“EAT up your greens” is the exasperated cry of many a parent when faced with a fussy child. But the paradox of vegetables is that they are both good and bad for you. The cultivated plants consumed by all folks except hunter-gatherers have evolved an ambiguous relationship with people, in which they exchange the risk of being eaten by a human for the reproductive security that domestication brings. But the wild plants from which these cultivars are descended are very definite about the matter. They do not want to be consumed and they make that opinion known by deploying all sorts of poisonous chemicals to discourage nibbling herbivores. In many cases, those poisons have persisted into the cultivated varieties, albeit at lower levels.

Animals, of course, have evolved ways of dealing with these poisons. The best of these, from a plant's point of view, is when an animal can taste, and thus reject, a poisonous chemical. This has long been assumed to be the basis of the taste of bitterness, but that theory has only now been put to a clear test. In a paper just published in Current Biology, Mari Sandell and Paul Breslin, of the Monell Chemical Senses Centre, in Philadelphia, have looked at the phenomenon in that bête noire of presidents and parents alike: broccoli.

Bitter tastes are detected by receptor proteins that are, in turn, encoded by a family of genes known collectively as TAS2R. Humans have around 200 TAS2R genes, each sensitive to different groups of chemicals. That variety, in itself, indicates the range of the plant kingdom's weaponry. Dr Sandell and Dr Breslin, though, focused their attentions on just one of these receptor genes, called hTAS2R38. The protein derived from this gene is known, from laboratory experiments, to be sensitive to a substance called phenylthiocarbamide (PTC). This compound contains a molecular group called thiourea. And thiourea-containing substances are known from other studies to inhibit the function of the thyroid gland.

Cruciferous vegetables, such as watercress, turnips and—most pertinently—broccoli, are rich in a group of thiourea-containing compounds called glucosinolates. Dr Sandell and Dr Breslin wondered if there might be a connection. And, since different versions of hTAS2R38 code for proteins that have different levels of reaction to PTC, they wondered if that might be reflected in the fact that some people like broccoli, and others do not.

The two researchers assembled a group of volunteers and checked which versions of the hTAS2R38 gene they had. They then fed the volunteers vegetables and recorded their reactions.

All of the vegetables were thought by at least some people to be bitter, but not all of them were cruciferous plants. The non-cruciferous ones were plants which, so far as is known, do not contain glucosinolates.





Mild and bitter

The results were clear. All volunteers found the non-cruciferous vegetables equally bitter, but their reactions to the cruciferous ones depended on their genes. Those with two copies of the version of hTAS2R38 coding for the protein that binds best to PTC (one copy having been inherited from each parent) thought broccoli and its cousins the most bitter. Those who had two copies of the poorly binding version thought they tasted fine. Those with one of each had an intermediate reaction.

Despite broccoli's bad reputation, the most bitter vegetables, according to this research, are swedes and turnips. That accords well with work which has shown that eating these vegetables suppresses the uptake of iodine into the thyroid gland. Iodine is an essential ingredient of thyroxine, a hormone produced by that gland.

The upshot of all this is that the complaints of children (and, indeed, of many adults) that green vegetables are horrid contains a lot of truth. There is no doubt that such vegetables are good for you. But they are not unequivocally good. As is often observed in other contexts, there is no free lunch.