Cholesterol

Cholesterol

Cholesterol

Cholesterol is a misunderstood molecule that is essential for life. It is frequently confused with plasma lipoproteins, which are complex structures serving as a transport medium for triglyceride and cholesterol molecules within the circulation. A component of all animal cell membranes, cholesterol is necessary for both structural integrity and fluidity. Cholesterol also serves as a precursor for steroid hormones, bile acid and vitamin D. All animal cells produce cholesterol with the amount produced within the body usually substantially more than that consumed. About 80% of total body cholesterol production occurs in the liver.

 

Ancel Keys, an American physiologist and epidemiologist, popularised the hypothesis that serum cholesterol (and saturated fat) were causally related to cardiovascular disease(104). Acceptance of this theory led to the introduction of dietary guidelines recommending dietary restriction of both saturated fat and cholesterol. More than 50 years later, the balance of evidence no longer supports this hypothesis, yet changing dietary guidelines to reflect the evidence has been a slow and contentious process(105). The recommended limit on dietary cholesterol has been removed from the most recent editions of the Australian and American dietary guidelines.

 

The consumption of eggs has long been controversial given their cholesterol content. Observational studies examining egg consumption specifically, rather than dietary cholesterol overall, have not found it to be associated with cardiovascular disease, except maybe in people with diabetes(106). However, randomised controlled trials are yet to find such an association. Current evidence suggests daily egg consumption may even be healthful. A 2018 large prospective cohort study of over 500 000 middle-aged Chinese adults found that daily egg consumption was associated with lower risk of cardiovascular disease, ischaemic heart disease, major coronary events, haemorrhagic stroke and ischaemic stroke. Daily egg consumption was found to be associated with an 18% reduced risk of death by cardiovascular disease and a 28% reduced risk of death by haemorrhagic stroke.

 

Recommended fat intake: a brief history

Excess dietary fat, particularly saturated fat, has long been considered to increase the risk of coronary heart disease. This perspective gained prominence in the 1970s thanks to the influence of Keys who promoted his ‘diet-heart hypothesis’. This was based on the belief that total serum lipoprotein, and later more specifically low-density lipoprotein, were significant factors in the pathogenesis of coronary heart disease. In making the case to support his hypothesis, Keys presented data from six countries demonstrating a relationship between mortality and dietary fat. Four years after Keys’ original paper, Yerushalmy and Hillebroe published a paper rebutting Keys’ conclusion, presenting data from a further 15 countries. By this time however, preliminary work on Keys’ (in)famous Seven Countries Study was well underway and Keys’ hypothesis proceeded to influence decades of future dietary policy and guidelines.

 

Most current dietary guidelines continue to stipulate that saturated fats should represent less than 10% of dietary energy (11% in the UK). This concern regarding saturated fat may be ‘the single most influential recommendation in conventional dietary advice’, providing the basis to recommend low-fat dairy and lean meats over full fat dairy and fattier cuts of meat, and to recommend margarine and vegetable oils instead of butter and animal fats.

Despite the rapid acceptance of the ‘diet-heart hypothesis’, a systematic review and meta-analysis of studies available at the time that the first dietary guidelines were not consistent with the scientific evidence available at the time. It is now well understood that high-density lipoprotein (HDL) has a protective effect on cardiovascular disease, and the ratios between HDL and low-density lipoprotein (LDL), and triglycerides and HDL, are more predictive of cardiovascular risk than LDL in isolation. Increasing dietary saturated fat intake has been shown to increase both HDL and LDL, leading to more favourable ratios.