Cholesterol ingestion and absorption

In the UK, the average daily intake of cholesterol is 304 and 213 mg for males and females, respectively⁽ Reference Henderson, Gregory and Irving ^{23 )}, 10–15 % of which is in the esterified form⁽ Reference Iqbal and Hussain ^{24 )}. In the small intestine, esterified cholesterol is hydrolysed to form free cholesterol, which is more readily incorporated into bile acid micelles, which facilitate the absorption of cholesterol via Niemann–Pick C1-like 1 protein (NPC1L1)⁽ Reference Betters and Yu ^{25 ,} Reference Ikeda, Matsuoka and Hamada ^{26 )}. Additionally, phytosterols can also be absorbed via NPC1L1⁽ Reference Davis, Zhu and Hoos ^{27 )}. Intestinal absorption of cholesterol and phytosterols can be limited by heterodimer ATP-binding cassette (ABC) G5/G8, which effluxes these sterols back to the intestinal lumen⁽ Reference Yu, Qian and Jiang ^{28 )}. Acyl CoA: cholesterol acyltransferase 2 esterifies internalised cholesterol, which is then incorporated into a nascent chylomicron via microsomal TAG transfer protein⁽ Reference Chang, Li and Chang ^{29 ,} Reference Atzel and Wetterau ^{30 )}. The nascent chylomicron then exits the enterocyte by exocytosis into the lymphatic system before entering the blood stream⁽ Reference Van Dyck, Braem and Chen ^{31 )}. The nascent chylomicron is converted to a mature chylomicron upon acquisition of apo C-II and E from HDL. Apo C-II activates lipoprotein lipase on the capillary endothelium of adipose or muscle tissue, which in turn catalyses the hydrolysis of TAG⁽ Reference Kersten ^{32 ,} Reference Olivecrona and Beisiegel ^{33 )}. Apo C-II is then returned to HDL, and hepatic LDL receptors (LDLr) and LDLr-related protein recognise apo B-48 and E, initiating the absorption of the chylomicron remnants⁽ Reference Cooper ^{34 )}.

Cholesterol synthesis

Cholesterol is synthesised endogenously in all nucleated cells in the body from acetyl CoA⁽ Reference Bloch ^{35 )}. Renfurm et al.⁽ Reference Renfurm, Bandsma and Verkade ^{36 )} observed endogenous cholesterol was synthesised at a rate of 9·8 (sd 6·2) mg/kg per d in healthy adults with a mean age of 32 years and mean weight of 64 kg. This equates to 627·2 mg/d synthesised cholesterol, a similar value to the 710 mg/d observed by Jones and Schoeller⁽ Reference Jones and Schoeller ^{37 )}. Interestingly, cholesterol consumption can influence the synthesis of endogenous cholesterol; an increase from 173 to 781 mg/d dietary cholesterol has been observed to decrease the rate of sterol synthesis by 34 %⁽ Reference Parker, McNamara and Brown ^{38 )}.

Cholesterol synthesis commences when acetoacetyl CoA thiolase catalyses the interconversion of acetyl CoA and acetoacetyl CoA. One molecule acetyl CoA and one molecule acetoacetyl CoA undergo a condensation reaction by 3-hydroxy-3-methylglutaryl (HMG) CoA synthase to form a molecule of HMG CoA. HMG CoA reductase, with the addition of two NADPH molecules, and then catalyse the conversion of HMG CoA to mevalonate. As the rate-limiting enzyme of cholesterol biosynthesis, HMG CoA reductase is the therapeutic target of statins, for the treatment of hypercholesterolaemia, and the prevention of atherosclerosis⁽ Reference Sirtori ^{39 )}. Phosphorylation of mevalonate by the enzyme mevalonate kinase forms mevalonate-5P, which undergoes further phosphorylation to form mevalonate-5PP via the enzyme phosphomevalonate kinase. Decarboxylation and dehydration by mevalonate-5PP decarboxylase create isopentenyl-PP and thus its isoform dimethylallyl-PP via isopentenyl diphosphate delta isomerase. Farnesyl diphosphate synthase initiates the condensation of dimethylallyl-PP with one molecule of isopentenyl-PP and NADPH to create geranyl-PP. Further condensation and the addition of another molecule of isopentenyl-PP and NADPH create farnesyl-PP. Condensation of two farnesyl-PP molecules by squalene synthase and NADPH forms squalene, which is then converted to squalene epoxide by squalene epoxidase, NADPH, and O₂, before undergoing cyclisation by oxidosqualene cyclase to form lanosterol⁽ Reference Hoshino, Chiba and Abe ^{40 )}. A series of reactions, including the branching of 7-dehydrodesmosterol to either desmosterol or 7-dehydrocholesterol, both of which can then be converted to the end product cholesterol via the enzymes 24-dehydrocholesterol reductase and 7-dehydrocholesterol reductase concludes the de novo synthesis of cholesterol⁽ Reference Luu, Hart-Smith and Sharpe ^{41 ,} Reference Risley ^{42 )}.

Lipoprotein dynamics and reverse cholesterol transport

VLDL-cholesterol is formed from the hepatic pool of cholesterol to transport endogenously synthesised TAG to the tissues⁽ Reference Havel ^{43 )}. Partial hydrolysis of VLDL by lipoprotein lipase forms intermediate density lipoprotein, with subsequent hydrolysis of intermediate-density lipoprotein by hepatic lipase forming LDL, which acts to deliver cholesterol to the peripheral tissue⁽ Reference Havel ^{43 )}. VLDL-cholesterol, intermediate-density lipoprotein-cholesterol and LDL-cholesterol can be removed from the circulation by hepatic LDLr, while LDL-cholesterol can also be absorbed independently⁽ Reference Spady, Turley and Dietschy ^{44 ,} Reference Veniant, Zlot and Walzem ^{45 )}. Reverse cholesterol transport transfers cholesterol from the tissues to the liver via HDL, reducing the risk of cholesterol accumulation and atherosclerosis⁽ Reference Shen, Peng and Peng ^{46 )}. Cholesterol can be effluxed from the tissues by the receptors ABC-A1, and scavenger receptor class B member 1, or via receptor independent passive diffusion to nascent HDL⁽ Reference He, Zhang and Li ^{47 –} Reference Gillotte, Davidson and Lund-Katz ^{49 )}. The incorporated cholesterol is then esterified by lecithin-cholesterol acyltransferase⁽ Reference Sorci-Thomas, Babiak and Rudel ^{50 )}. Cholesterol enters the liver either directly, via the receptor scavenger receptor class B member 1, or via the enzyme cholesteryl ester transfer protein (CETP). CETP mediates the 1:1 exchange of cholesterol from HDL with TAG from VLDL and LDL⁽ Reference Zhang, Charles and Tong ^{51 )}. Once in the liver, cholesterol can be removed from the body.

Cholesterol excretion

Cholesterol can be removed from the body by two mechanisms: directly via the hepatic ABCG5/G8 receptor and effluxed to the gall bladder or alternatively, cholesterol can be converted to bile acids for faecal excretion⁽ Reference Brown and Goldstein ^{52 ,} Reference Repa, Berge and Pomajzl ^{53 )}. Approximately 98 % of bile acids are conjugated to either taurine or glycine, as conjugation increases polarity, which reduces passive transport from the intestinal lumen into enterocytes, and allows the movement of bile acids to be tightly regulated, and under receptor control; in addition to improving solubility⁽ Reference Aldini, Montagnani and Roda ^{54 )}. Removal of the amino acid from conjugated bile acids, by bacterial bile salt hydrolase (BSH), decreases reabsorption efficiency, thus unconjugated bile acids make up 98 % of the 5 % of bile acids that are excreted daily⁽ Reference Batta, Salen and Rapole ^{55 ,} Reference Gérard ^{56 )}. This modification is of significant interest, as the production of more readily excreted bile acids, may lead to the increased conversion of cholesterol to bile acids to replace those lost, in turn lowering serum cholesterol⁽ Reference Oner, Aslim and Aydas ^{57 )}.

Intrinsic ageing

The ageing process has been associated with an increase in both total cholesterol (TC) and LDL-cholesterol. For instance, Ericsson et al. reported an increase in TC from 4·8 mm/l in the young (aged 20–39 years), to 5·14 mm/l in the middle aged (aged 40–59 years), and to 5·44 mm/l in old aged (aged 60–80 years) healthy Scandinavian volunteers⁽ Reference Ericsson, Eriksson and Vitols ^{58 )}. Furthermore, LDL-cholesterol increased with age, from 3·37 in the young, to 3·76 in the middle aged and to 4·05 mm/l in the old aged. Additionally, VLDL-cholesterol has been observed to either remain steady or increase with age, while HDL-cholesterol appears to be unaffected by the ageing process⁽ Reference Ericsson, Eriksson and Vitols ^{58 ,} Reference Abbott, Garrison and Wilson ^{59 )}. Abbott et al. also found sex influences the lipoprotein profile. For example, females exhibited higher levels of LDL-cholesterol, especially in those using oestrogen hormones, and increased HDL-cholesterol, whereas VLDL-cholesterol was greater in males⁽ Reference Abbott, Garrison and Wilson ^{59 )}. The age-associated dysregulation of cholesterol metabolism, and accumulation of LDL-cholesterol, has been associated with alterations to several key mechanisms, including cholesterol absorption, LDL-cholesterol clearance, bile acid synthesis and subsequent intestinal bacterial modification (Fig. 5)⁽ Reference Morgan, Mooney and Wilkinson ^{5 )}.

Fig. 5. (Colour online) Overview of age-related changes to cholesterol metabolism.

Intestinal cholesterol absorption varies greatly among individuals, with estimates ranging from 20·0 to 80·1 %⁽ Reference Abbott, Garrison and Wilson ^{59 ,} Reference Bosner, Lange and Stenson ^{60 )}. Evidence from murine models suggests cholesterol absorption increases significantly with age⁽ Reference Wang ^{7 )}. Rodent studies have demonstrated this is mediated by a significant increase in NPC1L1 in both the duodenum and jejunum, while ABCG5/G8 expression is suppressed⁽ Reference Duan, Wang and Ohashi ^{61 )}. It has been estimated this increase in cholesterol absorption and concurrent reduction in efflux from enterocytes, confers a 19–40 % increase in cholesterol absorption with age⁽ Reference Duan, Wang and Ohashi ^{61 )}. Moreover, it was observed that high levels of oestrogen up-regulated NPC1L1 and ABCG5/G8 mRNA expression. Oestrogen and ageing have been reported to enhance cholesterol absorption via the estrogen receptor-α pathway⁽ Reference Duan, Wang and Ohashi ^{61 )}. It is important to note that these findings have not yet been observed in human individuals⁽ Reference Bosner, Lange and Stenson ^{60 )}.

Bile acid metabolism is also affected by the ageing process, most significantly, there is a reduction in bile acid synthesis. Wang⁽ Reference Wang ^{7 )} found ageing resulted in a significantly reduced biliary bile acid output, from 192 to 211 to 124–157 µm/h per kg in mice. Additionally, Wang⁽ Reference Wang ^{7 )} demonstrated intrinsic ageing resulted in a reduction in bile acid synthesis, with a 33·3–57·1 and 41·7–56·3 % decrease in cholesterol 7α-hydroxylase (CYP7A1) activity in male and female mice, respectively, dependent on dietary and genetic factors. Similarly in human subjects, an inverse correlation between age and CYP7A1 expression has been described⁽ Reference Bertolotti, Gabbi and Anzivino ^{62 )}. For instance, cholesterol 7α-hydroxylation rates were reduced by 50 % for individuals over 65 years, compared with individuals below 65 years of age in one Italian cohort⁽ Reference Bertolotti, Abate and Bertolotti ^{63 )}. Bertolotti et al.⁽ Reference Bertolotti, Abate and Bertolotti ^{63 )} estimated by linear regression analysis a 60 mg/d (about 150 µm/d) decline every 10 years, in cholesterol undergoing cholesterol 7α-hydroxylation, while Einarsson et al.⁽ Reference Einarsson, Nilsell and Leijd ^{64 )} estimated an 80 mg/d (200 µm/d) reduction over the same time period. Bertolotti et al.⁽ Reference Bertolotti, Gabbi and Anzivino ^{62 )} propose the age-related reduction in CYP7A1 expression could be related to the concomitant decline in hepatic nuclear factor four and co-activator CYP7A1 promoter-binding factor/liver receptor homologue-1, mediated by the decline in growth hormone and insulin-like growth factor.

Once bile acids reach the small intestine, modification by digestive microflora occurs influencing enterohepatic circulation. Many digestive bacteria produce the enzyme BSH, which deconjugates bile acids, decreasing reabsorption efficiency, and enhancing excretion. For example, Tanaka et al.⁽ Reference Tanaka, Doesburg and Iwasaki ^{65 )} reported 59 and 98 % of Lactobacillus and Bifidobacteria strains, isolated from faeces are BSH positive. With age there are several changes to the gut microflora, including a decline in the number and species diversity of Lactobacillus and Bifidobacterium ⁽ Reference Hopkins and Macfarlane ^{8 ,} Reference Woodmansey, McMurdo and Macfarlane ^{66 )}. Therefore, it is possible that the age-related decline of these bacterial species reduces bile acid deconjugation, and in turn reduces the conversion of cholesterol to bile acid. This may play a role in the accumulation of cholesterol with age⁽ Reference Morgan, Mooney and Wilkinson ^{5 )}.

It has also been reported, the clearance rate of LDL-cholesterol is affected by the ageing process. The apo B-100 containing lipoproteins, LDL-cholesterol and VLDL-cholesterol are removed from the blood via hepatic LDLr, for elimination, either by direct efflux or conversion to bile acids. Millar et al.⁽ Reference Millar, Lichtenstein and Cuchel ^{6 )} determined the mean LDL apo B-100 residence time was 2·42 d for younger male adults (mean age 31 (sd 6) years) and 3·46 d for older male adults (mean age 61 (sd 10) years). With age, a decline in LDLr activity and/or numbers is thought to be responsible for the reduction in LDL clearance rate and increase in residence time. The reduced conversion of cholesterol by CYP7A1 may play a role in reduction of LDLr. Additionally, proprotein convertase subtilisin/kexin type 9, a proprotein convertase responsible for the degradation of LDLr has been correlated with age⁽ Reference Lagace, Curtis and Garuti ^{67 )}. Interestingly, proprotein convertase subtilisin/kexin type 9 has also been correlated with BMI, TC, LDL-cholesterol and TAG⁽ Reference Cui, Ju and Yang ^{68 )}. Furthermore, Millar et al.⁽ Reference Millar, Lichtenstein and Cuchel ^{6 )} observed, that although VLDL apo B-100 residence time was not affected by age, the production rate of VLDL apo B-100 was correlated with age. The age-related increase in body fat and elevated plasma free fatty acids were attributed to this increase in VLDL apo B-100 production rate.

Cholesterol metabolism and diet

Herron et al.⁽ Reference Herron, Vega-Lopez and Conde ^{69 )} examined the effect of about 640 mg/d cholesterol feeding on men aged 18–57 years, and determined that 37·5 % of subjects behaved as hyper-responders, with an increase of ≥0·06 mm/l in TC, while 62·5 % behaved as hypo-responders, with an increase in TC of <0·05 mm/l. Hyper-responders exhibited a significant 23·0, 7·8 and 18·0 % increase in LDL-cholesterol and HDL-cholesterol and LDL:HDL ratio, respectively, whereas changes to LDL-cholesterol, HDL-cholesterol, TG and LDL:HDL ratio were NS for hypo-responders. Interestingly, hyper-responders exhibited elevated lecithin-cholesterol acyltransferase and CETP activity, suggesting the up-regulation of reverse cholesterol transport as a compensatory mechanism, to reduce the risk of atherosclerosis. Quintao et al.⁽ Reference Quintao, Grundy and Ahrens ^{70 )} propose tissue pools of cholesterol may rapidly expand in response to cholesterol feeding, even in the absence of aberrations to plasma cholesterol levels. Typically, there are two main mechanisms to compensate for an increase in dietary cholesterol; elevated cholesterol excretion and decreased cholesterol synthesis. It has been suggested a reduction in cholesterol intake should be considered unnecessary for individuals who have already reduced SFA intake, and increased PUFA:SFA ratio⁽ Reference Edington, Geekie and Carter ^{71 )}. Edington et al.⁽ Reference Edington, Geekie and Carter ^{71 )} determined a 2-fold increase or decrease in dietary cholesterol in participants who also reduced dietary fat with an increased PUFA:SFA ratio had no effect on TC or LDL-cholesterol after 8 weeks. This is due to the significant impact SFA had on serum LDL-cholesterol, by influencing several regulatory mechanisms. Firstly, it has been observed there is a reduction in LDLr; resulting in reduced LDL-cholesterol clearance and increased LDL-cholesterol⁽ Reference Mustad, Etherton and Cooper ^{72 )}. Mustad et al.⁽ Reference Mustad, Etherton and Cooper ^{72 )} demonstrated an 8-week reduction in SFA resulted in a 10·5 % increase in LDLr and subsequent 11·8 % decrease in LDL-cholesterol. It has been estimated for every 1 % increase in LDLr, there is a 0·74 % reduction in LDL-cholesterol. Secondly, SFA may influence cholesterol synthesis⁽ Reference Glatz and Katan ^{73 )}. Glatz and Katan⁽ Reference Glatz and Katan ^{73 )} determined a low PUFA:SFA ratio diet resulted in increased cholesterol synthesis, compared with a high PUFA:SFA ratio diet (1·86 v. 1·55 mm/d). Additionally, Jones et al.⁽ Reference Jones, Lichtenstein and Schaefer ^{74 )} demonstrated maize oil increased absolute cholesterol synthesis from 13·9 mg/kg per d at baseline, to 21·3 mg/kg per d. Thirdly, it has been demonstrated SFA influences the concentration of CETP. For example, Jansen et al.⁽ Reference Jansen, López-Miranda and Castro ^{75 )} observed that CETP concentrations were significantly elevated by 12 and 11 %, in individuals on a high SFA diet, compared with individuals on the National Cholesterol Education Program StepI diet and MUFA diet, respectively. Additionally, an elevation of SFA from 8·4 to 11 % decreased lecithin-cholesterol acyltransferase activity from 56 to 74 nm/ml per h, which may result in decreased reverse cholesterol transport and influence CVD risk⁽ Reference Berard, Dabadie and Palos-Pinto ^{76 )}.

Cholesterol metabolism and disease

The age-related accumulation of both TC and LDL-cholesterol has been associated with the pathogenesis of several diseases⁽ Reference Sjogren and Blennow ^{77 –} Reference Sharrett, Ballantyne and Coady ^{79 )}. In a meta-analysis of sixty-two studies, a 17·5 % reduction in relative risk for all-cause mortality for every 1 mm/l decrease in TC was reported, while each 1 mm/l decrease in LDL-cholesterol was associated with a 15·6 % relative risk reduction for all-cause mortality⁽ Reference Gould, Davies and Alemao ^{4 )}.

CHD has been associated with significantly elevated TC, LDL-cholesterol, TAG, apo B and Lp(a) and reduced HDL-cholesterol and apo A–I in both American males and females⁽ Reference Sharrett, Ballantyne and Coady ^{79 )}. Gould et al.⁽ Reference Gould, Davies and Alemao ^{4 )} reported a 24·5 and 29·5 % reduction in relative risk for CHD-related mortality and CHD event respectively for every 1 mm/l decline in TC. Additionally, each 1 mm/l decline in LDL-cholesterol was associated with a 28·0 and 26·6 % decline in relative risk for CHD-related mortality and CHD event, respectively. Sharrett et al.⁽ Reference Sharrett, Ballantyne and Coady ^{79 )} found that each 1 mm/l increase in LDL-cholesterol was associated with a relative risk for CHD of between 1·36 and 1·44 for males and 1·19 and 1·32 for females, while a 0·4 mm/l increase in HDL-cholesterol was associated with a 0·64–0·72 and 0·64–0·76 relative risk for CHD for males and females, respectively. Interestingly, the decreased risk for mortality with reduced TC has been observed to decline with age. For example, a meta-analysis conducted by Lewington et al.⁽ Reference Lewington, Whitlock and Clarke ^{80 )} concluded a 1 mm/l reduction in TC was associated with an about 50 % reduction in IHD mortality for 40–49 year olds, which decreased to an about 33 % reduction for 50–69 year olds and about 17 % for 70–89 year olds.

Intriguingly, several studies have described an inverse relationship between cholesterol and mortality⁽ Reference Ravnskov, Diamond and Hama ^{81 )}. Lv et al.⁽ Reference Lv, Yin and Chei ^{82 )} observed a 19 % decrease in relative risk for each 1 mm/l increase in LDL-cholesterol in Chinese ≥80 year olds. Additionally, a 40 % lower risk for mortality was associated with those with abnormally high LDL-cholesterol (≥3·37 mm/l), compared with those with a lower plasma concentration of LDL-cholesterol. Similarly, Takata et al.⁽ Reference Takata, Ansai and Soh ^{83 )} observed a 0·8 % decrease in mortality for each 1 mg/dl (0·026 mm/l) increase in LDL-cholesterol in Japanese ≥80 year olds. In addition, each 1 mg/dl increase in TC was associated with a 0·9 % reduction in mortality.

Pharmacological intervention

Lipid-lowering drugs are often utilised in the treatment of CVD (Fig. 6). Statins are often recommended for those diagnosed with CVD, or those with a high risk of developing the disease, due to their tolerability and efficacy⁽ Reference Weng, Yang and Lin ^{86 )}. Statins act by competitively inhibiting HMG CoA reductase, the rate-limiting enzyme of cholesterol synthesis in order to lower serum cholesterol, thus reducing CVD risk⁽ Reference Law, Wald and Rudnicka ^{87 )}. Simivastatin reduced LDL-cholesterol  by 33 % after 12 weeks, in patients with a history of myocardial infarction, while atorvastatin exhibited a 49 % reduction⁽ Reference Pedersen, Faergeman and Kastelein ^{88 )}. A meta-analysis of four trials demonstrated standard-dose statin therapy reduced LDL-cholesterol  by 22 %, to a mean of 2·59 mm/l (101 mg/dl), while high-dose statin therapy reduced LDL-cholesterol by 42 %, to a mean of 1·92 mm/l (75 mg/dl)⁽ Reference Cannon, Steinberg and Murphy ^{89 )}. The standard-dose therapy was associated with a coronary death or myocardial event rate of 9·4 %, whereas high-dose therapy was associated with an 8·0 % event rate. Significantly, overall statin therapy resulted in a 16 % odds reduction in coronary death or myocardial infarction⁽ Reference Cannon, Steinberg and Murphy ^{89 )}. Ridker et al.⁽ Reference Ridker, Cannon and Morrow ^{90 )} observed that patients whose statin therapy reduced LDL-cholesterol to <70 mg/dl (<1·81 mm/dl), had a reduced rate of recurrent myocardial infarction or death from coronary events (2·7 events per 100 person-years), than individuals whose statin therapy did not reach this goal (4·0 events per 100 person-years). However, it has also been demonstrated non-statin therapy (diet, bile acid sequesterants and ileal bypass surgery) reduced CHD risk equally to that of statins, with each 1 % reduction in LDL-cholesterol corresponding to a 1 % decrease in CHD risk⁽ Reference Robinson, Smith and Maheshwari ^{10 )}. Statins can also be used in combination with drugs such as Ezetimbibe, which targets NPC1L1 to lower cholesterol absorption. Cannon et al.⁽ Reference Cannon, Blazing and Giugliano ^{84 )} observed that combination therapy lowered LDL-cholesterol by a further 24 % than statin therapy alone. Combination therapy also significantly reduced myocardial infarction and ischaemic stroke risk.

Fig. 6. (Colour online) UK prescription drugs for CVD. Data from British Heart Foundation⁽ Reference Townsend, Bhatnagar and Wilkins ^{134 )}.

Dietary intervention

A reduced intake of SFA and an increase in MUFA and PUFA may be useful in maintaining a healthy lipid profile, and reducing CHD risk⁽ Reference Mozaffarian, Micha and Wallace ^{11 )}. An 8-week reduction in SFA was observed to reduce TC and LDL-cholesterol by 9·3 and 11·8 %, respectively, in healthy males and females aged between 20 and 65 years⁽ Reference Mustad, Etherton and Cooper ^{72 )}. Additionally, Berry et al.⁽ Reference Berry, Eisenberg and Haratz ^{91 )} reported a 12-week MUFA or PUFA diet reduced TC by about 10 and 16 %, respectively, while reducing LDL-cholesterol by 14 and 21 %. A meta-analysis of 13 614 participants participating in diets where SFA was replaced with PUFA, found that on average there was a 0·76 mm/l (29 mg/dl) reduction in TC, and that each 1 mm/l reduction in TC was associated with 24 % reduced risk for a CHD event. Moreover, each additional year of diet was related to a further 9·2 % risk reduction⁽ Reference Mozaffarian, Micha and Wallace ^{11 )}.

Additionally, the use of phytosterols is recommended as a therapeutic strategy for the treatment of hypercholesterolaemia. It is estimated a 2 g/d dose of phytosterols lowers LDL-cholesterol by 10 %⁽ Reference Katan, Grundy and Jones ^{92 )}. A meta-analysis of 124 studies concluded 0·6–3·3 g/d phytosterols reduced LDL-cholesterol by 6–12 %, with stanols and sterols showing similar LDL-cholesterol lowering efficacies⁽ Reference Ras, Geleijnse and Trautwein ^{85 )}. The effect of phytosterols diminishes above about 2–3 g/d⁽ Reference Katan, Grundy and Jones ^{92 )}, consequently 2 g/d is generally accepted as the daily amount required to help treat hypercholesterolaemia^{( 93 )}.

Furthermore, meta-analysis of sixty-seven trials revealed that soluble fibre could lower TC and LDL-cholesterol by 0·045 and 0·057 mm/l per g soluble fibre, respectively⁽ Reference Brown, Rosner and Willett ^{94 )}. There are a variety of mechanisms that dietary fibre could influence; for example, bile acid micelle formulation, fat excretion, intestinal motility, SCFA production and absorption of macronutrients⁽ Reference Weickert and Pfeiffer ^{95 –} Reference Slavin ^{97 )}. Interestingly, diet may not only impact cholesterol metabolism directly, and may also interact with the gut microbiome⁽ Reference Gibson, Beatty and Wang ^{98 )}. For example, David et al.⁽ Reference David, Maurice and Carmody ^{99 )} describe that short-term changes to diet could significantly influence both the microbial community structure and bacterial gene expression. For instance, a 5 d animal-based-diet significantly altered the abundance of twenty-two clusters, elevated faecal bile acid concentration, and increased BSH expression⁽ Reference David, Maurice and Carmody ^{99 )}. Everard et al.⁽ Reference Everard, Lazarevic and Gaia ^{100 )} describe that high-fat feeding significantly affected twenty genera. This may have been influenced by a decline in the antimicrobial peptide regenerating islet-derived protein three gamma. Interestingly, prebiotic treatment increased regenerating islet-derived protein three gamma  about 6-fold within the colon. Targeting the digestive microbiota with pre- and probiotics is a potential avenue for the treatment of diseases including hypercholesterolaemia.

Targeting the gut microbiome

There is emerging evidence indicating the important role for the gut microbiome in regulating many biological systems, including cholesterol metabolism⁽ Reference Cénit, Matzaraki and Tigchelaar ^{101 ,} Reference Kinross, Darzi and Nicholson ^{102 )}. Ageing is associated with the alteration of the gut microbiome. These changes differ between individuals and populations, as diet, lifestyle, host health and antibiotic use likely play a role in composition⁽ Reference Claesson, Jeffery and Conde ^{103 ,} Reference O'Sullivan, Coakley and Lakshminarayanan ^{104 )}. However, it has been observed Lactobacillus and Bifidobacterium species diversity and total count decline with age⁽ Reference Hopkins and Macfarlane ^{8 ,} Reference Woodmansey, McMurdo and Macfarlane ^{66 )}. Supplementation with probiotics may have the ability to partially ameliorate changes associated with ageing, such as immunosenescence, neurodegeneration, carcinogenic transformation and hypercholesterolaemia⁽ Reference Patel, Singh and Panaich ^{105 ,} Reference Moroti, Souza Magri and de Rezende Costa ^{106 )}.

With regard to hypercholesterolaemia treatment, supplementation with probiotics has been associated with a decline in TC and LDL-cholesterol⁽ Reference Guo, Liu and Zhang ^{9 )}. A meta-analysis of probiotic administration studies concluded probiotic administration resulted in a mean 6·40 mg/dl (0·17 mm/l) reduction in TC and 4·90 mg/dl (0·13 mm/l) reduction in LDL-cholesterol. Additionally, probiotics were observed to have beneficial effects on TC and LDL-cholesterol for individuals with high, borderline high and normal cholesterol levels⁽ Reference Guo, Liu and Zhang ^{9 )}. Probiotics have also been an effective treatment strategy for individuals with underlying conditions. For example, Bernini et al.⁽ Reference Bernini, Simão and Alfieri ^{107 )} observed ingestion of 80 ml probiotic milk containing 3·4 × 10⁸ colony-forming units/ml Bifidobacterium lactis HN019 for 45 d, significantly lowered TC from 209 to 194 mg/dl (5·40 and 5·02 mm/l), and LDL from 128·5 to 111 mg/dl (3·32–2·87 mm/l) in patients with metabolic syndrome, in addition to lowering BMI. Moreover, the proinflammatory cytokines TNF-α and IL-6 were significantly reduced with probiotic treatment.

There are several proposed mechanisms to explain the lipid-lowering ability of probiotics⁽ Reference Ishimwe, Daliri and Lee ^{108 )}. For example, some probiotics produce the bile acid deconjugating enzyme BSH, which may increase bile acid excretion, and up-regulate the conversion of cholesterol to bile acids to replace those lost⁽ Reference Tsai, Lin and Hsieh ^{109 )}. It has also been observed that probiotics increase SCFA, which inhibit the hepatic rate-limiting enzyme in cholesterol synthesis, HMG CoA reductase⁽ Reference Ishimwe, Daliri and Lee ^{108 )}. Furthermore, there may be a reduction in cholesterol absorption due to the assimilation and possible incorporation of cholesterol into the bacterial cell membrane⁽ Reference Brashears, Gilliland and Buck ^{110 )}. Additionally, some bacteria may act by destabilising cholesterol micelles resulting in the coprecipitation of cholesterol with deconjugated bile acids⁽ Reference Brashears, Gilliland and Buck ^{110 )}.