by DR CHRISTOFFER VAN TULLEKEN
Published: Mail Online, 21st October 2014
Something that is good for us can also be bad for us. It may sound illogical, but it's why one week we can read that something is a universal panacea only to become a powerful carcinogen the next.
Take oily fish. It's full of good things such as long-chain omega-3 fatty acids, but also contains pollutants. How good or bad it is depends on who you are and how much you eat. (Check out the NHS Choices website for information on oily fish).
And what about the claims for 'superfoods' such as blueberries and goji berries?
Read the smallprint of the research and inevitably it will say something like, 'at some unspecified point in the future the chemical in this superfood may provide benefit to some patients'.
That's fine, but all this means is that by now many of us have a healthy scepticism about the next claim to come along.
We start to suspect that quite apart from making the people who eat them hugely dull, foods that are 'good for us' may not be quite as good for us as we've been lead to believe.
So when beetroot hit the headlines a few years ago with claims about improved athletic performance and lower blood pressure, many probably dismissed them. I certainly did.
Then I met Dr Andrew Webb.
Dr Webb is a consultant physician at Guy's and St Thomas' in London, specialising in blood pressure management.
He's been studying beetroot for several years (funded partly by the British Heart Foundation).
He's found that it does lower blood pressure and tomorrow night, on BBC2's Trust Me, I'm A Doctor, we put his findings to the test - comparing beetroot against foods about which there have been similar claims: garlic and watermelon. The setting for our test was the historic Cardiff Arms Park rugby stadium; the participants, naturally, men of the Cardiff Arms Park Male Choir.
All 28 had slightly elevated blood pressure. Ideally your reading should be less than 130/80 when resting; the first number, the systolic pressure, is the pressure in the blood vessels when the heart contracts, the second is when the heart relaxes.
Our participants' systolic readings were, to a man, 130.
They were divided into three groups, with each group given a different fruit or vegetable for a week; they then swapped. Depending on the week, they had to start each day with two whole beetroot, or two cloves of garlic (eaten as they liked), or two large slices of watermelon.
These quantities were felt to be realistic amounts for someone to incorporate in their diet.
A number claimed they ate masses of the stuff until one of the baritones said sternly: 'Lads, if we all eat so much of this stuff how come we're all overweight with hypertension? I've only ever seen any of you drink beer and eat pies!'
During the test, each volunteer had to take their blood pressure three times each morning and evening. Here's the theory about how these foods might help with blood pressure. Garlic contains allicin, an organosulfur compound which makes garlic smell.
It's also believed to act on our kidneys, changing levels of hormones and causing the blood vessels to open up.
Watermelon and beetroot are thought to boost the levels of nitric acid, a compound that causes blood vessels to open up. Watermelon contains amino acids that can help generate nitric oxide.
With beetroot, as Dr Webb explained, it comes down to its high concentration of nitrates: when you eat beetroot these nitrates are absorbed into the blood, then secreted in your saliva.
In the saliva, nitrates are metabolised by the bacteria on the back of your tongue into nitrite - this is then further metabolised into nitric oxide.
Other sources of nitrates include celery, lettuce, watercress, rocket, spinach, chard, fennel and radish.
Nitrates are best preserved by eating vegetables raw, but if you want cooked beets, don't peel or top and tail them before boiling - otherwise the nitrates will escape into the water (and make sure you drink the water too, or make soup!).
After learning all this, I wasn't surprised to see that the lowest blood pressure in each group was recorded during their week eating beetroot.
On average, systolic pressure went down to 128.7, compared with 129.3 in the garlic week, and 129.7 on the watermelon. All these results were comparable to those of a low dose of a blood pressure drug: over a lifetime and combined with other lifestyle changes, beetroot will reduce the risk of strokes and heart attacks.
In fact, the nitrates in beetroot are so active that it's being investigated for controlling blood pressure on intensive care units.
One of the advantages of nitrates may be that they won't have to go through expensive and time-consuming licensing procedures in the same way a new drug would.
Of course, beetroot on its own isn't going to send the big drug companies into bankruptcy. However, it does fit into a toolbox of lifestyle changes that could do just that.
Chris is Infectious disease specialist at University College London.