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Spotlight on the cholesterol-lowering effect of oats

Linda Main
BSc SD
Dietetic Adviser
Heart UK - The Cholesterol Charity

Cholesterol is the single biggest risk factor for coronary heart disease and a major risk factor in stroke and other circulatory disorders.1 Despite this, two thirds of the population have raised cholesterol above desirable levels2

Beta-glucans are a type of soluble fibre present in the bran of cereal grains. They dissolve in the gastrointestinal tract and become thickened and viscous, resembling the consistency of wallpaper paste. Barley and oats contain far more beta-glucans than other grains, boasting 7 g and 5 g per 100 g respectively.

How do beta-glucans lower cholesterol?
The reduction of cholesterol by beta-glucans is probably the result of several effects; the largest of these is due to the decreased absorption of bile acids. Bile acids are secreted into the duodenum where their role is to emulsify fats and cholesterol. Fatty acids are then packaged into small particles known as micelles, which can be absorbed at the brush border of the gut surface.

Bile acids themselves are not absorbed with the micelles but, under normal circumstances, are actively reabsorbed lower down in the terminal ileum where they then enter the portal vein and are recycled by the liver. This recycling of bile acids is known as the enterohepatic circulation.

When beta-glucans are present in the gut they bind with bile acids, interrupting their re-absorption and causing their excretion via faeces. This stimulates the uptake of LDL cholesterol by the liver and the synthesis of additional bile acids from the breakdown of cholesterol. This interruption of the enterohepatic circulation is the main mechanism for reducing plasma total and LDL cholesterol.

What is the evidence?
The cholesterol-reducing effect of beta-glucans has been the subject of a number of reviews. The resulting consensus of opinion confirms that both total and LDL cholesterol are lowered in individuals with either normal or moderately raised cholesterol levels (see Table 1). HDL cholesterol and triglyceride levels appear to be unaffected by soluble fibre.3,4

[[Tab 1 oats]]

De Groot first observed in 1963 that rolled oats could decrease total cholesterol levels by as much as 11%. This initial finding provided a stimulus to subsequent metabolic studies, which showed significant cholesterol-lowering effects (13-26%).

Studies in free-living populations followed, showing more variable results, some of which can be explained by the size of the study, the amount of beta-glucans in the treatment arm, the food matrix in which beta-glucans was present, or the poor design of trials.

A number of systematic reviews and meta-analyses provide an overview of the accumulated science and take the quality of individual trials into account. The first, published in 1992, concluded that there was strong evidence to support the view that oat products in the diet can bring about modest reductions in blood cholesterol.5 An intake of 3 g per day of soluble fibre was estimated to lower total cholesterol by 0.13 to 0.16 mmol/l.

In 1999, Brown et al examined the effects of a range of soluble fibres on blood cholesterol levels.4 The Harvard researchers showed that all types of soluble fibres lowered total and LDL cholesterol by similar amounts, with 3 g of soluble fibre from oats providing a reduction of total and LDL cholesterol of 0.13 mmol/l. In 2007 a Cochrane review found that consuming oats appeared to lower both total and LDL cholesterol.6 The authors expressed some reservations about the size and quality of some studies but, once pooled in a meta-analysis, a significant effect on total and LDL cholesterol was observed.

Two further reviews took place in 2008. In an evidence-based review of soluble fibres, Theuwissen and Mensink identified a dose-response effect on cholesterol reduction.3 They estimated that every gram of soluble fibre eaten would have an additive effect on cholesterol reduction. Ruxton and Derbyshire in the most recent systematic review evaluated a total of 21 studies, concluding that regular consumption of oats and oat products was necessary to bring about reductions in total and LDL cholesterol with oat bran having the strongest effect.7

How big is the effect?
The US National Institutes of Health National Cholesterol Education Program (NCEP) estimates that soluble fibre can reduce cholesterol levels by 5%. Their published recommendations endorse oats and other sources of viscous (soluble) fibre (5-10 g per day) as part of a healthy cholesterol lowering diet (see Table 2).8

[[Tab 2 oats]]

Soluble fibre can be combined with other cholesterol-lowering foods for an additive effect. Evidence for this comes from Jenkins. He researched a combination of foods (low saturated fat diet plus soluble fibres, plant sterols, almonds and soy protein) in patients with raised cholesterol under controlled conditions.9 His initial results showed a reduction in LDL cholesterol of 28-35% when this "portfolio" approach was strictly adhered to, an effect similar to a first-generation statin. 

The practical aspects of such a diet have been tested during a one-year trial in 66 patients with raised cholesterol.10 The participants self-selected their diets, buying all ingredients from local stores. Only the sterol-enriched spread and bread (containing oat-bran) were specially provided as, at that time, these would have been difficult to obtain otherwise. Overall, a 13% reduction of LDL cholesterol was achieved and sustained over one year with approximately one third of respondents having a reduction of 20% or more.

Health claims
In 1997 the US Food and Drink Administration approved a health claim for oats and the reduction in risk of CHD. Since then, at least four other similar health claims have been approved: UK (2004), Sweden (2002) Malaysia (2006) and France (2008).

In 2004, the UK Joint Health Claims Initiative approved the following health claim for soluble fibre: "The inclusion of oats as part of a diet low in saturated fat and a healthy lifestyle can help reduce blood cholesterol". In order to carry the claim, any one food had to provide at least one quarter (0.75 g) of the 3 g deemed necessary for the effect in a single serving.

With the advent of recent health claims legislation across Europe there has been a push to consolidate a list of claims for all member states supported by well-established science. Once fully evaluated by the European Food Safety Authority (EFSA) Panel on Dietetic Products, Nutrition and Allergies, these article 13 claims will eventually form an authorised list of published health claims for use on food products that comply with the conditions of use.

Opinions have recently been published for just over 900 of the several thousand health claims submitted. A large proportion of these were negative, but the relationship between beta-glucans and the reduction of blood cholesterol concentrations was approved. Once the authorised list is finally published, foods wishing to bear the claim "regular consumption of beta-glucans contributes to maintenance of normal blood cholesterol concentrations" must contribute at least 3 g per day of beta-glucans from oats, oat bran, barley, barley bran or from mixtures of non-processed or minimally processed beta-glucans in one or
more servings.

Summary   
Modifying diet can have a significant effect on total and LDL cholesterol levels. The baseline for these changes is a diet low in saturated fat. Further reductions in cholesterol levels can be made through the regular consumption of soluble fibres (including beta-glucans).

Wholegrain barley, or its bran, is not readily available either as a food ingredient or in commercially prepared foods in the UK, and consequently most people do not eat sufficient barley bran to have an effect on cholesterol levels. Oat bran and oatmeal are, therefore, the most practical and palatable sources of beta-glucans in the UK.

Three grams of beta-glucans can be provided by 40 g oat bran or 60 g of oatmeal, a practical amount which can be eaten on a regular daily basis. Some patients will be very open to consuming porridge oats or oat-based muesli every day; but what about those who do not favour such worthy cereals? Thankfully, a greater range of oat-based breakfast cereals that comply with the compositional guidance from EFSA are now available to suit other preferences.

Early attempts at making bread from oat flour proved unpalatable, but better techniques now mean that bread can be an acceptable source of oat flour and oat bran. Thus, an increased range of consumer-friendly products (such as breakfast cereals and breads) have started to extend the range of meal options available for health professionals to recommend.     

References
1. HEART UK - The Cholesterol Charity. Cholesterol and the Ageing Population. London: Heart UK; 2007.
2. British Heart Foundation (BHF). Coronary Heart Disease Statistics 2008. London: BHF; 2008. Available from: www.heartstats.org
3. Thuewissen E, Mensink RP. Water-soluble dietary fibers and cardiovascular disease. Physiol Behav 2008;94:285-92.
4. Brown L, Rosner B, Willett WW, Sacks FM. Cholesterol-lowering effects of dietary fiber: a meta-analysis. Am J Clin Nutr 1999; 69(1):30-42.
5. Ripsin CM, Keenan JM, Jacobs DR Jr. Oat products and lipid lowering. A meta-analysis. JAMA 1992;267(24):3317-25.
6. Kelly SA, Summerbell CD, Brynes A, Whittaker V, Frost G. Wholegrain cereals for coronary heart disease. Cochrane Database Syst Rev 2007; 18(2) Art:CD005051.
7. Ruxton CHS, Derbyshire E. A systematic review of the association between cardiovascular risk factors and regular consumption of oats. British Food Journal 2008;110(11):1119-32.
8. The National Institutes of Health National Heart, Lung and Blood Institute. Third Report of the Expert Panel on the Detection, Evaluation and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III). MD: NHLBI; 2002.
9. Jenkins DJ, Kendall CW, Faulkner D et al. A dietary portfolio approach to cholesterol reduction: combined effects of plant sterols, vegetable proteins and viscous fibres in hypercholesterolaemia. Metabolism 2002;51(12): 1596-604.
10. Jenkins DJ, Kendall CW, Faulkner DA et al. Assessment of the longer-term effects of a dietary portfolio of cholesterol-lowering foods in hypercholesterolemia. Am J Clin Nutr 2006; 83:582-91.