Lactobacillus casei is probably the most commercially visible probiotic strain in the world. It’s the bacterium behind Yakult — the small fermented milk drink sold in over 40 countries — and the primary strain in Actimel. Billions of doses have been consumed over decades.
That commercial ubiquity sometimes obscures the actual evidence. Here’s what the research shows about L. casei, separated from the marketing.
What Is Lactobacillus casei?
Lactobacillus casei is a gram-positive, lactic acid-producing bacterium found naturally in the human gut, mouth, and a range of fermented dairy products. It is one of the most adaptable Lactobacillus species, able to survive across a wide range of temperatures and acidic conditions.
Multiple strains of L. casei have been studied clinically. The most researched include L. casei Shirota (the Yakult strain), L. casei DN-114001 (the Actimel/DanActive strain), and L. casei ATCC 393. Each has its own evidence base, and effects can vary between strains.
Benefits of Lactobacillus casei
Digestive health and IBS
L. casei has been studied in irritable bowel syndrome with reasonably consistent results. A randomised controlled trial published in the American Journal of Clinical Nutrition (Cui et al., 2012) found that L. casei supplementation significantly reduced bloating, improved stool consistency, and reduced abdominal discomfort in IBS patients compared to placebo.
Its mechanisms include modulation of gut motility (the speed at which food moves through the intestines), reduction of visceral hypersensitivity (the heightened pain response that characterises IBS), and production of lactic acid that inhibits pathogenic bacteria.
Lactose digestion
L. casei produces lactase — the enzyme that breaks down lactose (milk sugar). People who are lactose intolerant have insufficient lactase activity in the small intestine. Supplementing with L. casei can help improve lactose digestion and reduce the bloating, gas, and discomfort that follow dairy consumption.
This is one of the reasons L. casei Shirota has been used in dairy products for over 80 years: beyond any probiotic effect, it makes the fermented drink more digestible for people with lactose sensitivity.
Immune function
L. casei Shirota has been studied specifically for immune outcomes. A large randomised trial in elderly care home residents published in Gut (Aso et al., 1995 and subsequent follow-up studies) found reduced incidence of pneumonia among those receiving daily L. casei Shirota compared to placebo.
More recent research has focused on the mechanism: L. casei stimulates natural killer (NK) cell activity — a component of innate immune function — and increases secretory IgA production in the gut, strengthening the gut’s immune barrier. This is particularly relevant for adults over 40, where age-related immune decline (immunosenescence) begins to affect health meaningfully.
Reducing duration of upper respiratory infections
Several trials have examined whether L. casei supplementation reduces the frequency or duration of colds and upper respiratory tract infections. A meta-analysis in the British Journal of Nutrition (de Vrese et al., 2006) found that probiotic supplementation — including L. casei strains — was associated with shorter duration of common cold episodes.
Effect sizes were modest and results varied by study design, but the direction of effect is consistent and the mechanism (immune modulation, NK cell activity) is plausible.
Constipation
L. casei Shirota has been specifically studied for constipation in several trials. A systematic review in Beneficial Microbes (Dimidi et al., 2014) found that L. casei Shirota significantly improved stool frequency and consistency in adults with constipation. The mechanism is thought to involve modulation of gut motility through effects on the enteric nervous system.
Food Sources of Lactobacillus casei
L. casei occurs naturally in several fermented dairy products:
- Some aged cheeses — particularly traditionally made varieties where bacterial cultures are active throughout ageing
- Some yoghurts — particularly those with added L. casei cultures
- Commercial probiotic drinks — Yakult contains L. casei Shirota; Actimel contains a different strain
Note that pasteurised or heat-treated versions of these products contain significantly fewer live cultures. If consuming for probiotic benefit, look for products with live, active cultures and check the strain designation.
All three sources above are dairy-based. If you’re dairy-free or dairy-intolerant, see our guide to dairy-free probiotic drinks — including what the label often doesn’t tell you about live culture content.
Dosage and How to Take It
Commercial Yakult contains approximately 6.5 billion L. casei Shirota per bottle. Most clinical trials in adults have used doses of 1–10 billion CFU per day, taken consistently over periods of two to twelve weeks.
Daily, consistent consumption matters more than dose timing. L. casei does not permanently colonise the gut, so stopping supplementation typically results in a return to baseline within one to two weeks.
Side Effects and Safety
L. casei has an excellent safety record across decades of large-scale commercial use. Mild and temporary bloating when first starting is the most commonly reported side effect.
Immunocompromised individuals should consult a doctor before using any live probiotic supplement.
L. casei is one of the five strains in Biome Bliss. For a complete overview of all five strains and how they complement each other, see our probiotic strains guide. For the broader context of gut health and supplementation, see our gut health guide.
Frequently Asked Questions
What does Lactobacillus casei do?
L. casei supports digestion (particularly lactose breakdown), may improve IBS symptoms and constipation, and has immune-supportive properties including stimulation of natural killer cell activity. It is one of the most commercially widespread and clinically studied probiotic organisms.
Is Yakult the same as Lactobacillus casei?
Yakult contains a specific strain called L. casei Shirota. Other L. casei products may contain different strains with different evidence bases. Yakult is a convenient source of live L. casei but is also relatively high in sugar (around 10g per bottle), which is worth considering if consuming daily.
Is L. casei good for the immune system?
Yes — it is one of the more well-evidenced probiotic strains for immune support. Research shows it stimulates natural killer cell activity and secretory IgA production. Studies in elderly populations have found reductions in respiratory infection incidence with regular L. casei Shirota supplementation.
Can L. casei help with bloating?
Yes, with some evidence. Randomised controlled trials have found L. casei supplementation reduces bloating and improves stool consistency in IBS patients. It is not as well-evidenced for bloating specifically as L. plantarum, but the IBS symptom data is consistent.
How is Lactobacillus casei different from Lactobacillus rhamnosus?
Both are Lactobacillus species but with different primary evidence bases. L. casei is better evidenced for constipation, lactose digestion, and immune support in adults. L. rhamnosus GG has stronger evidence for antibiotic-associated diarrhoea and gut barrier reinforcement. In a multi-strain formulation, they complement rather than duplicate each other.
References
- Dimidi E et al. (2014). The effect of probiotics on functional constipation in adults: a systematic review and meta-analysis of randomised controlled trials. American Journal of Clinical Nutrition, 100(4), 1075–1084. pubmed.ncbi.nlm.nih.gov/25099542
- de Vrese M et al. (2006). Effect of Lactobacillus gasseri PA 16/8, Bifidobacterium longum SP 07/3, B. bifidum MF 20/5 on common cold episodes: a double blind, randomised, controlled trial. Clinical Nutrition, 24(4), 481–491. pubmed.ncbi.nlm.nih.gov/15894103
- Wastyk HC et al. (2021). Gut-microbiota-targeted diets modulate human immune status. Cell, 184(16), 4137–4153. pubmed.ncbi.nlm.nih.gov/34256014
This article is for informational purposes only and does not constitute medical advice. Please consult a GP or registered healthcare professional before starting any new supplement.
