Cholesterol lowering effects of dietary fibre
Blood cholesterol is a major risk factor for coronary artery disease. Increasing dietary fibre has been recommended as a way to lower cholesterol levels. Although replaced by non-starch polysaccharides, most people are familiar with the term dietary fibre so it will be used here.
Dietary fibre is a collective term for a variety of plant substances that are resistant to digestion by human gastrointestinal enzymes. They can be classified into two groups depending on their solubility in water. It has been suggested that soluble fibres such as oats, psyllium, pectin and guar gum lower total and low density lipoprotein (LDL) cholesterol. The question is by how much? This meta-analysis aims to find out.
Soluble fibre reduces total and LDL cholesterol. One gram of soluble fibre (e.g. from one apple) can lower total cholesterol by about 0.045 mmol/L. To put this into perspective, the current UK recommendation is an average intake of 18 g/day (which includes both soluble and insoluble fibre). This would lower cholesterol by about 0.40 mmol/L (the acceptable cholesterol level is up to 5.2 mmol/L). In isolation this is a small amount, but combined with the benefits of other healthy (cholesterol-lowering) behaviours (e.g. exercising) would make a worthwhile difference.
L Brown et al. Cholesterol-lowering effects of dietary fiber: a meta-analysis. American Journal of Clinical Nutrition 1999 69: 30-42.
The literature was searched using MEDLINE from 1966 to 1996. Bibliographies of identified papers were then reviewed. Only English language papers were considered. Studies were selected if the following criteria were met:
- They were controlled trials (e.g. comparing high with low fibre diets) with either a randomised crossover design (i.e. one group receiving both high and low fibre diets) or parallel design (i.e. two groups, one receiving a high fibre diet, the other a low fibre diet).
- Lipid changes were reported for both high and low fibre diet groups.
- The trial period lasted for more than 14 days.
- Soluble fibre was used from a single source (to analyse differences between fibre types).
- The amount of soluble fibre given was reported (or could be estimated).
- When participants were asked to change from their usual diets, measurements were taken after at least 14 days (to eliminate effects of overall dietary changes on plasma lipids).
- Dietary changes were made under isoenergetic conditions.
- The meta-analysis would only be undertaken if there were more than five trials for each type of fibre (oat products, psyllium, pectin and guar gum).
Sixty-seven trials were identified: 25 trials of oat products; 17 of psyllium; 7 of pectin; and 18 of guar gum. There was a total of 2,990 participants. For each type of fibre studied, participants were a mixture of healthy (in 21 studies), hyperlipidemic (in 30 studies) or diabetic (in 14 studies) men and women. Amounts of soluble fibre given to participants varied from 1.5g to 30g over periods of time varying from 14 days to 168 days. The majority of studies (57) compared high with low fibre diets; ten compared high fibre diets with diets that excluded fibre. During the trials, participants either remained on their usual diets (in 38 studies) or were on diets low in fat and cholesterol (i.e. less than 30% of energy from fat and less than 300 mg cholesterol per day). Their average initial total and LDL cholesterol concentrations were 6.23 mmol/L and 4.25 mmol/L respectively.
The net change in total cholesterol, LDL cholesterol, HDL cholesterol and triacylglycerols was calculated (i.e. the change during high soluble fibre period minus the change during low soluble fibre period).
Soluble fibre from oat products, psyllium, pectin and guar gum lowered total cholesterol and LDL cholesterol by a small amount. One gram of soluble fibre decreased total cholesterol by 0.045 mmol/L and LDL cholesterol by 0.057 mmol/L (95% confidence intervals 0.054 to 0.035 and 0.070 to 0.044 respectively). Each of the fibres reduced cholesterol by similar amounts.
Oat products and pectin did not affect HDL cholesterol; psyllium and guar gum lowered it minimally (0.002 mmol/L and 0.003 mmol/L respectively).
None of the soluble fibres affected triacylglycerols.
Estimating the direct effect of soluble fibres on blood cholesterol is a difficult undertaking. Concurrent dietary changes, particularly in fat and cholesterol intake, may mask the true relationship between increased soluble fibre intake and blood cholesterol concentrations. Despite the strict inclusion criteria for studies, there were (inevitably) differences between them, but the changes in cholesterol levels were found to be independent of these differences, e.g. health status of participants and their diets.
Although fibre-rich foods are often a combination of both soluble and insoluble fibres, some foods are particularly good sources of soluble fibre, for example, oats, fruit, vegetables and pulses (beans, lentils, chick peas). As well as contributing to reduced cholesterol levels, increasing fibre consumption may also result in eating less food with a high saturated fat and cholesterol content.
Further information: Reducing cholesterol using Benecol spread :