver high-quality monounsaturated fats alongside fibre, micronutrients, and protein. Walnuts in particular are a meaningful plant source of omega-3s.
• Stop avoiding full-fat dairy if you enjoy it — the evidence that full-fat dairy causes harm has weakened substantially. Some studies suggest fermented full-fat dairy (yoghurt, cheese) may be modestly protective. The case for low-fat dairy on health grounds is much weaker than it was in 1990.
• Read "low-fat" labels with scepticism — low-fat processed foods routinely replace fat with added sugar, refined starch, or both to restore palatability. Check the ingredients, not just the fat content.
• Reduce seed oil consumption where practical — refined seed oils (sunflower, corn, soybean) are high in omega-6 and prevalent in processed and restaurant food. Reducing processed food consumption reduces omega-6 intake without requiring detailed calculation.
• Eliminate trans fats from your diet — check ingredient labels for "partially hydrogenated oils." Though heavily regulated, some processed foods (particularly imported or artisan baked goods) still contain them.
• Do not fear fat in whole foods — eggs, nuts, seeds, avocado, and oily fish are whole foods where fat is an integral part of a complete nutritional package. The goal is not to count fat grams; it is to favour whole food fat sources over industrial ones.

The 100 Great Years perspective

The low-fat era is a useful reminder that dietary consensus can be wrong, and that the incentives shaping nutritional research are not always aligned with public health. 100 Great Years takes a whole-food framing to nutrition precisely because it side-steps much of this noise: if your diet is built primarily around minimally processed food — which naturally includes healthy fats in their whole-food context — the question of which specific fats to optimise becomes secondary. The more actionable question is not "how much fat?" but "how much of your diet comes from foods that have been industrially processed?" The NOVA score in the Nutrition tracker is designed to answer that question directly.

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This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making decisions about your health.