Beta-sitosterol

What it is, and what it is used for

Beta-sitosterol is the most common plant sterol, found in nuts, seeds, avocados and vegetable oils. You already eat a few hundred milligrams a day without trying.

It has two separate uses that get confused. At two grams a day it is a cholesterol-lowering agent, which is what the plant sterol spreads in the supermarket are for. At 60 to 130 mg a day it is used for the urinary symptoms of an enlarged prostate, which is the use the prostate trials tested and the one this page is about. The doses are not interchangeable and neither are the products.

It is also the reason some ingredients overlap. Beta-sitosterol is part of what saw palmetto and pumpkin seed oil contain, which is why a prostate complex listing all three may be delivering much less of three things than it appears to.

What the trials found

The systematic review pooled four double-blind trials covering 519 men, running from 4 to 26 weeks. Against placebo, beta-sitosterol raised peak urinary flow by 3.91 mL/s and cut residual volume after urinating by 28.6 mL. Only two of the four trials used the International Prostate Symptom Score, and across those two it improved by a weighted mean of 4.9 points.

For scale: on a 35-point scale where 8 to 19 counts as moderate and under 8 as mild, 4.9 points is a change most men would notice. It comes from two trials, though, which is a thin base.

The largest single trial, published in the Lancet in 1995, randomised 200 men to 20 mg three times a day or placebo for six months. Symptom score fell by 7.4 points against 2.1 on placebo. Peak flow rose from 9.9 to 15.2 mL/s. Residual volume fell from 65.8 to 30.4 mL. Flow and residual volume did not change in the placebo group.

An 18-month follow-up of that trial found the benefit held, with an important caveat: it was an open extension in which men chose whether to continue, and only 117 of the original 200 could be analysed. The 38 men who kept taking it had stable values throughout. The 41 who stopped had slightly worse symptom scores and more residual urine, though their flow rate held. And 27 men from the original placebo group who started beta-sitosterol afterwards improved by the same amount the treated group had.

Does beta-sitosterol shrink the prostate?

No, and this is the clearest finding in the whole evidence base. The systematic review states flatly that beta-sitosterol did not reduce prostate size, and the Lancet trial measured prostate volume as a secondary endpoint and found no relevant reduction in either group.

That is not a failure. It tells you what the compound is not doing: it eases symptoms without shrinking the gland. How it does that is not known, so any comparison with alpha blockers is about the outcome, not the mechanism. The drugs that genuinely shrink the prostate are the 5-alpha-reductase inhibitors, finasteride and dutasteride, and they take six months to a year to do it.

The practical consequence is that beta-sitosterol does not alter the course of the condition. Stop taking it and the symptoms drift back, which is what the 18-month follow-up showed in the men who stopped.

Is beta-sitosterol safe?

On the evidence available, yes, at these doses and over these durations. Withdrawal rates in the pooled trials were 7.8 percent on beta-sitosterol and 8.0 percent on placebo, a difference of nothing. People were not dropping out because of side effects.

The trials report side effects as mild, with withdrawal rates the same as placebo. At the much higher intakes used to lower cholesterol, around 2 to 2.5 g a day, plant sterols lower blood carotenoids such as beta-carotene by around 10 percent, though vitamins A, D and E are unaffected and levels stay within normal ranges. More is still not better.

Two real limits. The trials ran 4 to 26 weeks, with one 18-month follow-up, so long-term safety is not established, it is simply untested. And anyone with sitosterolemia, a rare inherited disorder in which plant sterols accumulate, should not take it.

The important caveat is not about the compound at all. An enlarged prostate and prostate cancer can produce the same early symptoms, and a supplement that improves your flow will not tell you which one you have. Get the symptoms assessed before you treat them.

What to look for in a product

The review found something most product pages leave out. Three of the four trials used non-glucosidic beta-sitosterol preparations and those are where the benefit came from. The one trial using a pure beta-sitosterol-beta-D-glucoside preparation showed no improvement in urinary flow measures at all.

So the form matters, not just the milligrams. Look for a label stating beta-sitosterol in milligrams. Be wary of 'plant sterol complex' with only a combined total, because that total includes campesterol and stigmasterol and you cannot tell how much of the sterol that was tested you are getting.

Check the dose against the use. A cholesterol product delivering two grams of mixed sterols is not the same purchase as a prostate product delivering 60 to 130 mg of beta-sitosterol, even though both are honestly labelled.

The honest weakness

Everything above rests on trials conducted in the 1990s. They were double-blind and placebo-controlled, which is more than most of this category can say, and nothing published since has contradicted them. But nothing has confirmed them either, no trial has used modern outcome measures, and the preparations were not standardised between studies.

The reviewers' own conclusion is the right summary: beta-sitosterol improves urological symptoms and flow measures, and its long-term effectiveness, safety and ability to prevent the complications of an enlarged prostate are unknown.