Despite being around since Neolithic times, fermented foods are a relative new guy to the nutrition trend party, at least in North America. With ringing endorsements cropping up all over the place promising everything from improved digestion to an enhanced immune system and longevity, it’s no wonder they are popular! Most suggest the proposed beneficial effects of eating fermented foods is due to the presence of natural probiotics, thought to bolster your intestinal population of those friendly little gut microorganisms.
But is this all hoopla or is this ancient food preservation method really the path to better health? Are fermented foods any better for you than their non-fermented brethren?
In part 1 of Foods with Funk: Fermented Foods 101, we debunked some of the probiotic myths – not all fermented foods, or even those fermented with bacteria, are good sources of probiotics. Now it’s time to tackle the “health halo” of fermented foods. Most of the buzz regarding the health benefits of fermented foods stems from their probiotic, or live, beneficial microorganism content (1,2). But are all fermented foods good for you? Which foods have earned the health buzz and which haven’t?
What are the proposed benefits of fermented foods? Although some people suggest that consuming fermented foods is a cure-all for absolutely everything, most research has focused on intestinal health and immune function (2). Interestingly, a lot of articles on the internet reference studies of probiotic SUPPLEMENTS when they are discussing the benefits of fermented food, a huge leap in logic to us! So, does clinical research data support the beneficial effects of regularly consuming fermented foods?
Intestinal health. There is growing evidence that gut microbiota have a tremendous influence on gastrointestinal health and risk of intestinal disorders (see our previous A Gut Feeling? post). Stands to reason that improving gut bacteria composition would be beneficial, but more research is needed. Patients with inflammatory bowel diseases like Crohn’s and ulcerative colitis, as well as IBS, may frequently REPORT improved symptoms with fermented dairy products like yogurt in observational studies (3), but there is limited data from clinical trials.
In an eight week randomized controlled trial comparing daily kefir and milk consumption, patients with IBS reported improved symptoms with both, but no differences between the two products (4). To our knowledge, there are no studies looking at the effect of other probiotic, fermented foods on intestinal health… though somehow we doubt kimchi and IBS would be a good combo!
Immune system. The proposed immune-modulating effects of probiotics are typically thought to be linked to reducing gut permeability (so-called “leaky gut”) and increasing T-cell production (5). No clear and direct connection between probiotics and immune function outcomes, however, has been made (see our previous post Nature vs. the Common Cold).
Similarly, the effect of fermented foods on immunity is pretty new (6) with some promising data from in vitro and animal models, as well as humans. In a small study looking at daily yogurt consumption in healthy women, both regular yogurt and probiotic-enhanced varieties boosted markers of immune function over two weeks (7). What about actual sickness? Interesting, but small scale. Two studies in the elderly suggest that 90 g (about 1/4 cup) of yogurt daily may significantly reduce the risk of catching the common cold (8).
Metabolic benefits. The effect of food fermentation on glycemic control, blood pressure and cholesterol level is gaining some traction, but remains controversial. Results from a recent meta-analysis (9) found that fermented dairy consumption – yogurt and kefir – was associated with reductions in blood pressure among hypertensive and pre-hypertensive people; doses ranged from 100 – 450 ml per day. Perhaps yogurt is best? A Cochrane Review published in 2012 on fermented milk and hypertension found no link to support including kefir as a dietary intervention for blood pressure (10).
Aside from fermented dairy, the evidence is equivocal. Higher kimchi intake – as a part of a traditional Korean diet – has been linked to both reduced risk of metabolic syndrome (11) and increased risk of obesity (12). A traditional Korean diet also includes lots of vegetables (yay!) and tons to white rice (not-so-yay), so laying either the claim or blame at the feet of kimchi alone is a stretch! The role of soy products is similarly murky with respect to metabolic benefits, with one study singling out fermented soy products – in this case, natto – as beneficial for blood sugar control, though the study did not control for test meal glycemic index (13).
Nutrition bioavailability. It’s not news that the bacterial synthesis of vitamins is crucial for human health – you have your gut microflora to thank for vitamin K! But is there any evidence that fermented foods are superior with respect to enhancing bioavailability of nutrients? In some cases, absolutely! Bacterial synthesis of B-vitamins may improve nutrient status, as evidenced by a small study in women given yogurt or probiotic supplement daily – their thiamin and riboflavin status improved with both yogurt and supplement (14).
With respect to minerals, bacterial enzymes reduce the phytate content of fermented grains, legumes and nuts (15) – phytic acid binds and prevents the absorption of minerals – thereby increasing bioavailability of iron, magnesium and zinc in foods like natto and tempeh. What about fermented dairy and calcium? Unfortunately, there is no evidence that fermentation increases the bioavailability of calcium in kefir and yogurt.
Reducing symptoms of intolerance and allergy. The bacteria cultures found in both yogurt and kefir help with lactose digestion and have been shown to improve tolerance and reduce symptoms in people with lactose maldigestion (16), but there is no other data to support the use of fermented foods to combat other food intolerances or allergies. In terms of allergic conditions such as dermatitis, eczema and asthma, the research is limited to probiotic supplementation as a possible adjunct treatment strategy (17) – there is no data on fermented foods themselves.
Reducing risk of cancer. Once again, fermented dairy is looking pretty good! In a study of over 45,000 people with 12 years of follow-up, higher yogurt intake was significantly linked to reduced risk of colorectal cancer (18). Ditto cultured milks such as kefir. Adequate intake of dairy products is consistently linked to reduced risk of bladder, breast and colon cancers, with the strongest evidence for yogurt and cultured milk (19).
When it comes to fermented soy, however, the picture is not so rosy. In a meta-analysis of Asian populations, fermented soy was actually associated with an INCREASED risk of gastric cancer (20), perhaps due to salt content, whereas non-fermented soy consumption was protective. Similar story for prostate cancer – fermented soy was associated with increased risk of the disease (21).
No Baloney’s advice? If you want to get the most benefit from consuming fermented foods, we suggest opting for fermented dairy products like yogurt and kefir - just watch the added sugar content. While there are a lot of PROPOSED benefits of eating other fermented foods, the evidence does not support the claims at this time.
What’s our verdict on the others?
- Kombucha: Stick with plain old tea – the only published data in humans involves kombucha toxicity (22).
- Fermented soy: No research on benefits aside from isoflavones. Natto and tempeh are great sources of protein and micronutrients. As for miso paste, it contains loads of salt so use sparingly as a flavouring agent.
- Kimchi: proponents try to link kimchi to a host of benefits (23) but the evidence only supports the “tasty” side dish claim!
- Pickles and sauerkraut: Absolutely no research and high in salt – keep portions small and use as a condiment.
There is plenty of evidence supporting the beneficial effects of probiotic SUPPLEMENTATION on intestinal health, immunity and allergies (5, 23), likely through immunomodulatory effects. But just as we harp on supplement manufacturers for trying to market pills based whole food research, making the reverse leap from supplement research to whole foods is likewise unsound. Just because probiotic supplements have shown positive study results doesn’t mean natural probiotic-containing foods will carry identical benefits. We’re all for whole foods, but we are also for evidence!