Recent evidence suggests that dietary intake of cholesterol can influence plasma and serum levels, but not significantly. However, this is still subject to debate and further study [ 3 ]. Plasma cholesterol levels along with the levels of LDL cholesterol, HDL cholesterol, and serum triglycerides are currently used as biomarkers of the so-called standard ‘lipid profile’ for each individual. The standard lipid profile has been widely used as a traditional biomarker, not only for cardiovascular health but also for other lipid-related abnormalities and disorders [ 4 ].

Cholesterol can enter the blood through the digestion of dietary fat via chylomicrons. However, since cholesterol has an important role in cellular function, it can also be directly synthesised by each cell in the body. Notably, LDL particles are thought to act as a major transporter of cholesterol to the peripheral tissues, as at least two-thirds of circulating cholesterol resides in LDL. Conversely, HDL molecules are thought to do the opposite. They take excess cholesterol and return it to the liver for excretion [ 1 2 ].

Since cholesterol is mostly a lipophilic molecule, it does not dissolve well in blood. For this reason, it is packed into lipoproteins that are composed of a lipid core (which can contain cholesterol esters and triglycerides) and a hydrophilic outer membrane comprising phospholipids, apolipoprotein, and free cholesterol. This allows for the transport of the nonpolar lipid molecules such as cholesterol and triglycerides around the body through the blood to cells that require them. Plasma lipoproteins are separated into five major classes: chylomicrons, very-low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL) [ 1 2 ].

Cholesterol, an unsaturated alcohol of the steroid family, is essential for the normal function of all animal cells. It is also a fundamental element for the normal structural makeup and the fluidity of all cell membranes. Cholesterol interacts with phospholipid bilayers in the cell membrane and increases membrane packing. Cholesterol also takes part in signal transduction, intracellular transport, nerve conduction, and signalling pathways through lipid rafts and caveolae. Cholesterol has various other biological functions, i.e., it is a precursor molecule for several biochemical pathways such as the synthesis of vitamin D, steroid hormones (e.g., cortisol, aldosterone, and adrenal androgens), and sex hormones (e.g., testosterone, oestrogens, and progesterone). Cholesterol is also a constituent of bile salts, which are crucial constituents of digestion, as they facilitate the absorption of lipids, fats, and fat-soluble vitamins A, D, E, and K [ 1 ].

1.2. Cholesterol Levels: Demonising a Risk Factor but Not the Causative Mechanisms of Chronic Diseases

Several modifiable and non-modifiable risk factors (genetic, environmental, nutrition, and lifestyle, etc.) are thought to influence the balance between health and disease by inducing mechanisms related to disease onset, development, and the manifestations of symptoms. The presence or coexistence of these risk factors seem to trigger underlying molecular and cellular mechanistic pathways that can lead to continuous chronic manifestations and the long-term loss of tissue homoeostasis and tissue dysfunction. These continuous chronic manifestations can develop over time before cellular disturbances manifest and cause tissue disorders, while, if not counterbalanced by our immune system and by specific preventive measures such as a healthy diet and lifestyle, the subsequent symptomatic disease finally appears, and medical treatment may be required to reduce the risk of mortality. Elucidating these molecular and cellular mechanistic pathways and acquiring the mechanistic evidence of the underlying multifactorial causes of a chronic disease can lead to suitable preventive targets against these diseases with fewer side effects, which is an ongoing difficult and demanding task. Such difficulties have misled the scientific and medical community to often and lightly extrapolate the easily acquired observed statistical and epidemiological correlations of traditional risk factors to several chronic diseases, towards matching these risk factors as the causative agents of these diseases.

According to the ‘cholesterol hypothesis’, high blood cholesterol is a major risk factor, while lowering cholesterol levels can reduce risk [ 5 ]. Dyslipidaemias (i.e., hypercholesterolaemia or hyperlipidaemia) are abnormalities of lipid metabolism characterised by increased circulating levels of serum total cholesterol, LDL cholesterol, triglycerides, and decreased levels of serum HDL cholesterol. High levels of LDL cholesterol and non-HDL cholesterol have been associated with cardiovascular risk, while other cholesterol-related serum markers, such as the small dense LDL cholesterol, lipoprotein(a), and HDL particle measurements, have been proposed as additional significant biomarkers for CVD risk factors to add to the standard lipid profile [ 6 ]. HDL cholesterol has been considered as the atheroprotective ‘good’ cholesterol because of its strong inverse correlation with the progression of CVD [ 7 ]; however, it is the functionality of HDL cholesterol, rather than its concentration that is more important for the preventative qualities of HDL cholesterol in CVD. In general, dyslipidaemias have been ranked as significant modifiable risk factors contributing to prevalence and severity of several chronic diseases including aging, hypertension, diabetes, and CVD. High serum levels of these lipids have been associated with an increased risk of developing atherosclerosis [ 8 ].

11,12, Furthermore, dyslipidaemias have been characterised by several studies not only as a risk factor but as a “well-established and prominent cause” of cardiovascular morbidity and mortality worldwide [ 9 ]. Even though such an extrapolation is not adequate, it was, however, not surprising that this was made, because since the term arteriosclerosis was first introduced by pioneering pathologists of the 19th century, it has long been believed that atherosclerosis merely involved the passive accumulation of cholesterol into the arterial walls for the formation of foam cells. This process was considered the hallmark of atherosclerotic lesions and subsequent CVD. Moreover, one-sided interpretations of several epidemiological studies, such as the Seven Countries Study (SCS), have highlighted outcomes that mostly concerned correlations between saturated fat intake, fasting blood cholesterol concentrations, and coronary heart disease mortality [ 10 13 ]. Such epidemiological correlations between dyslipidaemias and atherosclerosis led to the characterisation of atherosclerosis as primarily a lipid disorder, and the “lipid hypothesis” was formed, which would dominate thinking for much of the 20th century.

In the clinical setting, in order to address the lipid hypothesis, the levels of cholesterol related plasma lipoproteins and triglycerides (lipid profile) have been used as traditional biomarkers for cardiovascular risk, but also for dietary and treatment guideline designs [ 5 ]. Dietary and medical guidelines have focused on the reduction of cholesterol and lipid levels as the best way to prevent chronic diseases such as CVD [ 5 9 ]. Such guidelines suggest the application of statin therapies in order to reduce the levels of cholesterol (through inhibition of cholesterol synthesis by HMG-CoA reductase inhibitors); however, numerous side effects have been reported, including the development of other chronic diseases such as diabetes mellitus [ 14 ]. Moreover, specific dietary strategies for reducing cholesterol intake are the mainstay of management in most cases of dyslipidaemia, prior to, or simultaneously with, the initiation of a lipid lowering agent [ 9 ]. Dietary fats, cholesterol, and the levels of serum cholesterol in relation to the development of CVD have been somewhat demonised.

On the other hand, since cholesterol is an essential biomolecule for the normal function of all our cells, an emerging question has recently surfaced: “how much do we need to lower the levels of cholesterol”? Furthermore, given the fact that cholesterol plays a crucial role in several of our cellular and tissue mechanisms, it is not surprising that there are several consequences due to the aggressive reduction of cholesterol levels in the body, which has been common practice over the last few decades. In addition, targeting cholesterol and fat intake by introducing diets with low-fat products and by reducing the intake of high-fat foods can lead to less absorption and lower bioavailability of other lipids containing high value nutrients, such as several lipid soluble vitamins (especially vitamin D) and other lipid molecules. Such lipids have exhibited a plethora of beneficial bioactivities, not only related to reducing the risk of chronic diseases but also through a wide range of important bio-functionalities and anti-inflammatory properties [ 3 ]. Therefore, lower cholesterol levels do not equate to better health, or to lower risk of chronic diseases such as CVD. Homeostasis must be maintained, even with regard to cholesterol, both HDL and LDL [ 15 ].

17, Moreover, recent systematic reviews and meta-analyses have started to question the validity of the lipid hypothesis, as there is lack of an association or an inverse association between LDL cholesterol and both all-cause and CVD mortality in the elderly [ 15 ] and several cancers such as lung, prostate, and breast cancer [ 16 18 ]. Such studies provide the rationale for more research about the causes (and not only the risk factors) of chronic diseases such as atherosclerosis, CVD, and cancer, but also for a re-evaluation of the guidelines for cardiovascular prevention, in particular because the benefits of statin treatments have been exaggerated [ 15 ].

Statistical and epidemiological extrapolations often lack fully clarified biochemical mechanistic evidence, while associations and correlations do not necessarily mean causation. In addition, a follow-up by systematic reviews and meta-analyses often present contradictory outcomes against the initial results that were introduced by early stage epidemiological studies lacking consistency, biological gradient, and coherence. Thus, such extrapolations can lead to one-sided, premature targeting of risk factors accompanied with consequences, often without the desirable outcomes. Targeting a risk factor such as high serum cholesterol may decrease the probabilities for a disease, but usually cannot prevent the causation of chronic diseases.