30 probably isn't the magic number

Planting ideas on uncertain ground

Waitrose has a new favourite number. Shoppers now have their visits punctuated by signs emphasising the importance of eating 30+ different plants a week, while packaging now comes with little numerical targets. If you want a shortcut, you can also buy a ‘Daily30+’ supplement for £13.50 per week.

But is 30 really a magic number? And where did it come from anyway?

The number 30 has been popularised by the nutrition company Zoe, makers of Daily30+ (as well as the famous subscription-based app). In turn, the original source of the number is cited as a 2018 citizen science study, which measured different aspects of participant microbiomes. In a sub-analysis, the research team divided participants into two groups based on how many plants they said they ate in an average week: ‘individuals answering “more than 30” (n = 41) and “less than 10” (n = 44)’. The study didn’t look at health outcomes, but found that, on average, certain components of the microbiome had higher abundance in the ‘more than 30’ group.

At the individual level, though, there was a huge amount of variation in some of these measurements. Take conjugated linoleic acid, for which a handful of individuals pulled up the average in the ‘more than 30’ group:

Source: McDonald et al, 2018

As I’ve written about before, the above is also a good reminder that the average effect measured in a population study usually has little predictive value if looking at individual outcomes.

So, why less than 10 and more than 30 as the two groups? As far as I can tell from the paper, the numbers were chosen arbitrarily – just a simple way of putting a number on ‘few’ and ‘many’. Maybe picking different cutoffs would have left too few people to compare? Either way, it means we don’t know from that study whether 30 is any more special than 25, or 20, or even 15.

Indeed, here’s what Zoe’s head nutritionist told The Times last year:

“The science behind ‘is 30 the magic number?’ is not that strong”, Amati says. “It is from just one study, although it’s a very good study. But the wonderful thing about this is it has captured people’s attention and increased their fibre intake, which many people are deficient in.”

If I’ve understood this correctly, it seems to imply that the thing people are doing for a perceived benefit doesn’t necessarily have that benefit. But it has a different benefit, so it’s OK to let them believe it. And if the main benefit is fibre, you can obviously buy sources of that much more cheaply. Take flaxseed; even if you were determined to buy it at Waitrose, it would still be almost ten times cheaper than the Daily30+ supplement.

Last autumn, Zoe announced they’d run a randomised controlled trial of their supplement versus a control group, who ate the same number of grams of croutons instead. The announcement also noted that some major benefits were probably down to fibre:

Looking at digestive symptoms overall, 70% of participants in the Daily30+ group reported improvements. This is likely due to Daily30+’s fibre content.

Given all this, I think it’s a shame that we’re now seeing signs with the number 30 all over a supermarket. Because if much of the benefit really is down to increased fibre intake, then that’s something people could improve much more easily and cheaply – without the worry of whether to put turmeric on their porridge to hit a magic target.

Cover image source: Waitrose magazine.