FAQ

What are plant sterols?

Sterols are present in plants and animals. The major sterol in animals is cholesterol. Several sterols are present in plants, primarily beta-sitosterol (80 per cent), campesterol and stigmasterol (Figure 1). These phytosterols, or plant sterols, are essential components of the cell membranes of plants - a similar role to that played by cholesterol in animal cells. The structure of plant sterols is very similar to that of cholesterol, differing only in the side chain.

Like cholesterol, plant sterols are minor components in the human diet. On average, Australians eat about 200-300mg of plant sterols per day. This is roughly equivalent to the intake of cholesterol. Strict vegetarians may consume twice the average intake. Major sources of plant sterols are vegetable oils and foods made from vegetable oils such as margarines and salad dressings. Nuts and seeds are also good sources. Cereals provide approximately 20 per cent of plant sterol intake.

Figure 1: Sterol Structures
Figure

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Why enrich spreads with plant sterols?

Increased intake of plant sterols reduces blood cholesterol levels - this was first observed in humans in 1953. There are many studies into the effects of plant sterols on blood cholesterol. To achieve this benefit plant sterols need to be consumed in moderate amounts on a daily basis. Enriching staple foods like margarines or spreads with plant sterols provides a simple means of realising the cholesterol lowering potential of plant sterols.

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Is there evidence that plant sterol spreads lower cholesterol? In recent years, about thirty studies have been conducted using plant sterol-enriched foods, primarily spreads. Effects on serum LDL-cholesterol levels were very consistent with reductions falling in the range of 8-14%. In 2000, Malcolm Law published a pooled analysis of 14 randomised, double blind trials in which plant sterol spreads were compared with standard margarines (1). The mean LDL-cholesterol reduction was approximately 12%. A study of similar design, conducted in Australia at CSIRO (2), compared Flora pro-activ with a standard polyunsaturated margarine and found a reduction in serum LDL of 9.6% (Figure 2).

Figure 2: CSIRO study: Effects on serum cholesterol when poly margarine is replaced with plant sterol spread
Figure 2: CSIRO study: Effects on serum cholesterol when poly margarine is replaced with plant sterol spread
Noakes M et al. Am J Clin Nutr 2002;75:79-86

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How do plant sterols work in the human body?

Plant sterols partially block the absorption of cholesterol from the intestine reducing the amount that reaches the bloodstream plus any cholesterol consumed in food.

Figure 5: How plant sterols work in the human body
Figure 5: How plant sterols work in the human body

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What is the mechanism of cholesterol lowering?

Plant sterols act by reducing the absorption of dietary and biliary cholesterol from the gastrointestinal tract. During digestion, cholesterol from both sources competes with plant sterols for incorporation into the mixed micelles formed in the gut. Cholesterol predominates because of its higher concentration and is absorbed into the lining of the gut from the micelles. Any cholesterol that cannot be incorporated into the micelles is not absorped and passes out of the gut.

When appreciable amounts of plant sterols are consumed they compete with cholesterol and displace it from the micelles. As a result less cholesterol is absorbed and more is excreted through the bowel (Figure 6). The fall in cholesterol absorption is between 25% and 50%(1). The liver responds to the decreased supply of cholesterol from the gut by increasing uptake of cholesterol from the bloodstream and increasing in cholesterol synthesis. The overall result is a fall in serum total and LDL-cholesterol but no change in triglycerides or HDL-cholesterol.

Figure 6: Mechanism of Action of Phytosterols on Cholesterol Metabolism
Figure 6: Mechanism of Action of Phytosterols on Cholesterol Metabolism

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How much plant sterol spread is needed to reduce blood cholesterol?

The amounts of plant sterols consumed in most studies using spreads fall in the range of 1.6 to 3.0g/day. Studies with larger and smaller intakes provide a clear picture of the relationship between plant sterol intakes and a fall in serum LDL-cholesterol to be developed (Figure 7). The figure shows that near-maximal blood cholesterol lowering occurs with an intake of plant sterols of 2g/day. This is equivalent to 25g/day of plant sterol spread. Substantial cholesterol-lowering occurs with a plant sterol intake of half this level. At intakes above 2g/day, further falls in LDL-cholesterol occur but are marginal. Based on these results an intake of plant sterols of 2g/day appears optimal.

Figure 7: Reduction in LDL-Cholesterol Levels with Plant Sterol Consumption
Figure 7: Reduction in LDL-Cholesterol Levels with Plant Sterol Consumption
Unilever Studies

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Is the cholesterol-lowering effect of plant sterol spreads additive to that of statin medication?

Yes. This was recently confirmed in a multi-centre, randomised, double-blind study conducted in 10 research centres around Australia (4). Men and women with primary hypercholesterolaemia were commenced on a low saturated fat diet then provided with either standard polyunsaturated margarine or plant sterol spread (Flora pro-activ), plus either statin or placebo. The results are shown in Figure 4. When the plant sterol spread replaced the standard polyunsaturated spread LDL-cholesterol fell by approximately 8%, again confirming that the effects of plant sterol spreads are additive to those of a low saturated fat diet.

When the plant sterol spread was taken in addition to the statin LDL-cholesterol fell by a further 6%, indicating an additive effect. The total fall in LDL-cholesterol when the statin and plant sterol spread were taken together was nearly 39%. The additional cholesterol-lowering provided by the plant sterol spread was equivalent to at least doubling the dose of statin.

Figure 4: Percentage changes in LDL cholesterol
Figure 4: Percentage changes in LDL cholesterol
Simons LA et al. Am J Cardiol 2002;90:737-40

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Is the cholesterol-lowering effect of plant sterol spreads additive to that of reducing saturated fat?

Yes. The mechanisms of action of plant sterols and dietary fats are different and complementary. The additive effect of plant sterols and dietary fatty acid change was clearly shown in a study in Professor Jim Mann's laboratory in Dunedin New Zealand (3).

Subjects were commenced on an otherwise healthy diet containing 20g of butter. The butter was replaced firstly with a standard polyunsaturated margarine and then with a plant sterol spread (Flora pro-activ) in four week intervention phases. As Figure 3 shows, LDL-cholesterol fell appreciably (5.5%) when the polyunsaturated margarine replaced butter ?an effect due entirely to a change in dietary fatty acid intake. When the plant sterol spread replaced the polyunsaturated margarine, LDL-cholesterol fell (6.8%) due entirely to the plant sterols. This confirmed the additive effect. The total fall in LDL-cholesterol when butter was replaced by Flora pro-activ was 12.3%.

Figure 3: % Reduction in total and LDL-cholesterol when margarine and plant sterol spread replace butter
Figure 3: % Reduction in total and LDL-cholesterol when margarine and plant sterol spread replace butter
Cleghorn CL et al. Eur J Clin Nutr 2003;57:170-176

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Are plant sterols spreads recommended for people with diabetes?

Yes. People with diabetes, especially Type 2, are at very high risk of developing coronary heart disease. The Heart Protection Study (5) recently showed that cholesterol-lowering (with statins) was associated with highly significant falls of about one-quarter in coronary death, other cardiovascular outcomes and all-cause mortality. Importantly, benefits in these high-risk subjects were evident irrespective of initial cholesterol levels. There was no threshold below which cholesterol lowering did not reduce coronary risk.

The importance of lowering LDL-cholesterol in this high risk group is reflected in the recent dietary recommendations of the American Diabetes Association (6) which include:
  • reducing saturated fat to <7% energy
  • increasing plant sterols/stanols (2g/day)
  • increasing soluble fibre (10-25g/day)
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What do other health authorities say about plant sterol spreads?

The National Heart Foundation's position statement on plant sterols concludes "For people with an increased risk of coronary heart disease, plant sterols and stanols provide an additional option for risk reduction through lowering the level of plasma cholesterol"(7).

The Third Report of National Cholesterol Education Program Expert Panel on Detection, Evaluation and Treatment of High Blood Cholesterol in Adults in the United States has endorsed plant sterols for the management of cholesterol (8). The NCEP recommends a multifaceted lifestyle approach, the 'essential features' including:
  • Reduced intakes of saturated fats (<7% of calories) and cholesterol
  • Plant sterols/stanols (2g/day) and increased viscous (soluble) fibre (10-25g/day)
Alice Lichtenstein from Tufts University in Boston recently summarised data generated in the last couple of years that contributed to the NCEP decision (9).

Very similar advice is offered by the American Heart Association (10).

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Are plant sterols absorbed into the bloodstream?

About 3-5% of ingested plant sterols is absorbed from the gastrointestinal tract. The remaining 95-97% of plant sterols pass through the gut and is excreted. Campesterol is slightly better absorbed than beta-sitosterol, the predominant plant sterol. Absorbed plant sterols do not accumulate in the body - they are rapidly excreted through the bile. Under normal dietary conditions, plant sterols comprise less than 1% of all sterols in plasma, by far the majority being cholesterol. When plant sterol-enriched spreads are consumed, serum levels of plant sterols rise but remain at less than 1% of total sterols.

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Are there any side effects of plant sterol spreads?

Other than lowering total and LDL-cholesterol, the only other known effect of plant sterol-enriched spreads is a modest fall in serum levels of some carotenoids, most often ß-carotene. Levels of ß-carotene remain in the normal range and the reduction is well within the seasonal variation seen in serum ß-carotene levels. It is not known whether this change in serum ß-carotene levels has any implications for health, positive or negative. It has been suggested that ß-carotene may protect against cancer. However, supplementation trials have been associated with an increase or no change in risk. The large Heart Protection Study recently found no association between supplementation with ß-carotene and any health outcome (11).

In a CSIRO trial, advice to consume five serves of fruit and vegetables each day, including one carotenoid-rich serve, maintained serum ß-carotene at baseline levels i.e. negating the carotenoid-lowering effect (2). Encouraging patients to eat extra fruits and vegetables would appear to be prudent advice for people consuming plant sterol-enriched spreads or indeed for those at risk of cardiovascular disease.

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Has the safety of plant sterol spreads been investigated?

These safety data have been extensively and favourably reviewed by regulatory bodies in twenty two countries, including Food Standards Australia New Zealand (FSANZ).

A number of other lines of evidence indicate plant sterols are safe:
  • All humans are exposed to plant sterols at low levels through their everyday diet. No adverse side effects have been observed in population groups with higher intakes of plant sterols, such as vegetarians.
  • Following the discovery of their blood cholesterol-lowering effect, plant sterol preparations were used in over 100 studies, often at high doses, without recorded adverse side effects.
  • Plant sterols are poorly absorbed from the gastrointestinal tract and rapidly excreted from the bloodstream.
  • A plant stanol (hydrogenated sterol) margarine was introduced in Finland in the early 1990s. No adverse effects have been observed following widespread use in the Finnish population over the intervening years.
Prior to the introduction of Flora pro-activ, Unilever initiated a series of its own safety studies, all of which are now published (12-18). These included toxicity studies and two-generation studies. None of these studies suggested safety concerns with plant sterols at high levels of intake, let alone the low levels used in plant sterol spreads.

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Are plant sterol spreads suitable for children?

There is no evidence to suggest that plant sterols are unsafe for any population group, including children. Food Standards Australia New Zealand has recommended that members of at-risk groups in the community, such as pregnant/lactating women and children under 5, seek medical advice before using plant sterol spreads. This is prudent advice given that these groups have specialised nutrient requirements.

Children with familial hypercholesterolaemia are at very high coronary risk and may go on to suffer myocardial infarctions in their twenties or thirties. Cholesterol-lowering medication is seldom recommended for people under about 18 years of age. Plant sterol-enriched spreads may be a useful adjunct to a low saturated fat diet for these children (19). Again, the spreads should be used under medical supervision.

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Are plant sterol spreads contraindicated in any groups of people?

Yes. There is a rare generic condition, beta-sitosterolaemia, in which plant sterols are readily absorbed from the gut and poorly excreted through the bile. Plant sterols accumulate and subjects experience premature coronary heart disease. Beta-sitosterolaemia is extremely rare - about fifty cases are known world-wide.

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Can plant sterol spreads be part of a weight management program?

25g of Flora pro-activ per day provides maximum cholesterol lowering. This amount can easily be included in a 6000 KJ per day weight loss plan. Flora pro-activ light can be used if greater restrictions on total kilojoules are considered. 25g of Flora pro-activ or Flora pro-activ light is sufficient for three to four slices of bread a day.

Very low fat diets may be undesirable as they compromise intake of essential fatty acids and fat-soluble vitamins, lower palatability of the diet and also reduce compliance.

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References
  1. Law M. BMJ 2000;320:861-64.
  2. Noakes M et al. Am J Clin Nutr 2002; 75: 79-86.
  3. Cleghorn CL et al. Eur J Clin Nutr 2003;57:170-176.
  4. Simons LA et al. Am J Cardiol 2002;90:737-40.
  5. HPS Collaborative Group. Lancet 2002;360:7-22
  6. American Diabetes Association. Clinical Practice Recommendations 2003.Diabetes Care 2003;26: Supplement 1.
  7. http://www.heartfoundation.com.au/ (health professionals section)
  8. Executive Summary. Third report of NCEP Expert Panel. JAMA 2001;285:2486-97.
  9. Lichtenstein AH. Curr Opin Clin Nutr Metab Care 2002;5:147-52.
  10. American Heart Association. AHA Guidelines for the Primary Prevention of Cardiovascular Disease and Stroke: 2002 Update. Circulation 2002:106:388.
  11. HPS Collaborative Group. Lancet 2002;360:23-33
  12. Ayesh R et al. Food Chem Toxicol 1999;37:1127-1138.
  13. Baker VA et al. Food Chem Toxicol 1999;37:13-22.
  14. Hepburn PA et al. Food Chem Toxicol 1999;37:521-532.
  15. Sanders DJ et al. Food Chem Toxicol 2000; 38:485-491.
  16. Waalkens-Berendsen DH et al. Food Chem Toxicol 1999; 37:683-696.
  17. Weststrate JA et al. Food Chem Toxicol 1999; 37:1063-1071.
  18. Wolfreys AM et al. Food Chem Toxicol 2002; 40: 461-470.
  19. Amundsen AL et al. Am J Clin Nutr 2002, 76: 338-44.
  20. J Jones PJ et al. J Lipid Res 2000;41:697-705.
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