Chlorella is a single-celled freshwater green algae that has become one of the most popular nutritional supplements worldwide. The cells are tiny, roughly 1 to 4 micrometers in diameter, and packed with an unusually dense concentration of nutrients, especially protein and iron. It’s sold as tablets, capsules, and powder, and used primarily as a daily supplement to fill nutritional gaps and support overall health.
What Chlorella Actually Is
Chlorella belongs to a family of microscopic green algae called Chlorellaceae. The two species you’ll encounter most often in supplements are Chlorella vulgaris and Chlorella pyrenoidosa. These round, single-celled organisms reproduce rapidly through photosynthesis, which is part of why they’re so rich in chlorophyll, the green pigment that gives them their deep color.
Unlike spirulina, which is technically a cyanobacterium (blue-green algae), chlorella is a true green algae with a rigid cellulose cell wall. That cell wall matters: humans can’t digest it in its natural state, so virtually all chlorella supplements are processed to crack or break down the cell wall before packaging. Without this step, your body would pass most of the nutrients right through. Once the wall is disrupted, animal studies show more than 80% of chlorella’s protein becomes digestible.
Nutritional Profile
Chlorella’s standout feature is its protein density. It’s 50 to 60% protein by weight, which is exceptionally high for any whole food, let alone a plant source. A 1-ounce (28-gram) serving provides about 16 grams of protein and 115 calories. It also produces essential fatty acids, including omega-6 (linoleic acid) and omega-3 (alpha-linolenic acid).
The iron content is where chlorella really separates itself. That same 1-ounce serving delivers roughly 202% of the daily value for iron. For comparison, the same amount of spirulina provides 44% of the daily value. This makes chlorella one of the most iron-dense supplements available, which is particularly relevant for people on plant-based diets or those prone to iron deficiency.
Chlorella also contains a range of antioxidant compounds: chlorophyll (in unusually high concentrations), vitamin C, beta-carotene, lycopene, and lutein. Some varieties may contain vitamin B12, though the evidence on whether it’s a reliable source of bioavailable B12 is still mixed. Depending on the specific product, iron content can range from 6 to 40% of the daily value per supplement dose, so checking the label matters.
Immune System Effects
One of the better-studied benefits of chlorella is its effect on immune function. In a randomized, double-blind, placebo-controlled trial, healthy adults who took 5 grams of chlorella daily for eight weeks showed significantly increased natural killer cell activity compared to the placebo group. Natural killer cells are part of your body’s first line of defense, responsible for identifying and destroying virus-infected cells and abnormal cells before they become a larger problem.
The same study found that chlorella supplementation raised levels of several immune signaling molecules, including interferon-gamma and interleukin-1 beta. These increases were significantly larger in the chlorella group than in the placebo group, and the changes in these signaling molecules correlated directly with the boost in natural killer cell activity. In practical terms, this suggests chlorella may help prime your immune system to respond more effectively to threats.
Cholesterol and Heart Health
A systematic review and meta-analysis of randomized controlled trials found that chlorella supplementation produced a meaningful reduction in both total cholesterol and LDL (“bad”) cholesterol. Pooling results across multiple studies, total cholesterol dropped by an average of 7.47 mg/dL and LDL cholesterol dropped by 7.71 mg/dL compared to placebo groups. These aren’t dramatic numbers on their own, but for a food supplement with minimal side effects, they represent a consistent and statistically significant shift.
Interestingly, the dose-response analysis revealed that the cholesterol-lowering effect on LDL was most apparent at doses up to 1,500 mg per day. At higher doses, the benefit plateaued and was no longer statistically significant, suggesting that more isn’t necessarily better when it comes to cardiovascular effects. Triglyceride levels did not change significantly with chlorella supplementation, and HDL (“good”) cholesterol remained essentially unchanged.
Heavy Metal Binding
Chlorella has a reputation as a detoxifier, and there’s a biological basis for this, though the evidence in humans is still developing. Chlorella’s polysaccharides can act as adsorbents for heavy metals, essentially binding to them. The algae also produces compounds called phytochelatins, short peptides with a high content of the amino acid cysteine, which help neutralize heavy metals. Cysteine appears to be particularly effective at promoting mercury detoxification in chlorella-exposed environments.
In one clinical study, participants who took algae extract (including chlorella) and amino acid supplements for 90 consecutive days showed reductions in hair levels of mercury, tin, silver, and lead compared to their baseline measurements. Mercury levels dropped from a median of 1.9 to 1.15 micrograms per gram of hair, and tin levels fell from 0.11 to 0.03. These are promising results, but researchers note that the exact mechanism by which regular chlorella consumption removes heavy metals in humans hasn’t been fully established yet.
Dosage Used in Studies
Clinical trials have generally used chlorella at doses between 3 and 10 grams per day. The immune study used 5 grams daily, while other trials have gone as high as 8 grams per day for lutein absorption research. A dose of 6 grams daily was used in studies looking at immune markers in saliva. If you’re starting chlorella for the first time, beginning at the lower end of this range and increasing gradually gives your digestive system time to adjust.
Side Effects and Digestive Adjustment
The most common side effects are digestive: constipation and diarrhea. In one 12-week study, about 31% of participants with a history of constipation found their symptoms worsened during the first two weeks. Similarly, about 15% experienced mild diarrhea in the first week. In both cases, symptoms were mild to moderate and resolved on their own within the first two weeks without any intervention. No participants in that study reported abdominal pain, headaches, fever, or other complaints over the full 12-week course.
Because chlorella is extremely rich in iron and vitamin K, people taking blood thinners or those with iron overload conditions should be cautious. Chlorella’s high chlorophyll content also means the supplement can turn stool green, which is harmless but sometimes alarming if you’re not expecting it. Product quality varies, so choosing supplements that have been tested by an independent lab for heavy metal contamination is worth the extra cost, since algae can absorb contaminants from the water they’re grown in.