public health
by Tracy Purbrick Specialist Primary Care Dietitian
They’re everywhere. Lurking on super- market shelves, peering out at us from magazines and if we’re not careful, they’ll end up in our fridges and down our gullets too. The seemingly innocu- ous cure for that bothersome high cholesterol, the phytosterols (AKA plant sterols and stanols), could well be less benign than their manufactur- ers are letting on. So what exactly are these poten- tial ‘triffids’? Phytosterols are the plant world’s version of cholesterol, just as phytoestrogens are to our oestrogen, and chlorophyll is to our haemoglobin. Phytosterols are especially rich in fatty plants and their oils, but still only provide a typical westerner with around 200- 400mg per day, similar to the 200-500mg coming from cholesterol itself. Since they chiefly lower cholesterol by competing with it for absorption, intakes need to be ramped up 10 fold, to 2.5g per day, to have the desired effect. So far, so good. However, having conned the body into absorbing xenosterols in place of cho- lesterol, what’s the body now to do with all of these unwanted substances? Well, relax, say their manufacturers; the body will simply pump them back out, which is why retention levels of phytosterols are only generally one percent compared to 50 percent for cholesterol. Except that this pump doesn’t always work optimally and, for all people taking these products, plasma phytosterol levels will increase to some degree. But what happens to those people who accumulate these sub- stances most - people with the condi- tion of phytosterolaemia.
Warning signs Phytosterolaemia is an exceedingly
rare disorder, affecting around one in five million people, though under- diagnosis is suspected to be probable, with the afflicted getting lumped under the familial hypercholesterolaemia umbrella. But it is often the rare disease that shines the light on more common illnesses. Indeed, disease is often just one end of the spectrum of increasing body dysfunction. Just as, for example, there are many stages to poor handling of blood sugar before the definition of
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Tracy Purbrick has over 12 years experience working in the NHS and has spent the last 10 years as a primary care dietitian at the Wembley Centre for Health & Care (NHS Brent), North West London. Her work includes HIV, general practice clinics, special- ist obesity clinics and home enteral tube feeding. She is a keen triathlete.
Plant sterols – are we being led down the garden path?
diabetes is reached. So, whilst phytos- terolaemia isn’t rife, it could well prove to be the canary in the coal mine. What are the warnings that should be heeded by this condition? Well, phytosterolaemia is caused by a faulty gene, which prevents the normal pumping mechanism that the body uses from ridding itself of unwanted sterols, both cholesterol and phytoster- ols. However, the body has a particular aversion to phytosterols and preferen- tially expels them over cholesterol. This begs the question, should we be trying to undermine the way that the gut has evolved to reject these substances? If we look at the consequences of
phytosterolaemia, what we have are very raised plasma sterol levels accom- panied by extremely premature cardio- vascular disease. Yet, this is often on a background of normal cholesterol lev- els. So, not surprisingly, it has led some researchers to wonder whether phytos- terols are potentially more atherogenic than cholesterol itself.
Evidence suggests… What other evidence is there to raise
concern? Well, to linger a little longer on phytosterolaemics, we have the intrigu- ing phenomena of their tendon xan- thomas. These are the same as the hard, fatty lumps found in people with familial hypercholesterolaemia. The interesting thing is, that even in phytosterolaemics, they are mostly composed of choles- terol. Which has raised the question, do phytosterols somehow facilitate entry of cholesterol, in view of the nondescript background cholesterol levels? There is certainly some rationale for thinking that phytosterols could be more treacherous than cholesterol, since they are actually more apt to oxidize.
But what of people with ‘normal’
phytosterol metabolism, who have far lower plasma levels? Well, even within ‘normal’ population plasma ranges, higher levels have been associated more strongly with a family history of coronary artery disease. Indeed, one study found that even a modest in- crease in phytosterol levels in hypercho- lesterolaemic patients, doubled their risk of coronary events. And recent studies have found phytosterols lurking in the
atheromatous plaques of apparently normal adults.
Of course, not all studies have found
harmful effects. It is an area that, in the oft quoted researchers parlance, ‘needs further research’. What I find particularly disturbing however, is that many of us charged with advising on heart healthy diets, are blissfully unaware that any safety issue exists. Let alone the fact that there is raging controversy out there in the world of coronary boffins. With the friendly face of Gloria Hunniford and the depiction of cholesterol as a multitude of people in yellow scrubs being shooed away by the stanol saviours in blue, the TV campaigns of the two big brands are seeping into the nation’s conscience and, increasingly, their bloodstreams.
Much to learn…
In spite of their healthy heart claims, it must be remembered that these products have only been found to lower LDL cholesterol. They have not been found to lower actual cardiovascular end points, i.e. heart disease. How many times have surrogate end points been used, only to lead us up the garden path. The medical literature is awash with predictions of how this or that drug will lower the incidence of one disease or another, based purely on surrogate end points. We often find that millions of pounds later, study results are chalked up as ‘disappointing’ because they’ve failed to live up to all the calculations and hype. Or more worryingly still, they’ve been exposed to cause an- other, equally serious condition. Phytosterols are not inert sponges, cleaning out cholesterol. They have physiological ramifications. At the very least, they lower absorption of some fat soluble vitamins, such as the carotenoids and, of course, there are different types. Just as the paradigm that ‘all dietary fat is bad’ has been increasingly refined, so too have we much to learn about the differing effects of individual phytosterols. Whilst any sort of shadow hangs over
these products, should a moratorium on them be called? Cardiovascular disease is in any case, about so much more than cholesterol. But that’s an- other article…
NHDmag.com Aug/Sept '10 - issue 57
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