"The foods with the most evidence for cholesterol reduction are nuts, legumes, whole cereals rich in soluble fiber, and cocoa and its main commercial product, chocolate."
Coronary heart disease (CHD) is a leading cause of death and disability worldwide. As shown in landmark epidemiological studies, unhealthy lifestyles (smoking, lack of exercise and bad dietary habits) contribute nearly 80% of population-attributable CHD risks.[1,2] Hence, lifestyle modification is the cornerstone of population-based strategies for CHD prevention. An abnormal lipid profile, particularly high LDL-cholesterol levels, is a critical contributor to the CHD burden that can be significantly influenced by dietary changes.[3] As first recognized more than 50 years ago, dietary fatty acids are major determinants of plasma cholesterol concentrations.[4] Since then, a wealth of epidemiologic studies have assessed associations between dietary exposures and CHD,[5] while many clinical trials have shown the impact of dietary changes on cardiovascular risk factors, including hypercholesterolemia.[6]
Strong evidence supports the notion that a cardioprotective dietary pattern is high in vegetables, fruits, legumes, nuts, whole grains and lean protein sources such as white meats, fish and low-fat dairy products, while being low in saturated and trans-fatty acids, and foods with a high glycemic index. This concept fits both the traditional Mediterranean diet and so-called 'prudent' dietary patterns, as opposed to harmful 'western' diets.[5,7] Adherence to a healthy dietary pattern has long been recommended as the first line of treatment to decrease LDL-cholesterol levels and reduce cardiovascular risk in patients with hypercholesterolemia.[3]
For the dietary management of hypercholesterolemia, it is important to note that some natural foods possess cholesterol-lowering properties by themselves, independently of the background diet. The foods with the most evidence for cholesterol reduction are nuts,[8] legumes,[9] whole cereals rich in soluble fiber,[10] and cocoa and its main commercial product, chocolate.[11] All such foods share the common characteristic of being edible seeds. Seeds are composed of complex matrices in the outer layer and the germ and are rich in minerals (potassium, calcium and magnesium, but little sodium), vitamins (particularly tocopherols and folic acid), phytosterols, polyphenols and other bioactive phytochemicals, which among other functions, serve to protect the plant's DNA from oxidative stress and thus preserve the potential of perpetuating the species.[12] On the other hand, the endosperm of seeds contains nutritive components aimed at sustaining the growth of the embryo, with a variable mixture of high-quality protein, complex carbohydrate and fat, depending on the type of seed. While some seed components (soluble fiber, unsaturated fatty acids and phytosterols) have the potential to reduce blood cholesterol, the whole seed provides a wide array of bioactive molecules that are likely to have significant health benefits. This goes some way to explain why frequent consumption of edible seeds and/or derived products has been associated with salutary effects beyond cholesterol lowering on intermediate cardiovascular risk markers in clinical trials and, more importantly, with protection from development of CHD in observational cohort studies.[8–11] A brief review of the lipid and lipoprotein changes associated with regular consumption of these foods follows.
The scientific evidence behind the proposal of nuts being cardioprotective food stems from both epidemiological observations suggesting a consistent inverse association between the frequency of nut intake and development of CHD and numerous short-term clinical trials demonstrating that nut-enriched diets reduce blood cholesterol[8] and also have beneficial effects on other intermediate cardiovascular risk markers.[13] As shown in a recent pooled analysis of 25 feeding trials, tree nuts and peanuts (technically a legume, but included in the nuts group because of a similar nutrient composition) have a cholesterol-lowering effect that is dose-related and independent of the type of nut tested.[14] Specifically, consumption of 67 g (2.4 oz) of nuts daily produced estimated mean reductions of 10.9 mg/dl (5.1%) in total cholesterol and 10.2 mg/dl (7.4%) in LDL-cholesterol. Nuts had no significant effect on HDL-cholesterol or triglycerides, except in participants with high serum triglycerides, in whom a significant 10.2 mg/dl reduction was observed. Noticeably, more pronounced lipid-lowering effects were observed in subjects with higher baseline LDL-cholesterol levels and lower BMI. Nuts are high-energy foods because they contain on average 50% fat, although most fatty acids are unsaturated. Contrary to expectations, evidence from both epidemiological studies and clinical trials suggests that regular nut consumption does not induce weight gain, which is mainly attributable to their satiating effect.[8]
Legumes are a good source of complex carbohydrate and soluble fiber, in addition to minerals, polyphenols, phytosterols and other seed phytochemicals. They are also rich in saponins, compounds that naturally interfere with intestinal cholesterol absorption. Soy beans and some of their constituents, namely soy protein and isoflavones (phytoestrogens), have been much studied for hypocholesterolemic and other health effects, but other nonsoy legumes (other beans, peas, lentils, among others) are preferentially consumed in the western hemisphere. As reported in a recent meta-analysis of ten randomized, controlled trials, daily consumption of one serving of nonsoy legumes is associated with mean total and LDL-cholesterol reductions of 12 mg/dl and 8 mg/dl, respectively, with small increases in HDL-cholesterol and reductions in triglycerides.[9] Of note, frequent legume consumption has also been associated with reduced CHD rates in prospective studies.[15]
The lipid effects of whole grains depend on their soluble fiber content. Barley and oats, but not whole wheat, rye or rice, contain β-glucan, a subtype of soluble fiber that becomes gel-like in the intestine and binds bile acids, thereby promoting their fecal loss.[10] Bile acids are derived from cholesterol and the liver compensates for the losses by increasing synthesis, thus using extra cholesterol from enhanced uptake of circulating LDL particles, resulting in reduction of LDL-cholesterol levels. Oat diets, usually as breakfast cereal, have been the most studied against control diets in controlled clinical trials and results have demonstrated mean reductions of total cholesterol of 7.7 mg/dl and of LDL-cholesterol of 7 mg/dl. Interestingly, increased consumption of whole grains of any kind, even those not containing soluble fiber, has been consistently associated with a reduced risk of CHD and other cardiometabolic outcomes,[10] probably because all whole grains are seeds rich in beneficial nutrients and phytochemicals.
Cocoa is the seed of the tree Theobroma cacao and has the characteristic composition of beneficial nutrients, minerals and phytochemicals found in all seeds. Like nuts, cocoa is a fat-rich food, but here the predominant fatty acid is stearic, a saturated fatty acid that the organism rapidly transforms into the monounsaturated oleic acid. Cocoa also contains the potent polyphenolic antioxidants catechin and its isomer epicatechin, flavonoids to which much of the beneficial properties of chocolate and other vegetable foods containing them are attributed. Both white and dark chocolate are rich in cocoa fat (butter), but only dark chocolate retains the phytochemicals of cocoa seeds, hence its bitter taste, which is typical of all polyphenol-rich foods. Several recent controlled clinical trials have shown that daily consumption of 50–100 g of dark chocolate reduces total and LDL-cholesterol between 5 and 10% compared with white chocolate, without consistent effects on triglycerides or HDL-cholesterol.[11] Similarly to nuts and whole grains, prospective studies suggest that frequent consumption of cocoa products protects from CHD, but also from stroke and diabetes. Besides cholesterol-lowering, the benefits ascribed to cocoa are probably linked to blood pressure reduction, improvement of insulin resistance and anti-inflammatory effects of flavonoid-rich chocolate products.[11]
In addition to these natural seed-foods, dietary management of hypercholesterolemia may include industry-transformed foods in the form of cholesterol-lowering supplements and other food products, such as virgin olive oil, which cannot be described here due to space constraints. The cholesterol-lowering supplements include margarines, dairy products and other foods enriched with gram doses of phytosterols or stanols; soluble fiber products, such as psyllium, pectin and guar gum; soy protein and red yeast rice.[16] Virgin olive oil, the culinary fat preferentially used in the Mediterranean diet, is a pure 'juice' of olives containing both the fat (mainly oleic acid) and the minor bioactive components of olives, such as phytosterols, tocopherols and phenolic compounds, and has recently emerged as another cardioprotective food, with both cholesterol-lowering and HDL-raising properties.[17] A bold approach to dietary cholesterol-lowering is to use a combination of these foods. The portfolio diet described by Jenkins and colleagues comprised four key components:[18]
Foods rich in soluble fiber
Soy protein
Plant sterols
Almonds
Their combined effect resulted in a 29% LDL-cholesterol reduction, comparable with that observed with a small dose of a statin.[18]
Finally, one must pay attention to the overall dietary pattern of individuals at risk, such as those with hypercholesterolemia. Exposure to the Mediterranean diet, in which nuts and olive oil are key food items, stands out as one of the dietary factors with stronger evidence for a causal link with CHD prevention.[5] Ongoing research, such as the large, randomized PREDIMED trial, wherein Mediterranean diets supplemented with one daily serving of mixed nuts or up to 50 ml of virgin olive oil are tested against a control diet in participants at high cardiovascular risk,[19] might eventually settle the critical issue of whether, in addition to improving blood cholesterol and other cardiovascular risk factors,[20] these dietary approaches also prevent cardiovascular events.
Acknowledgements
CIBERobn is an initiative of ISCIII, Spain.
Clin Lipidology. 2012;7(5):489-492. © 2012 Future Medicine Ltd.