杏吧原创

EATING healthily can be tricky, especially for avid followers of the latest food- related headlines. Even fare touted as a superfood one day can be vilified as a cancer risk the next. Perhaps nowhere is this more the case than in recent claims about rocket, a salad leaf.

It is 鈥渁t the centre of cancer concerns鈥�, said one story, a strong accusation to level at a humble leaf. The claim hinges on high levels of nitrates that rocket contains. Is there any clear evidence to back it up? No, not when you dig a bit deeper into the complex findings around this.

Nitrates are naturally occurring mineral compounds that plants need for healthy growth. These are drawn up from the soil and become concentrated in the leaves in some species. It has long been known that rocket is a particularly potent accumulator of these substances. So what is the issue?

Well, from the late 20th century, we started becoming concerned that nitrates might pose a health hazard. This was largely about their presence in drinking water as a consequence of fertiliser that washes off farmland and their use as food additives, for example to preserve processed meats. Those worries were sparked by experiments that involved feeding large quantities of nitrates to rats, which suggested a raised risk of conditions such as stomach cancer.

Based on these findings, legislation was brought in decades ago by the likes of the World Health Organization and the European Union to limit nitrates in food to what were considered safe levels. Given nitrates are in rocket, the rules applied to it too.

How strong was the evidence on nitrates though? It is important to remember that, in science, not all evidence is created equal. The results of animal studies are often a poor guide to what will happen in people. Furthermore, feeding lab rats heavily nitrate-laced water isn鈥檛 exactly a great proxy for our salad consumption. Studies that look for a relationship between real-world behaviours in people (such as tracking actual nitrate consumption) and health are generally considered far better evidence. So what do these show?

One of the most comprehensive reviews of this type of evidence, conducted by China鈥檚 Fudan University, reported no clear positive association between overall cancer risk and nitrate intake.

鈥淚t is important to remember that, in science, not all evidence is created equal鈥�

In fact, in the case of gastrointestinal cancer a slightly lower risk was observed, particularly in the group whose consumption was the highest.

When it comes to looking for links between intake of nitrate-rich vegetables, such as rocket, beets and spinach, and broader health outcomes, results are encouraging. For example, the University of Plymouth, UK, reported that vegetarians, who can consume three times the average amount of dietary nitrate, tend to have a lower cardiovascular disease risk.

What鈥檚 more, since the original cancer-link research in rats, many human trials in the past decade have shown dietary nitrate in the form of vegetables can have significant health benefits, including reduced blood pressure, which is statistically associated with a far lower risk of dying from cardiovascular disease, and even improved athletic performance.

This has led some researchers to now go as far as to suggest that the naturally occurring dietary nitrate from vegetables 鈥渟hould be considered as a nutrient necessary for health, rather than as a contaminant鈥�. That鈥檚 fighting talk.

So does all this mean that public health officials who moved to limit nitrate intake in the past got it wrong? Not necessarily.

While the new evidence of potential health benefits of dietary nitrate is intriguing, it isn鈥檛 an effect that has been shown universally across all trials and most of these studies are relatively short term, so we lack data for the long-term effects. It is perfectly possible that the same compounds at low levels may be beneficial, but harmful at high ones. Indeed, if dietary nitrate is one day considered a nutrient, this would hardly be surprising, as many nutrients, such as vitamin A, are also toxins in too high a dose.

In all the relevant studies used for setting nitrate limits, the authors went to great lengths to point out the limitations of their data, as well as highlighting that the benefits of eating fruits and vegetables far outweigh any downsides of their nitrate content.

One more point: the existing limits on nitrate in foods aren鈥檛 meant to warn consumers off eating crops like rocket, as some headlines imply, but are more a guide to farmers to signal when they should change practices to help reduce nitrate levels.

The bottom line of all this? Eat your veg. Following standard dietary advice to aim for diversity will avoid the risk of having too much of one type of fruit or veg. There is so much more we need to uncover about nitrates, but for now, we can say your salad isn鈥檛 going to kill you.