This is the all too familiar advice: out with the bacon and eggs, and in with the bran flakes. But, in recent years there has been a backlash against this dietary wisdom. The Paleo Diet has surged in popularity, giving one license to gorge on meat like our ancestors. And, frequent media reports (4) now claim that the experts got it wrong: saturated fat is healthy.
Origins of the Lipid Hypothesis
The theory that saturated fat causes heart disease (the lipid hypothesis), stems from Ancel Key’s presentation at the 1955 World Health Organisation’s meeting on heart disease (5). It claimed that eating a diet high in saturated fat increases the ‘bad’ LDL cholesterol. This causes fatty material to build up on the inside of the arteries, causing them to narrow, which predisposes us to heart problems (atherosclerosis).
In 1958, Key’s started his 10 yearlong seven countries study which showed that countries where the diet was low in saturated fat, had low rates of heart disease and mortality (6). By the 1980s the case was closed (7), low-fat diets were in and even McDonald’s were offering low-fat burgers (8).
The Change in the Western Diet
Over time, we decreased our total fat and saturated fat intake and increased our carbohydrate intake. The graph shows that as dietary guidelines were introduced and the public consensus moved towards low-fat diets; our diet changed with it (9).
The death rates from heart disease have also decreased so it has appeared to have worked (10). But smoking, blood pressure and cholesterol have all decreased; and medical treatments have improved (11). So it is hard to tell what has worked.
Did the decrease in fat and saturated fat, decrease heart disease?
The Current Evidence on Saturated Fat
There has been a lot of research on this subject which has been nicely summarised by a number of meta-analyses. The latest and best designed of these concluded:
‘Saturated fats are not associated with all-cause mortality, cardiovascular disease, coronary heart disease, ischemic stroke, or type 2 diabetes’ (14)
This conclusion was in line with most of the previous meta-analyses (15). Saturated fats appear to have a neutral effect on health. But perhaps other fats are still better than them – Should I replace my butter with vegetable oil? A 2017 meta-analysis on the subject including only well-designed studies concluded:
‘replacing saturated fats with mostly n-6 polyunsaturated fats is unlikely to reduce coronary heart disease events, coronary heart disease mortality or total mortality’ (16)
What’s to Blame for Heart Disease?
The idea that eating fat clogs the arteries is just plain wrong. Clogged arteries – atherosclerosis, is a complex process driven by inflammation and insulin resistance, which have many different causes. The plague in the arteries resembles a pimple which can burst causing a heart attack. The major dietary culprits are sugar and refined carbohydrates which promote insulin resistance and inflammation (17).
Buttered Bacon is Back!
Not so fast. Just because saturated fats have a neutral effect on heart disease does not mean there are benefits to eating large amounts. The PURE study which ran for 10 years in 17 countries with 135,335 subjects showed that those with the lowest morality consumed 9.5% energy from saturated fat. Those with very high and very low intakes were worse off. It also showed that those with the highest total fat intakes (35.3%) and lowest carbohydrate intakes (46.4%) had the lowest mortality (18).
Overall, saturated fat intake doesn’t appear to be very significant, so long as one does not go too low. In the PURE study, those with the lowest intake (2.8%) had a 45% increase in mortality compared to the optimal 9.5% intake. Other issues such as a high sugar and trans-fat intake are more important and worthwhile to focus your energy on.