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7 Health Benefits of Kelp (Seaweed) + Side Effects

Written by Carlos Tello, PhD (Molecular Biology) | Last updated:
Jonathan Ritter
Puya Yazdi
Medically reviewed by
Jonathan Ritter, PharmD, PhD (Pharmacology), Puya Yazdi, MD | Written by Carlos Tello, PhD (Molecular Biology) | Last updated:

Kelp has been an essential component of East Asian diets for centuries. In fact, this diet introduced kelp into different regions across the world. The potential health benefits of kelp are wide-ranging – it can fight iodine deficiency, help with diabetes, and prevent blood clots. Read on to find out more.

What Is Kelp?

Kelp, or brown algae, is found on the coasts of Korea and Japan. Kelp can have many therapeutic and nutritional benefits, as it is a rich source of iodine, minerals, antioxidants, fiber, proteins, and healthy carbohydrates. Kelp may also improve diabetes, reduce blood clots, and even help fight hepatitis C and breast cancer [1].

Kelp Nutrition

Kelp is an excellent source of iodine, potassium, magnesium, iron, and calcium [2].

Depending on the type of kelp or seaweed, the nutrient profile can vary greatly [3].

Laminarin is a specific type of kelp in the brown algae family rich in nutrients (iodine, potassium, magnesium, calcium, and iron). Its compounds may also block tumor growth and spreading [2].

Another type of seaweed – Gracilaria changii – is rich in fiber content and essential amino acids (the building blocks of proteins) [4, 3].

Kelp also contains a complex long-chain carbohydrate (polysaccharide) called fucoidan. Fucoidan is responsible for several potential effects of kelp, including antioxidant, anti-inflammatory, and cancer fighting [5, 6, 7].

Kelp is also an excellent source of vanadium, used in clinical studies to lower glucose levels in individuals suffering from diabetes type 1 and 2. Fucoxanthin, a pigment found in brown seaweed, may also boost weight loss [8, 9, 10, 11, 12].

Mechanism of Action

The high iodine content in kelp supports the production of thyroid hormones T3 and T4. If iodine deficiency is severe and prolonged, the thyroid gland enlarges and forms a goiter. This can also lead to a lack of thyroid hormones (hypothyroidism) [13, 2].

Kelp has potential cancer-fighting properties. Fucoidan from kelp may kill cancer cells and stop their growth [5].

Nutrients from seaweed carry potential health benefits. Dietary fiber, peptides, lipids, and minerals protect the heart. They may help reduce markers of heart disease, protect the cells (reducing oxidative stress), reduce inflammation in blood vessels, reduces high blood pressure, and decrease blood clotting [14].

Health Benefits of Kelp

Effective for:

Inducing Labor and Facilitating Abortion Procedures

Sticks made of Laminaria (a type of kelp) are used to induce birth and perform abortions. Different methods and amounts are used depending on the trimester (inserted into the cervix). The sticks cause the release of prostaglandins, which act as hormones that help initiate womb contractions [15, 16].

Laminaria sticks can mechanically assist in terminating pregnancy from the first to the late second trimester of pregnancy. In one study (longitudinal), 171 late second-trimester abortions were performed using Laminaria (cervical preparation). Only one had serious complications (no contractions during delivery) and 9 required additional safety measures [17, 15, 18].

However, there are better and safer methods for inducing labor or abortion. Depending on the circumstances, doctors may [19]:

  • Ripen the cervix with synthetic prostaglandins
  • Rupture the amniotic sac
  • Give intravenous Pitocin (synthetic oxytocin)

Possibly Effective for:

1) Iodine Deficiency

Kelp has a high iodine content (200 to 400 µg). It improved thyroid function in a study of 7 patients with severe motor and intellectual disabilities and hypothyroidism due to iodine deficiency. Patients were given 1 to 2 grams of powdered kelp daily, and this treatment restored thyroid function, increasing the concentration of iodine in the urine [20].

In another trial on 36 healthy people, kelp increased the levels of the hormone that stimulates the thyroid gland (TSH) [21].

However, excessive amounts may have the opposite effect. In a Japanese clinical trial on 13 people, eating 15-30 grams of kelp per day suppressed thyroid function, resulting in low thyroid hormone levels [22].

All in all, the evidence suggests that appropriate kelp doses may improve iodine deficiency and thyroid function. Be sure to discuss with your doctor if it may be helpful in your case and how you should take it.

2) Diabetes

Powdered seaweed pills reduced sugar levels in a study of 20 subjects with type 2 diabetes (RCT), taken daily for 4 weeks. It decreased fasting and post-meal blood sugar levels and serum lipid (fatty acid) levels. Also, the pills increased HDL levels, which help prevent heart disease associated with diabetes [23, 24].

In another trial on 65 people, polyphenols extracted from two different kelp species lowered blood sugar, insulin resistance, and inflammatory markers [25].

However, an extract with seaweed polyphenols was ineffective at lowering blood sugar levels (both before and after meals) in a trial on 26 people [26].

Kelp is a great source of vanadium. Oral vanadium supplements (150 to 300 mg daily) given to 14 type 1 diabetic patients (longitudinal study) for 30 months decreased fasting blood sugar levels by over 30%. Vanadium also decreased cholesterol levels. It caused no major side effects, with the exception of mild diarrhea at the beginning of the treatment period [27, 10].

Vanadium mimicked insulin in animal studies. In one study with diabetic mice, a vanadium-based compound reduced blood sugar levels and diabetic symptoms (such as thirst, hunger, and weight loss), with no side effects [28].

Although limited, the evidence suggests that kelp and its compounds may help lower blood sugar and insulin resistance.

3) Weight Loss

A study of Xanthigen, which is a type of kelp, showed reduced body weight, waist circumference, and body and liver fat content in 151 non-diabetic obese women. It also improved liver function tests and increased energy use at rest [29].

Mice fed fats from seaweed had increased markers of weight loss in fat tissue. Fucoxanthin, a pigment from seaweed, produced these effects [12, 30].

In a cell study, alginate (a carbohydrate present in the walls of algae and seaweed) reduced the activity of a protein in the pancreas that breaks down fats (pancreatic lipase). Lower activity reduces fat breakdown, leading to fewer fats being absorbed after a meal [31].

Again, the results are promising but limited. Further clinical research is needed before concluding for certain that kelp helps with weight loss.

Insufficient Evidence for:

The following purported benefits are only supported by limited, low-quality clinical studies and some animal research. There is insufficient evidence to support the use of kelp for any of the below-listed uses until the existing results are replicated in larger, more robust clinical trials. Speak with a doctor before taking kelp supplements and never use them as a replacement for approved medical therapies.

1) Blood Clotting and Flow

In a clinical trial on 24 people, dietary fucoidan prevented the formation of blood clots by increasing the production of two messengers (hydrogen peroxide and prostacyclins) in the blood vessels [32].

Fucoidan infusion decreased clotting in bleeding in rats. The rats also had less inflammation around the area of swelling, moved easier, and had better memory retention after fucoidan treatment [33].

Fucoidan supplements prevented blood clotting in mice. The supplements also decreased the activity of blood clot stimulators (platelets and fibrin). In another study, fucoidan injections in mice led to enhanced cell survival and function in tissues with low blood supply (ischemia) [34, 35, 36, 37].

In tissues with low blood supply, fucoidan decreased cell death proteins (including MAPK, JNK, and caspase-3) and harmful compounds (reactive oxygen species) [35].

2) Cancer

Below, we will discuss some preliminary research on kelp’s potential anticancer. It’s mostly in the animal and cell stage and clinical trials have yet to determine if it may be useful in anticancer therapy.

Do not under any circumstances attempt to replace conventional cancer therapies with kelp, its active compounds, or any other supplements. If you want to use it as a supportive measure, talk to your doctor to avoid any unexpected interactions.

A study of 15 postmenopausal women, 10 of whom were breast cancer survivors, looked at the effects of brown seaweed supplementation over a 3-month period (alternating with placebo). Seaweed decreased an important marker of breast cancer recurrence by half (receptor uPAR) after 4 weeks [38, 39].

Fucoidan, a key component of seaweed, may fight cancer and stop tumor growth, based on both cell and animal models. In addition, seaweed supplements and algae extracts, including the brown seaweed Laminaria, reduced colon, breast, and prostate cancer activity [40, 41, 42, 43].

Fucoidan injections or fucoidan, when given in food, slowed tumor growth in mice. Fucoidan killed cancer cells by activating the immune system (via natural killer cells) [40, 44].

Fucoidan reduced the growth of leukemia cells and killed 2 out of 4 lines tested in a study. In another study in cells and mice, fucoidan stopped the growth and spread of lung cancer cells by blocking growth pathways (AktmTOR and NF-kB) [45, 44].

3) Hepatitis C

In a study of 15 patients with chronic hepatitis C, fucoidan from brown seaweed was used to treat virus-related liver diseases. After 8 to 10 months of treatment, hepatitis C virus (HCV) levels in the blood significantly decreased [46].

Additionally, this study also examined alanine aminotransferase levels, a protein whose presence correlates to a more severe HCV infection. The blood tests also present a decrease in alanine aminotransferase levels. Despite the positive laboratory findings, these results did not lead to significant clinical improvements [46, 47].

Animal and Cell Research (Lack of Evidence)

No clinical evidence supports the use of kelp for any of the conditions listed in this section. Below is a summary of the existing animal and cell-based research, which should guide further investigational efforts. However, the studies listed should not be interpreted as supportive of any health benefit.


In a rat model, fucoidan (present in kelp) was used to reduce inflammation caused by immune cells in the brain. Fucoidan improved animal behavior, reduced harmful compounds (TNF-alpha), prevented neuron loss, and protected the cells from damage (reducing reactive oxygen species) that can cause neurodegeneration [48, 49, 50]

In a brain and spinal cord cell study, fucoidan reduced inflammation (blocking nitric oxide (NO) and prostaglandin E2 production). Fucoidan also blocked inflammatory proteins (cytokines IL-1β and TNF-alpha) and the inflammation pathway (reducing NF-kβ and p38 MAPK) [6, 51, 49, 52, 53, 54, 55].

Fucoidan reduced all important actors in the inflammatory cascade in cell studies. In brain immune cells (microglia) fucoidan from brown seaweed showed promise for treating neurodegenerative diseases caused by inflammation [6, 56, 57].


Fucoidan blocked the growth of the herpes simplex virus type 1 (HSV-1) in infected mice. Mice given fucoidan had better survival rates. The treatment improved immune response (innate and adaptive), increasing antibody production [58].

Brain Protection

In a study (cell), seaweed extracts (including kelp) protected brain cells from death in cell models of Parkinson’s disease and improved cell survival. It also protected from toxins, helping the cells avoid death (via hydrogen peroxide and caspase-3) [59, 60, 61, 62].

Bone Growth and Strength

In a rabbit model, fucoidan helped create new vessels, essential for communication with bones and bone repair. It also partially improved bone growth in rabbits with defects in skull formation [63].

In human stem cells, fucoidan boosted the development of cells that build bones, called osteoblasts. Fucoidan also increased the growth of new vessels, improving communication with bones [63, 64].

In another cell study, fucoidan increased proteins that promote bone and mineral formation (via BMP-2, osteocalcin, and ALP). Fucoidan given to aged female mice increased bone density and weight suggesting that fucoidan may play a role in treating age-related bone loss [65, 66, 67, 68].

Blood Pressure

Ten protein extracts from a particular sea kelp (wakame) were given to rats with high blood pressure. Of the 10 extracts, 4 experienced decreased blood pressure after both a single dose and routine use [69].

In a cell study, 5 organic brown seaweed (kelp) extracts blocked an important enzyme that may contribute to high blood pressure (Angiotensin-converting Enzyme, ACE). This enzyme is often a target for blood pressure-lowering drugs [70, 71].

Blood Fat Levels

A 1% or 5% fucoidan (from kelp) diet reduced fat in mice that were fed a high-fat diet over 12 weeks. Kelp reduced the weight of liver and fat tissue, glucose, and fats (cholesterol and fatty acids) in the blood. It increased the activity of a protein that breaks down fatty acids (lipoprotein lipase), dissolving the plaque in arteries [72].


The antioxidant properties of fucoidan (from kelp) were confirmed in a cell study that tested its 2 major components, sulfate and fucose. Fucoidan showed antioxidant effects and has the potential to be used as a natural antioxidant [7].

Side Effects & Precautions

This list does not cover all possible side effects. Contact your doctor or pharmacist if you notice any other side effects.

Call your doctor for medical advice about side effects. In the US, you may report side effects to the FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada, you may report side effects to Health Canada at 1-866-234-2345.

1) Excess Iodine

A 45-year-old woman with no history of thyroid disease experienced an extended period of thyroid hyperactivity (hyperthyroidism) shortly after beginning a kelp-containing diet. This later developed into low thyroid activity (hypothyroidism) caused by the excess iodine in the seaweed [73, 74, 75, 76].

A 39-year-old woman presented with a case of hyperthyroidism after drinking a kelp-containing herbal tea. The tea consumption led to the formation of a goiter, an abnormal enlargement of the thyroid gland [77, 74].

Similarly, a 40-year-old woman developed liver damage (hepatotoxicity) after drinking an herbal tea that contained kelp 3 times a day over 2 months. From the excessive iodine, the thyroid glands swelled, leading to hypothyroidism and reducing liver function [78, 79, 74]

A 54-year-old woman experienced hair loss worsening, memory loss, fatigue, and nausea due to exceeding the prescribed amount of kelp supplements. This caused iodine toxicity, impairing thyroid function, and possible arsenic poisoning (present in the patient’s urine) that caused the symptoms [80, 74].

2) Risk of Heavy Metal Poisoning

Dried seaweed may have high heavy metal content, such as cadmium (Cd). Cadmium can cause toxicity in the body [81].

Levels of other toxic heavy metals, such as arsenic, lead, aluminum, and mercury in Korean seaweed were determined to be very low and not a health threat [82, 83, 84].

Still, another cellular study (initiated in response to the case of severe side effects) found detectable arsenic levels in 8 out of 9 tested kelp supplements. Levels of arsenic were higher than the FDA tolerance level for certain food products. None informed of possible arsenic contamination [85].

Kelp supplements have been linked to autoimmune disorders that cause red blood cells and blood clotting factors to become defective and attacked by the body’s immune system due to the possible presence of toxic metal (arsenic) [86, 87, 88, 89].

3) Inaccurate Food Labeling

A UK-based study examining the accuracy of food labeling on edible seaweed and algae products surveyed 224 products, and only 10% of them contained information regarding iodine content. Of these, 26 were deemed to potentially lead to iodine intake above the accepted level [90].

4) Allergic Reactions to Laminaria Sticks

Two women experienced anaphylactic shock, an extremely severe and possibly fatal allergic reaction when Laminaria (a type of kelp) sticks were used to terminate a pregnancy. The reaction included breathing difficulty, nausea, and dangerously low blood pressure. Previous laminaria tent procedures possibly caused this sensitivity [91].

Drug Interactions

Supplement/Herb/Nutrient-drug interactions can be dangerous and, in rare cases, even life-threatening. Always consult your doctor before supplementing and let them know about all drugs and supplements you are using or considering.

As a huge source of iodine, kelp supplements may interfere with thyroid replacement therapies. Excessive amounts of iodine, similar to deficiencies, can alter thyroid hormones released and impair overall thyroid function [92, 74, 93, 94].

Limitations and Caveats

Most potential health benefits are supported by only a few, small clinical trials. In the cases of inflammation, oxidative damage, herpes, brain protection, bone health, and blood pressure and fats, only animal and cell-based studies have been carried out.

Supplementing with Kelp


Because kelp supplements are not approved for any health conditions, there is no official dose. Users and supplement manufacturers have established unofficial doses based on trial and error. Consult your doctor about the best kelp dose in your case.

Importantly, the FDA-recommended dosage for daily intake of iodine is 225 μg. One gram of powdered kelp contains approximately 200 μg of iodine. Certain kelp-based supplements contain that in one tablet. Never exceed this recommended dosage without consultation [80].

Natural Sources/Forms of Supplementation

Kelp is a form of large brown sea algae. Different kelp supplements exist with varying amounts of proteins and other nutrients. In nature, different kelp species (such as Laminaria digitata, Laminaria Hyperborea, Saccharina latissima, Alaria esculenta) that carry more proteins may contain less long-chain carbohydrates (such as fucoidan), and vice-versa [95, 90].

You can take kelp as food, pill, powder, or tincture. Different types of kelp are used, including winged kelp, laminarin, and fingered tangle. Each species varies in iodine levels and nutrients, so the supplement strength and risk will also vary [90, 2, 77, 20].

Genetics Related to Kelp Metabolism

People in some regions of the world may digest large-chain carbohydrates present in seaweed more easily than others. The Japanese, whose diet includes more marine algae, have a gut bacterium for seaweed digestion that is not present in Americans or Europeans. It originates from a marine bacterium (via gene transfer) living on the surface of some sea algae species [95].

User Experiences

The opinions expressed in this section are solely those of kelp users, who may or may not have medical or scientific training. Their reviews do not represent the opinions of SelfHacked. SelfHacked does not endorse any specific product, service, or treatment.

Do not consider user experiences as medical advice. Never delay or disregard seeking professional medical advice from your doctor or other qualified healthcare providers because of something you have read on SelfHacked. We understand that reading individual, real-life experiences can be a helpful resource, but it is never a substitute for professional medical advice, diagnosis, or treatment from a qualified healthcare provider.

Users regarded kelp as an effective agent for weight loss and an essential ingredient in energy-boosting juices. Additionally, users with histories of thyroid issues claimed that the source of iodine present in kelp and its supplements worked better than previously prescribed iodine pills, even improving metabolism.

Certain kelp supplement users complained of gaining weight and experiencing depression after long periods of use. Some even complained of developing the dependency on the supplements for thyroid function, due to the resulting iodine imbalance. Others complained of excessive sweating and mucus formation after taking kelp-based supplements.

About the Author

Carlos Tello

Carlos Tello

PhD (Molecular Biology)
Carlos received his PhD and MS from the Universidad de Sevilla.
Carlos spent 9 years in the laboratory investigating mineral transport in plants. He then started working as a freelancer, mainly in science writing, editing, and consulting. Carlos is passionate about learning the mechanisms behind biological processes and communicating science to both academic and non-academic audiences. He strongly believes that scientific literacy is crucial to maintain a healthy lifestyle and avoid falling for scams.


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