Chlorella

What it is

Chlorella is a single-celled freshwater green alga that is grown commercially and sold worldwide as a dietary supplement in tablets and powders. It contains a range of nutrients, and some products contain vitamin B12, though the amount varies enormously between products and some of it is in inactive forms.

What the research says

The human evidence is a set of small, mostly short randomized trials pooled in meta-analyses. Pooled together, chlorella produced small reductions in total and LDL cholesterol and in systolic blood pressure (about 4 to 5 mmHg in the 2018 analysis), but no reliable change in triglycerides, and the 2025 GRADE-rated review found no effect on fasting glucose or insulin. Reviewers call for more and better trials. The main safety issues are vitamin K content disturbing warfarin control, a documented 1977 outbreak of light-sensitivity skin reactions from badly processed tablets, and the algae's tendency to take up contaminants.

Evidence grade: Limited evidence.

What goes wrong

A literature review of 32 reports of warfarin and vitamin K interactions found that vitamin K-containing supplements and foods, chlorella among them, disturbed warfarin control, with INR rising or falling when intake started or stopped suddenly. (Source 1)

• Systematic review, Low certainty.
• Size: 32 articles.
• Who: Patients taking warfarin.
• How long: Not applicable.
• Result: Significant falls or rises in INR reported.
• Funding: not stated.

The majority of these articles reported disturbances of the anticoagulation effect of warfarin by the interaction of dietary supplements and vitamin K-containing foods (such as vegetables, chlorella, and fermented soybean called natto) with warfarin.

In 1977 at least 23 people in the Tokyo area developed light-triggered skin reactions after taking one brand of chlorella tablets; the tablets had high levels of pheophorbide, a light-activated breakdown product of chlorophyll created during ethanol processing. (Source 2)

• Case series, Low certainty.
• Size: At least 23 patients; mouse experiments on the tablets.
• Who: People taking one brand of chlorella tablets, Tokyo, 1977.
• How long: Not applicable.
• Result: Tablets from another maker with less than a tenth as much pheophorbide caused no reaction in mice.
• Funding: not stated.

In 1977, there was an outbreak of photosensitivity dermatitis by among persons who had ingested a certain brand of chlorella tablets. At least 23 patients were identified in and around the Tokyo metropolitan area.

The same product analysis found powdered chlorella can make a meaningful contribution to aluminium intake. (Source 3)

• Survey study, Low certainty.
• Size: 52 products.
• Who: Commercial supplements.
• How long: Not applicable.
• Result: 17.13% to 28.92% of the TWI of aluminium from three chlorella products.
• Funding: not stated.

Limit of this finding: The paper gives the tolerable weekly intake (TWI) for aluminium in two forms that do not agree: here as '2.8 μg/kg bw/day' (a daily figure for a weekly limit) and elsewhere as '2 µg/g bw/week'. Treat the percentages as the authors' estimates only. Do not use them to judge how much aluminium is safe, and do not rely on either TWI figure without checking the official limit.

In the case of Al content, the consumption of powdered Chlorella preparations of supplement no 2 (Poland origin), no 16 (Chinese origin) and 23 (Chinese origin) provide a contribution percentage of 25.04%, 28.92%, 17.13% of the TWI of Al (2.8 μg/kg bw/day).

What the evidence supports

A 2018 meta-analysis of 19 randomized trials found chlorella lowered total and LDL cholesterol, systolic and diastolic blood pressure and fasting glucose by small amounts. (Source 4)

• Meta-analysis, Low certainty.
• Size: 797 subjects across 19 RCTs.
• Who: Adults of varied health status.
• How long: Trials of varying length; benefits concentrated in trials of 8 weeks or longer.
• Result: TC −9.09 mg/dl (95% CI −12.91 to −5.26); LDL-C −8.32 mg/dl; SBP −4.51 mmHg (95% CI −6.53 to −2.48); DBP −1.64 mmHg (95% CI −3.28 to −0.01, P = 0.049); FBG −4.23 mg/dl (P = 0.041)
• Funding: not stated in the abstract.

Meta-analysis on 19 RCTs with 797 subjects indicated that Chlorella administration significantly decreased the levels of total cholesterol (TC; −9.09 mg/dl, 95% CI: −12.91 to −5.26, P < 0.001)

A 2025 GRADE-assessed meta-analysis of 25 studies found modest improvements in cholesterol, blood pressure and weight, rated from high to low certainty depending on the outcome. (Source 5)

• Meta-analysis, Moderate certainty.
• Size: 25 studies (33 effect sizes)
• Who: Adults over 18.
• How long: Varied.
• Result: TC −5.26 mg/dl; LDL-C −7 mg/dl; SBP −3.68 mmHg (P = 0.005); DBP −2.04 mmHg (P = 0.04); weight −1.37 kg.
• Funding: not stated in the abstract.

GRADE analysis showed low-quality evidence for hs-CRP and FBG, high-quality evidence for TC, LDL-C, and BMI, and moderate-quality evidence for other parameters.

What the evidence does not support

The same 2018 meta-analysis found no significant effect of chlorella on triglycerides, HDL cholesterol or body mass index, and the authors called for more precise trials. (Source 6)

• Meta-analysis, Low certainty.
• Size: 797 subjects across 19 RCTs.
• Who: Adults of varied health status.
• How long: Varied.
• Result: TG +1.73 mg/dl (P = 0.706); HDL-C +1.54 mg/dl (P = 0.443); BMI −0.23 kg/m2 (P = 0.545)
• Funding: not stated in the abstract.

supplementation improved levels of TC, LDL-C, SBP, DBP, and FBG but the changes in TG, HDL-C, and BMI were not satisfactory

The 2025 GRADE-assessed meta-analysis found chlorella did not affect triglycerides, fasting glucose or insulin. (Source 5)

• Meta-analysis, Low certainty.
• Size: 25 studies.
• Who: Adults over 18.
• How long: Varied.
• Result: No significant effect on TG, FBG or insulin.
• Funding: not stated in the abstract.

Chlorella supplementation did not affect TG, FBG, and insulin levels.

A 2025 analysis of 52 spirulina and chlorella supplements found that none exceeded the EU limits for lead or cadmium. (Source 7)

• Survey study, Low certainty.
• Size: 52 products (29 conventional, 23 organic)
• Who: Commercial spirulina and chlorella supplements.
• How long: Not applicable.
• Result: No sample exceeded the EC limits for Cd (0.050 µg/g) or Pb (0.30 µg/g)
• Funding: not stated.

None of the analyzed samples from either group exceeded the maximum permissible concentrations for toxic metals such as Pb and Cd as specified by the European Commission regulations

Where the evidence is mixed

The only placebo-controlled trial of a whole greens blend we found (AG1, which is not chlorella alone) was a 20-person, two-week crossover funded by the manufacturer; it found more micronutrient targets met but no difference in digestive quality of life. (Source 8)

• Randomized trial, Low certainty.
• Size: 20 resistance-trained adults.
• Who: Healthy resistance-trained men and women.
• How long: 14 days per arm, crossover.
• Result: Micronutrient EARs met +2.8 vs placebo (p = 0.0011); digestive quality of life p = 0.777.
• Funding: industry-funded (AG1; several authors employed by AG1)

AG1® supplementation significantly improved nutritional adequacy by increasing the total number of micronutrient Estimated Average Requirements (EARs) met compared to placebo (2.8; p = 0.0011), with no significant differences in digestive quality of life between groups (p = 0.777).

Where the research disagrees

How strong the cardiovascular evidence is

• Shiri et al. 2025 meta-analysis, meta-analysis of 25 RCTs: GRADE analysis showed low-quality evidence for hs-CRP and FBG, high-quality evidence for TC, LDL-C, and BMI, and moderate-quality evidence for other parameters. (Source 5)
• Fallah et al. 2018 meta-analysis, meta-analysis of 19 RCTs: More precise RCTs on subjects with different health status is recommended to clarify the effect of Chlorella supplementation on cardiovascular risk factors. (Source 4)

How much

• Reference intake: No recommended intake exists for chlorella; it is not an essential nutrient. The reviews we read set none. (Source 9)
• Upper limit: No upper limit has been set for chlorella itself in the sources we read. (Source 9)
• Studied: In the 2018 meta-analysis, cholesterol and blood pressure reductions were seen mainly in trials giving more than 4 g a day for 8 weeks or longer. (Source 4)

A common belief, and what the research shows

The belief: Chlorella is a reliable plant source of vitamin B12.

What the research shows: B12 content varies from almost none to a lot between products, and some products carry inactive B12-like compounds: "Some of the high B12-containing Chlorella products contain inactive corrinoid compounds such as 5-methoxybenzimidazolylcobamide and cobalt-free corrinoid."

Questions and answers

What is it?

Chlorella is a single-celled green alga grown commercially and sold as tablets and powders. It is a staple ingredient of many greens powders. (Source 9)

What does it do in the body?

In pooled trials chlorella slightly lowered total and LDL cholesterol and systolic blood pressure. It did not change triglycerides, and the most recent review found no effect on blood sugar or insulin. (Source 5)

Is it good or bad for you?

Small benefits on cholesterol and blood pressure appear in short trials. It can be bad for people on warfarin because its vitamin K content can shift clotting control, and badly processed tablets have caused light-sensitive skin reactions. (Source 1)

How do you get more of it?

Chlorella is only eaten as a supplement (tablets or powder, alone or in greens blends). The trials that found effects mostly gave more than 4 g a day for 8 weeks or more. (Source 4)

If it is harmful, what reduces it?

Does not apply as a nutrient, since the body does not need chlorella. The risks come from the product, not from a lack of it. For warfarin users the literature points to sudden starting or stopping of vitamin K sources as the problem. (Source 1)

Why might someone be low in it or missing it?

Does not apply. Chlorella is not a nutrient the body needs, so nobody is deficient in it. (Source 9)

Which whole foods contain it or feed it?

Chlorella is not part of ordinary whole foods; it is sold as a supplement. Its nutrients, such as vitamin B12, vary a lot between products and between species. (Source 9)

What happens if you do not have it?

Nothing is known to happen. Chlorella is not an essential nutrient, and reviewers note that even its active compounds are poorly understood. (Source 9)

How can you test for it?

No test measures chlorella in the body. People on warfarin have their clotting (INR) monitored, and the literature shows vitamin K sources such as chlorella can move it. (Source 9)

We searched: Searched chlorella reviews and meta-analyses (Bito 2020, Fallah 2018, Shiri 2025) and warfarin interaction literature for any biomarker of chlorella intake; none described.

References

1. Japanese Journal of Clinical Pharmacology and Therapeutics (Rinsho Yakuri). Literature Search on the Interaction Between Warfarin and Vitamin K. 2009. DOI 10.3999/jscpt.41.43. Read the source
2. Food Hygiene and Safety Science (Shokuhin Eiseigaku Zasshi). Causal Substances of Photosensitivity Dermatitis due to Chlorella Ingestion. 1979. DOI 10.3358/shokueishi.20.173. Read the source
3. International Journal of Molecular Sciences. Spirulina and Chlorella Dietary Supplements—Are They a Source Solely of Valuable Nutrients?. 2025. PMID 41226507, DOI 10.3390/ijms262110468. Read the source
4. Clinical Nutrition. Effect of Chlorella supplementation on cardiovascular risk factors: A meta-analysis of randomized controlled trials. 2018. PMID 29037431, DOI 10.1016/j.clnu.2017.09.019. Read the source
5. Algal Research. Chlorella supplementation diminishes cardiovascular risk factors in adults: A GRADE-assessed systematic review and meta-analyses of randomized clinical trials. 2025. DOI 10.1016/j.algal.2025.104281. Read the source
6. Clinical Nutrition. Effect of Chlorella supplementation on cardiovascular risk factors: A meta-analysis of randomized controlled trials (Conclusions section of abstract). 2018. PMID 29037431, DOI 10.1016/j.clnu.2017.09.019. Read the source
7. International Journal of Molecular Sciences. Spirulina and Chlorella Dietary Supplements—Are They a Source Solely of Valuable Nutrients?. 2025. PMID 41226507, DOI 10.3390/ijms262110468. Read the source
8. Frontiers in Nutrition. Effect of AG1® supplementation on nutritional adequacy and gut microbial composition in trained adults. 2026. DOI 10.3389/fnut.2026.1783951. Read the source
9. Nutrients. Potential of Chlorella as a Dietary Supplement to Promote Human Health. 2020. DOI 10.3390/nu12092524. Read the source