The diet-heart hypothesis, as developed and promoted by Ancel Keys from the 1950s onwards, proposed that dietary saturated fat raised serum cholesterol and that elevated serum cholesterol caused cardiovascular disease. Based on this hypothesis, populations were advised to reduce total fat, saturated fat, and dietary cholesterol intake. The gap was filled by refined carbohydrates. The epidemics of obesity, type 2 diabetes, and metabolic syndrome that followed are at least partly attributable to this dietary shift.
What the Evidence Actually Shows
Recent meta-analyses of randomised controlled trials have consistently failed to find a significant association between saturated fat reduction and reduced cardiovascular mortality. The largest and most rigorous analysis, by Siri-Tarino and colleagues in the American Journal of Clinical Nutrition in 2010, pooled data from 21 prospective studies including over 347,000 subjects and found no significant association between saturated fat and cardiovascular disease or stroke. The critical distinction is what replaces saturated fat: replacing it with refined carbohydrates increases cardiovascular risk; replacing it with unsaturated fats from whole foods reduces risk.
The Current Evidence-Based Position
The current evidence supports a dietary pattern that prioritises fat quality over fat quantity. Monounsaturated and omega-3 polyunsaturated fats from extra-virgin olive oil, avocados, nuts, and oily fish have consistent cardiovascular benefits. Trans fats from industrial partial hydrogenation are harmful and should be avoided. Saturated fat from whole food sources including eggs, dairy, and unprocessed meat is not the primary driver of cardiovascular risk that it was assumed to be in the context of a whole food dietary pattern.