Lactobacillus plantarum 137 (Heat-killed - HK L-137)
Heat-killed Lactobacillus plantarum 137 (HK L-137) is a heat-killed microorganism or ‘post-biotic’ derived from the probiotic Lactobacillus plantarum (L. plantarum). This probably sounds bizarre at first. Why ‘kill’ beneficial life bacteria when we all know they support our ever-important gut flora? What possible health benefits can they have? Well, you might be surprised. Not only do postbiotics offer many health benefits, they actually stand out for a number of reasons. In this blog, we will explore what postbiotics are and do, why HK L-137 is such a special postbiotic, and what unique health benefits this proprietary postbiotic strain provides.
What Exactly are Postbiotics and What do They Do?
While probiotics became known in 1965 and prebiotics in 1995, the concept of postbiotics only emerged in 2010.[i] Hence, they are considered the newest ‘biotic’ in the field of gut health.
An official definition of postbiotics was released by the International Scientific Association for Probiotics and Prebiotics (ISAPP) in 2021, which explained that microbes don’t have to be alive to provide a health benefit, as in the case of postbiotics.
So, what are they, if they are not alive? Postbiotics only contain inactivated microorganisms, either whole or in fragments that are bacteria-free.[ii] To create a postbiotic, you start with a live microorganism, such as L. plantarum, which is then ‘killed’ or inactivated using heat or ultraviolet light. The resulting mixture then delivers a health benefit when consumed, as shown in numerous scientific studies.[iii] The ISAPP therefore defines a postbiotic as “a preparation of inanimate (non-living) microorganisms and/or their components that confers a health benefit on the host”.
Postbiotics are metabolites (products) generated by the gut flora (microbiota), such as SCFAs (short chain fatty acids), cell wall fragments, enzymes/proteins, and even B-vitamins. They can be produced when probiotics feed on prebiotics.2 However, the amount produced in this way may be insufficient and is entirely dependent on a person’s diet and gut flora. Apart from this, postbiotics are present in many fermented foods such as yogurt, sauerkraut, pickled vegetables, kombucha, and traditional Japanese dishes.[iv]
Research on postbiotics has uncovered many beneficial health effects, including antioxidant activity, effects on immunity and common mild infections, effects on the gut flora and the gut the lining (therefore gut health), general gut health and cholesterol (fat) metabolism.2,[v],[vi],[vii]
In many ways, they work in a similar way to probiotics.6,[i] However, postbiotics have several advantages over probiotics: they are stable with a long shelf-life enabling easy storage and transportation; they are also easy to take, and have no known risks, making them more appealing than probiotics alone.5,7,[ii] A particular advantage that postbiotics have over probiotics is that they are much less affected by processing and storage conditions such as temperature and humidity,6,[iii] and even stomach acid and digestive enzymes.1
HK L-137 – A Unique Postbiotic with Clinically Studied Benefits at A Low Dose
HK L-137 was first isolated from narezushi, a popular preserved delicacy in Japan that dates back to ancient times. This dish was traditional made by fermenting fish and rice at an ambient temperature (15 to 20 degrees Celsius) for about four weeks. During this process, the dominant Lactobacillus population increases dramatically, while the population of other bacteria remains unchanged. Its sour taste and deep flavour distinguish narezushi from modern sushi which is not fermented but eaten fresh.4
HK L-137 is really well-studied. More importantly, its unique health benefits are backed by strong ‘gold standard’ scientific studies in healthy individuals known as RCTs (Randomised Controlled Trials), all of which support a low daily dose of 10 mg. The gold standard of the evidence further assures us that the study results are highly relevant to the average adult.
Moreover, HK L-137 is very safe. Even intakes of HK L-137 at 5-times the effective dose (50 mg) for 4 weeks have been shown to be safe in adults. Taking HK L-137 for prolonged periods also appears to be safe and actually has beneficial effects on the intestinal tract.[iv]
So, what do the RCTs in healthy adults tell us?
HK L-137 Promotes Immunity
HK-137 was studied in a RCT in 60 healthy individuals about 56 years old to determine whether it could influence immune function. Study subjects received one capsule containing 10 mg of HK-137 or a matching capsule containing a placebo every day for 12 weeks. Biomarkers for innate immunity – which is our fast-acting, first line immune defence – and biomarkers for acquired immunity – the type of immunity we develop over our lifetime – were then measured at regular intervals between 4 and 12 weeks. The results showed that daily supplementation with HK L-137 enhances acquired immunity, especially immune functions that orchestrates the body's defence against viruses and certain bacteria.[v]
Another RCT examined whether HK L-137 could affect immunity. 78 healthy men and women about 50 years old, received either one 10 mg tablet of HK L-137 or a placebo tablet every day for 12 weeks. Immunity parameters (symptoms) were rated once daily, and immune functions were measured every 4 weeks. As a result of supplementation with HK L-137, these individuals suffered significantly fewer immune issues than those who received the placebo. This effect was thought to be due to improved immune functions.[vi]
HK L-137 relieves Mild Inflammation
In this RCT, 100 healthy but overweight men and women in their early 40s took either a tablet containing 10 mg of HK L-137 or a placebo tablet for 12 weeks so to study the effects of HK L-137 on mild inflammation. Blood samples were collected every 4 weeks and confirmed – at the end of the study – that daily intake of HK L-137 indeed relieved mild inflammation in those who are at risk.[i]
HK L-137 Helps Maintain Healthy Cholesterol
As body weight is related to inflammation in the body, the same RCT that studied the anti-inflammatory effect of HK -137, also studied the effects of this postbiotic on cholesterol (fat) metabolism. It found that daily intake of 10 mg HK L-137 for 12 weeks not only relieved inflammation, but also supported normal lipid metabolism in overweight men and women.14
HK L-137 and Skin Health Support
An RCT investigated whether HK L-137 could reduce skin dryness and increase its hydration. It involved 80 healthy people with dry skin, mostly women about 47 years old who received either a tablet containing 10 mg HK L-137 or a placebo tablet for 12 weeks. The study specifically assessed the skin of the face and forearm, as well as the outcome of a skin-related questionnaires. The final results demonstrated that HK L-127 was indeed able to improve skin moisture in people with dry skin; in fact, this positive result was described as significant. Overall, supplementation with HK L-137, especially in older people or people with dry skin, improves satisfaction with their current skin condition.[ii]
In summary, postbiotics are new exciting biotics with many advantages; they are stable with a long shelf-life which enables easy storage and transportation – perfect for travel and on-the-go. They are also low dose, easy to take, and have no known risks. Postbiotic benefits are strain specific and Lactobacillus plantarum 137 (HK L-137) is a proprietary clinically studied and hero postbiotic strain with evidence-based health benefits for promoting immunity, supporting skin health and helping to maintain healthy cholesterol.
About the Author | 📑
Hi, my name is Sandra. I am a naturopath, nutritionist and herbalist from the Sunshine Coast and am a Senior Consultant at Natural Product Centre.
I graduated 30 years ago and have worked in the health and wellness industry ever since. On a professional basis, I love being involved in the development of natural supplements, research and health writing and hope to make a positive difference to the wellbeing of all the consumers and readers I reach, directly and indirectly.
My hobbies include travelling, exploring new places and art galleries and creating my own art. I also love nothing more than spend time in nature and be surrounded by the beautiful local flora and fauna of the coast.
References
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- Thorakkattu P, Khanashyam AC, Shah K, Babu KS, Mundanat AS, Deliephan A, Deokar GS, Santivarangkna C, Nirmal NP. Postbiotics: Current Trends in Food and Pharmaceutical Industry. Foods. 2022 Oct 5;11(19):3094.
- International Scientific Association for Probiotics and Prebiotics (ISAPP). What is a Postbiotics. 2025. Available from: https://isappscience.org/resource/what-is-a-postbiotic/
- Kiyohara M, Koyanagi T, Matsui H, Yamamoto K, Take H, Katsuyama Y, Tsuji A, Miyamae H, Kondo T, Nakamura S, Katayama T, Kumagai H. Changes in microbiota population during fermentation of narezushi as revealed by pyrosequencing analysis. Biosci Biotechnol Biochem. 2012;76(1):48-52.
- Zhao X, Liu S, Li S, Jiang W, Wang J, Xiao J, Chen T, Ma J, Khan MZ, Wang W, Li M, Li S, Cao Z. Unlocking the power of postbiotics: A revolutionary approach to nutrition for humans and animals. Cell Metab. 2024 Apr 2;36(4):725-744.
- Salminen S, Collado MC, Endo A, Hill C, Lebeer S, Quigley EMM, Sanders ME, Shamir R, Swann JR, Szajewska H, Vinderola G. The International Scientific Association of Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of postbiotics. Nat Rev Gastroenterol Hepatol. 2021 Sep;18(9):649-667.
- Kumar A, Green KM, Rawat M. A Comprehensive Overview of Postbiotics with a Special Focus on Discovery Techniques and Clinical Applications. Foods. 2024 Sep 17;13(18):2937.
- International Scientific Association for Probiotics and Prebiotics (ISAPP). Episode 18: The definition of postbiotics. 2025. Available from: https://isappscience.org/resource/episode-18-postbiotics/
- Mosca A, Abreu Y Abreu AT, Gwee KA, Ianiro G, Tack J, Nguyen TVH, Hill C. The clinical evidence for postbiotics as microbial therapeutics. Gut Microbes. 2022 Jan-Dec;14(1):2117508.
- Zang T, Han L, Lu Z, Tan L, Liang D, Shen X, Liao X, Liu Y, Ren H, Sun J. The History and Prediction of Prebiotics and Postbiotics: A Patent Analysis. Nutrients. 2024 Jan 27;16(3):380.
- Nakai H, Murosaki S, Yamamoto Y, Furutani M, Matsuoka R, Hirose Y. Safety and efficacy of using heat-killed Lactobacillus plantarum L-137: High-dose and long-term use effects on immune-related safety and intestinal bacterial flora. J Immunotoxicol. 2021 Dec;18(1):127-135.
- Hirose Y, Murosaki S, Yamamoto Y, Yoshikai Y, Tsuru T. Daily intake of heat-killed Lactobacillus plantarum L-137 augments acquired immunity in healthy adults. J Nutr. 2006 Dec;136(12):3069-73.
- Hirose Y, Yamamoto Y, Yoshikai Y, Murosaki S. Oral intake of heat-killed Lactobacillus plantarum L-137 decreases the incidence of upper respiratory tract infection in healthy subjects with high levels of psychological stress. J Nutr Sci. 2013 Dec 6;2:e39.
- Tanaka Y, Hirose Y, Yamamoto Y, Yoshikai Y, Murosaki S. Daily intake of heat-killed Lactobacillus plantarum L-137 improves inflammation and lipid metabolism in overweight healthy adults: a randomized-controlled trial. Eur J Nutr. 2020 Sep;59(6):2641-2649.
- Yoshitake R, Nakai H, Ebina M, Kawasaki K, Murosaki S, Hirose Y. Beneficial Effect of Heat-Killed Lactiplantibacillus plantarum L-137 on Skin Functions in Healthy Participants: A Randomized, Placebo-Controlled, Double-Blind Study. Front Med (Lausanne). 2022 Jul 6;9:912280.
