From bone broth to cider vinegar shots, there’s always a new food trend claiming to be the ultimate gut health remedy. So, we asked dietitian Laura Tilt to dive into the science behind the claims.
Claim: kiwis can ease constipation
Fact! Constipation (having hard, lumpy poo, or pooing fewer than three times a week) affects more than 1 in 10 people. While it can be caused by certain health conditions or medications, diet and fibre intake can play a role too. Enter the humble kiwi! Kiwis contain a type of fibre that is especially good at holding water, making poo softer and easier to pass.
In a 2023 study, healthy people with constipation or constipation-predominant IBS ate either two kiwis daily or 7.5 grams of psyllium husk (about 2 teaspoons). Both provided around 6 grams of fibre, but the kiwi group saw a greater increase in ‘spontaneous poos’ (without laxatives). While this study was funded by kiwi brand Zespri, other studies back the results. Bonus: Kiwis are tastier than psyllium, high in vitamin C, low in FODMAPs (IBS-friendly), and count towards your five-a-day.
P.S. If constipation is a new symptom for you and lasts longer than a couple of weeks or is accompanied by symptoms like bleeding or tummy pain, speak to your GP.
Claim: apple cider vinegar improves digestion
Fiction. You don’t have to look too far on social media to find someone claiming apple cider vinegar (ACV) remedied their gut issues. First off, what is it? ACV is a punchy fermented tonic made in a two-step process: first, natural sugars in apples are converted to alcohol by yeast, then bacteria turn that into acetic acid, giving ACV its distinctive kick. Fans claim ACV eases digestion, reduces bloating, and soothes heartburn—but there’s no scientific evidence in humans that it can do any of these things.
In fact, ACV could make heartburn worse, and one case even reported a teenager’s throat being burned from daily vinegar drinks. Plus, acidic foods like ACV can damage tooth enamel over time. So, save ACV for salad dressings, and if you drink it, dilute it, use a straw, or rinse your mouth with plain water once you’re done.
Claim: bone broth heals the gut
Fiction. Bone broth, a savoury stock made by simmering meat bones, veggies, and herbs for up to 24 hours, is often hailed as a gut healer. Sure it’s tasty and often rich in protein, but there’s no strong evidence to support gut-healing claims. These claims hinge on its collagen content. Collagen gives structure to our connective tissues, and is produced by cells lining the gut, helping maintain a healthy gut barrier. But collagen from food doesn’t become collagen in your body; it’s broken down into smaller proteins and amino acids (protein building blocks), which the body uses as raw materials for building and maintaining various tissues. One study which analysed bone broth samples found that none of them contained reliable amounts of proteins used to make collagen.
Nutrient levels also vary widely, and broths are often lower in minerals like calcium and magnesium than claimed (less than 5% of your daily needs in some cases!). Bones also store heavy metals like lead which can leak out during prolonged cooking, with one study finding that bone broths may contain significant levels of lead. Another study found the levels of heavy metals were low, but it’s something to bear in mind.
Claim: fermented foods are the same as probiotics
Fiction. Strictly speaking, fermented foods aren’t the same as probiotics. Probiotics are live microbes (mostly bacteria and yeasts) that have a proven health benefit when consumed in the right amounts. While fermented foods—like yoghurt, kimchi, and sauerkraut—are made using live microbes, these microbes might not be alive when you eat the food. For example pasteurised yoghurt or baked sourdough.
We don’t have solid evidence yet that the microbes in fermented foods result in specific health benefits. That doesn’t mean they’re not worth adding to your diet -- a 2021 study showed links between fermented food intake and microbiome diversity. Beyond microbes, fermented foods offer important nutrients too. Made from veggies, kimchi and sauerkraut are a source of fibre, milk kefir and yoghurt give you calcium and protein. Try swapping in live yoghurt or kefir for your granola, or adding fermented veg like kimchi to your sandwiches or salads—my personal fav is a kimchi and cheese toastie!
Claim: gluten is behind your bloating
Not necessarily. Bloating affects around 30% of people and can be linked to many factors, not just gluten. While bloating can be a symptom of coeliac disease (an autoimmune condition where gluten damages the small intestine), there are plenty of other causes.
If you’re experiencing ongoing bloating, see your GP before self-diagnosing or cutting gluten from your diet. A simple blood test can check for coeliac disease—but you’ll need to be eating gluten for the test to be accurate. If you have IBS and think bread is a trigger, it could be FODMAPs - a group of short chain sugars that can trigger symptoms in IBS sensitive tums.
Claim: you’ve got to eat 30 plant foods a week for a healthy gut
If you're into gut health, you’ve probably heard the advice to aim for 30 different plant foods a week (yep, I’ve mentioned this before too!). There’s science behind it— a 2020 study found that adults eating 30 types of plant foods weekly had a more diverse gut microbiome compared to those eating 10 or fewer. But don’t stress if you don’t hit 30 every week—adding a few different fruits and veggies is still a win.
The main takeaway is to eat a big variety of plant foods. Different types of plant foods contain different nutrients, which help keep a variety of microbes thrive. And don’t forget that there are lots of other ways you can keep your gut healthy alongside eating 30 plants - regular feel good movement, managing stress, getting into green space and eating plenty of fibre can help too.
Claim: a gut healthy diet is one-size-fits-all
Fiction. Gut health is highly individual —what works for one person may not work for another, partly due to the unique makeup of your microbiome.
Much of the dietary advice we hear about gut health like eating plenty of fibre, prebiotics, and plant foods—is great for creating a healthy gut environment, but isn’t always realistic if you’re managing specific gut issues. If you have a gut condition like IBS, getting advice from a gut-specialist dietitian can help you figure out what a gut-healthy diet looks like for you.
Read more from our fact v fiction series
The truth behind 7 bloating myths
Ultra-processed foods: are they really harmful
Glucose spikes and gut health - what you need to know
Sources
Consumption of 2 green kiwifruits daily improves constipation and abdominal comfort—results of an international multicenter randomized controlled trial. The American Journal of Gastroenterology. (2023). Link.
Bone broth unlikely to provide reliable concentrations of collagen precursors compared with supplemental sources of collagen used in collagen research. PubMed. (2019). Link.
The risk of lead contamination in bone broth diets. Science Direct. (2013). Link.
Fermented-food diet increases microbiome diversity, decreases inflammatory proteins, study finds. Stanford Medicine. (2021). Link.
