The 30 plants concept has entered the mainstream, but now a leading scientist is questioning how seriously we should take the advice, reports Sophie Morris

woman in grocery section of a shop.

How many plants have nurtured your precious microbiome this week? I should know my number, because I’ve been counting. But I began the experiment with a spreadsheet, eager to show off, and then lost track. The modern mantra is that 30 plants a week is the number of plants we should aim to eat, because diversity is good for gut health, and gut health impacts the rest of our body and brain.

This has been popularised across the contemporary nutrition canon, by everyone from Professor Tim Spector, founder of the ZOE app, to chef and campaigner Hugh Fearnley-Whittingstall, whose most recent book, How to Eat 30 Plants a Week (Bloomsbury, £25) was inspired by the trend.

The Government’s five-a-day fruit and veg advice is more than 20 years old, but the 30-plants line widens the field to include many more foods, such as grains, nuts, beans, legumes, herbs, seeds and spices. It doesn’t specify portion-size, however, which means that anything from a pinch of paprika to a sprinkle of pumpkin seeds counts as one plant.

The concept has entered the mainstream over the past few years, but now a leading scientist is questioning how seriously we should take the advice. “What is the evidence behind it?” asks Dr Emily Leeming, a microbiome scientist at King’s College, London, registered dietitian and chef. “And is 30 really the magic number for your microbiome?”

Leeming says the 30 plants advice is based on weak research; that it is an arbitrary figure – and that adhering to the advice too rigorously could be problematic for anyone with disordered eating. Her upcoming book, Genius Gut: Eating for Your Second Brain (Michael Joseph, out 25 July) explores controversial health messaging such as the 30 plants idea, analysing the science to present her top gut-brain hacks for health, mood and brainpower.

Self-reported results

She first became suspicious when she read the original report back in 2018. “It’s an easy message and became absorbed into the gut health discourse quite quickly,” Leeming says. The advice was a small part of a report from the American Gut Project, a citizen science project with a UK arm, the British Gut Project, led by Tim Spector, who has become famous for persuading us to pay greater attention to our own poo, and offer it up for scientific investigation.

“Part of my research at King’s was exploring plant diversity,” explains Leeming. “When I originally read the study, I really wanted to understand how they did the research.” She got in touch with the author of the paper, who told her that the 10,000 participants who shipped their poop for the microbiome testing were asked to complete various surveys, one of which asked about their diet, including the question: “In an average week, how many plant species did you eat. “They had to roughly guesstimate,” points out Leeming. “This was not rigorously collected data.”

The author also gave Leeming examples of what might count as a plant species, explaining that a multi-grain bread would count as three, and a can of soup containing carrots, potatoes and onions, would also count as three. “We know what counts as a plant, but how much counts as a plant point? It seems potentially even just a smidge counts. Is that having any impact on your gut, health-wise?”

This was the message I’d understood when I began counting, so whatever I was eating, I looked through my herbs and spices to find something appropriate to sprinkle over the dish, thus upping my number by adding almost nothing to my diet. Fearnley-Whittingstall’s book has a handy index of around 200 foods that you can tick off week to week.

Why 30?

Is 30 a magic number? Leeming says it is random. The researchers grouped participants into three even groups, with the lowest reporting a consumption of 0-10 plants, the middle group 11-30 plants, and the top group more than 30 plants. This was in order to compare the microbiome diversity of the lowest and the highest groups, but using the markers of less than 10, and more than 30, plants, isn’t remarkable in and of itself. “They could have looked at 20 plants vs 10 plants, or 40 vs 10 plants, and had similar results,” she says. “The 30 came from how they did the analysis, rather than finding that 30 was the sweet spot.”

What’s more, the American Gut Project didn’t adjust for any other behaviours, whereas the longstanding five-a-day advice, says Leeming, comes from rigorous evidence. “Eating 10 plants or fewer, that’s really no variety. You’re basically just eating bread and soup. Those eating 30 plants could have been eating more fruit and veg overall, but there was no adjustment for these possibilities. The evidence is associative rather than causative.”

I asked Francesca Lyon, lead nutritionist at women’s health and hormone clinic FUTURE WOMAN, how she approaches the 30 plants concept. “It doesn’t need to be 30, but variety is important,” she says. “The research that claimed 30 different plant foods a day was an association study not a gold standard research study. What the research actually concluded was that eating 30 different foods a week in comparison to 10 different foods a week showed a greater increase in microbial diversity. Therefore 30 is a great guideline but not a strict rule.”

Like Leeming, Lyon puts the emphasis on diversity overall, rather than an exact number. “Increasing microbial diversity can help improve symptoms like PMS, heavy and painful periods, irregular periods and even endometriosis,” she says.

Health worries

Thirty plants gained traction on social media as a goal or a game, but aiming for a finite dietary outcome can easily turn sour for anyone who struggles with disordered eating. “If you take the advice verbatim and are counting your plant points and religiously noting down everything they eat, it could exacerbate those issues,” says Leeming. It can also cause shopping, spending and cooking stress, as well as lead to unnecessary food waste. “We’re all busy. The message should be to think about diversity as a principle, to eat fibre and polyphenols for our gut, but it’s not the be all and end all it’s made out to be.”

Simon Edwards, in-house sports nutritionist for Raise and Replenish, a plant-based health drink brand, points out that we should be aiming for improving our nutrient intake overall, as well as the amount and diversity. “I have a study comparing the nutrients of fruit and veg from 1940-1990 in the UK, showing a nutrient loss of 30-50 per cent,” he says, worryingly. “But the national average fruit and veg intake is something like 2.4/day so asking everyone to up that to 10-plus won’t be received well nationally.”

Some people are obsessing over numbers and others feel lectured over their diets. “If you can get more healthy foods into that popularised diet, great, but all nutrition benefits are about adherence first before you need to think about the ‘perfect diet’,” says Edwards.

“As an educator in women’s hormones, I get the need to make recommendations ‘sticky’ so that people remember them!” says Lyon. “But equally, food guidelines shouldn’t add stress to your day. We encourage clients to increase diversity of plant foods but not to stress about the number 30.”

Easy wins

“We need to talk about the science in a way that is true to the principle of diversity,” says Leeming. “But to use it as a guiding principle, and not to over-egg it.” Thankfully, she has lots of easy ideas for adding plants into your diet. “Think about diversity in a way that works for you. Buy mixed bags of salad leaves, mixed veg stir fry bags, and cans of beans that contain multiple varieties. You can buy frozen fruits that come in a fruits of the forest mix, instead of just raspberries.”

Other ideas include finding something new on each weekly shop, stocking up your store cupboard and freezer with jars that won’t go off, keeping a nuts and seeds snack jar (also great for heart health), and thinking about the ‘eat the rainbow’ advice to give yourself a visual picture of eating well over counting every plant.

“Use the evidence in a way that’s practical, and works for you,” she urges. “If you eat tomato on a Monday, you can still eat tomato on a Wednesday.”