For most of my pharmacy career, the advice was consistent: avoid fat, especially saturated fat. Eat low-fat. Substitute butter with margarine. Use vegetable oil instead of lard.
The science behind that advice was weaker than I realised at the time. Over the past two decades, the evidence has shifted significantly. What we were told about fat and disease risk turns out to have been largely wrong — and the substitutions we made may have caused more harm than the foods they replaced.
The Original Hypothesis
The diet-heart hypothesis — that saturated fat raises cholesterol and cholesterol causes heart disease — was proposed by Ancel Keys in the 1950s. It shaped dietary guidelines for the next 60 years. It was also built on selective data. Keys had data from 22 countries. He published results from 7 — the ones that supported his conclusion.
Subsequent research has repeatedly failed to confirm a strong causal link between saturated fat intake and cardiovascular events in healthy populations. The 2010 meta-analysis by Siri-Tarino et al., published in the American Journal of Clinical Nutrition, analysed data from 21 studies covering 347,000 subjects and found no significant association between saturated fat and cardiovascular disease.
This does not mean saturated fat is without any effect. It means the relationship is more complex than a simple "eat this, get heart disease" model.
The Fats That Actually Cause Problems
While saturated fat was being demonised, two genuinely problematic categories were largely ignored: trans fats and industrial seed oils.
Trans fats — found in partially hydrogenated vegetable oils, margarine, and many processed foods — are now accepted as genuinely harmful. They raise LDL, lower HDL, and promote inflammation. Most countries have banned or severely restricted them. They were what we were advised to substitute for butter. That substitution was the problem, not the butter.
Industrial seed oils — canola, soybean, sunflower, corn oil — are high in omega-6 polyunsaturated fatty acids. When consumed in excess and in the wrong ratio to omega-3s (the modern diet has an omega-6 to omega-3 ratio of around 20:1, where the evolutionary norm was closer to 4:1), they promote chronic systemic inflammation. Inflammation interferes directly with leptin and insulin signalling — the two hormonal systems at the centre of weight management.
The Fats That Support Fat Burning
A ketogenic diet is high in fat. Getting the types right matters.
Natural saturated fats (butter, ghee, coconut oil, animal fat, full-fat dairy): stable, resistant to oxidation, good for cooking at higher temperatures.
Monounsaturated fats (olive oil, avocado, most nuts): well-documented benefits for cardiovascular markers, anti-inflammatory, stable enough for low-to-medium heat cooking.
Omega-3 polyunsaturated fats (fatty fish, sardines, mackerel, flaxseed, walnuts): anti-inflammatory, essential for brain and cardiovascular function. Prioritise these.
Avoid: industrial seed oils, margarine, any product with "partially hydrogenated" on the label, and deep-fried foods cooked in vegetable oil (oxidised polyunsaturated fats are particularly damaging).
The Practical Point
You do not need to fear fat. What you need to do is eat the right kinds. The foods humans ate for most of their evolutionary history — animal fats, eggs, fish, nuts, olives — are not the cause of metabolic disease. The industrial products introduced in the 20th century are far more plausibly implicated.
On the NKFB protocol, fat is fuel. Eat it from natural sources, cook with stable fats, avoid the processed alternatives, and your body has what it needs to run on ketones.
