Confusion reigns. Is a diet rich in saturated fat risky for health or not? One of the oldest and most widely understood dogmas of human nutrition is that too much of this fat from meat, dairy and other animal sources is potentially harmful.
Given the for decades, it has been unsettling to see this advice recently by a loose confederation of doctors, nutritionists and popular writers.
Advertisement
These detractors say that saturated fat has no adverse effect on human health, and that overconsumption of carbohydrates and highly processed plant oils is instead fuelling an increase in obesity and metabolic disease. Much of this is based on high-quality published in recent years that found no statistically significant relationship between intakes of saturated fats and heart disease across Western populations.
They may have to think again. A major epidemiological study led by researchers at Harvard University sheds new light on this perplexing state of affairs.
Complex diets
The problem with such work in the past has been the sheer complexity of human diets, posing an enormous challenge in attempts to accurately probe their metabolic effects. For energy intake to remain the same when someone changes their diet by cutting down on one component, their consumption of others must increase. Tracking these changing dietary patterns is probably crucial to understanding human health, but they have rarely been modelled in previous studies.
Not so this time. The new study examined the effects of dietary fat intake on death and its causes in more than 126,000 men and women tracked in the US since the 1980s. The researchers used well-validated food questionnaires updated repeatedly over the decades, adjusted their results to account for other aspects of lifestyle and health, and logged details of over 33,000 deaths.
Harnessing the statistical power that such large numbers provide, they were then able to model the effects of replacing saturated fats with unsaturated fats, or with carbohydrates.
Finding good substitutes
The results suggest that the effects depend crucially on the identity of the replacements. The greatest benefits came from diets in which poly- and monounsaturated fats were the substitutes. When polyunsaturated replaced saturated fat equivalent to just 5 per cent of total calorie intake, the risk of death from heart disease, cancer and other causes fell by 27 per cent.
In contrast, there seemed to be little benefit when saturated fats were replaced with carbohydrates, perhaps because in the context of US diets it seems likely that much of the gap would have been filled by sugar, rather than potentially healthier wholegrain carbohydrates.
These results are consistent with and elsewhere, and do much to support the concept of a healthy Mediterranean-style diet rich in polyunsaturated fats from plant foods and fish, and in monounsaturated fats from olive oil.
Advice to reduce intake of meat and dairy products and fill the gap with plant-based products 鈥� including complex carbohydrates, vegetable oils and manufactured products derived from them 鈥� has become a hallmark of guidance across the developed world, and for now it stands. The notion that 鈥渂utter is back鈥� is melting away.
Journal reference: JAMA Internal Medicine, DOI:
