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Kelp is a group of large brown seaweeds that grow in cold coastal waters and show up in kitchens as kombu, salads, broths and powders. In USDA data, raw kelp is very low in calories and protein but supplies notable amounts of vitamin K, folate and magnesium. Its biggest nutritional story is iodine, and that is also its main caution: the NIH Office of Dietary Supplements (ODS) notes that iodine levels vary greatly between seaweeds, and food labels rarely list them. Below are the USDA kelp nutrition facts, what is known about kelp iodine, the kelp benefits that hold up, and who should be careful.
Peptide Labs does not sell kelp as a food or as a stand-alone kelp product. This article is general nutrition information, not medical advice.
Key Takeaways
- USDA lists raw kelp at 43 calories per 100 grams, with 55% of the Daily Value for vitamin K and 45% for folate.
- Seaweed iodine ranged from 16 to 2,984 micrograms per gram in a study cited by NIH ODS.
- Nutrition Facts labels usually do not list iodine, so kelp foods rarely tell you how much they contain.
- The adult upper limit for iodine is a regulatory reference, and heavy kelp use can exceed it.
- People with thyroid conditions, those on antithyroid medicines or warfarin, and pregnant people should talk to a clinician first.
Kelp nutrition per 100 grams
The table uses the USDA FoodData Central SR Legacy entry for raw kelp (FDC 168457), shown per 100 grams and per 2 tablespoons (10 grams), the portion USDA lists. The %DV column uses FDA Daily Values and refers to the 100-gram amount. SR Legacy does not report iodine for kelp.
| Nutrient (raw kelp, FDC 168457) | Per 100 g | Per 2 tbsp (10 g) | % Daily Value per 100 g |
|---|---|---|---|
| Calories | 43 | 4 | Not applicable |
| Protein | 1.7 g | 0.2 g | 3% |
| Carbohydrate | 9.6 g | 1.0 g | 3% |
| Fiber | 1.3 g | 0.1 g | 5% |
| Vitamin K | 66 micrograms | 6.6 micrograms | 55% |
| Folate | 180 micrograms | 18 micrograms | 45% |
| Magnesium | 121 milligrams | 12 milligrams | 29% |
| Calcium | 168 milligrams | 17 milligrams | 13% |
| Iron | 2.9 milligrams | 0.3 milligrams | 16% |
| Zinc | 1.2 milligrams | 0.1 milligrams | 11% |
| Copper | 0.13 milligrams | 0.01 milligrams | 14% |
| Riboflavin (vitamin B2) | 0.15 milligrams | 0.02 milligrams | 12% |
| Sodium | 233 milligrams | 23 milligrams | 10% |
| Vitamin B12 | 0 | 0 | 0% |
Kelp is mostly water, so the amounts in a small portion are modest. Per 100 grams, vitamin K is the standout, which matters for anyone on warfarin (see the safety section). Folate and magnesium are also meaningful, and calcium and iron make smaller contributions. Sodium is moderate in the USDA entry for plain kelp, though seasoned kelp salads and snacks may be saltier. USDA lists no vitamin B12 for kelp. Dried kelp and kombu are more concentrated per gram than the raw values shown here, so check the package Nutrition Facts.
Kelp iodine: why the numbers swing so much
ODS explains that iodine is an essential part of the thyroid hormones T4 and T3 and is needed for skeletal and nervous system development in fetuses and infants. It lists seaweed, including kelp, nori, kombu and wakame, among food sources. Unlike most nutrients, though, seaweed iodine is hard to pin down.
ODS cites a 2004 study in the journal Thyroid that measured commercially available seaweeds sold whole or as sheets and found iodine concentrations from 16 to 2,984 micrograms per gram. Species, harvest location and processing all play a part. ODS adds that Nutrition Facts labels usually do not list iodine unless it was added, while Supplement Facts labels do show whether iodine is present and how much. In practice, that means a bag of dried kelp or kombu usually gives you no iodine number at all.
Kelp benefits: what the evidence supports
The kelp benefits that are well supported are nutritional: a low-calorie food that adds vitamin K, folate, magnesium and iodine. ODS says the general US population appears to get enough iodine, while people who do not use iodized salt, pregnant people, vegans and people who eat little dairy, seafood or eggs are more likely to fall short. Kelp can contribute for those groups, but because its iodine content is unpredictable, ODS-listed sources such as iodized salt, dairy, eggs and fish are easier to gauge.
Claims that kelp speeds metabolism, causes weight loss, cleanses the body or fixes thyroid problems are not backed by good human trials, and too much iodine can itself cause thyroid problems, as the next sections explain. Kelp-containing supplements are covered in our iodine guide, which explains how to read the iodine line on a Supplement Facts label.
Kelp vs kombu, wakame and nori
Kombu is a kelp, dried and used to make the Japanese broth dashi. Wakame is another brown seaweed, common in miso soup and seaweed salad. Nori, the sushi wrap, is a red seaweed that USDA calls laver; see our nori guide for its full profile. The table compares raw entries in USDA SR Legacy per 100 grams.
| Per 100 g raw (USDA SR Legacy) | Kelp (168457) | Wakame (170496) | Laver or nori (168458) |
|---|---|---|---|
| Calories | 43 | 45 | 35 |
| Protein | 1.7 g | 3.0 g | 5.8 g |
| Sodium | 233 milligrams | 872 milligrams | 48 milligrams |
| Vitamin K | 66 micrograms | 5.3 micrograms | 4 micrograms |
| Folate | 180 micrograms | 196 micrograms | 146 micrograms |
| Calcium | 168 milligrams | 150 milligrams | 70 milligrams |
| Iodine | Not reported | Not reported | Not reported |
Kelp has far more vitamin K than wakame or nori in these entries, wakame is much saltier, and nori has the most protein. None of the USDA entries carry an iodine value, which is why the ODS figures above matter. Sea moss is a red seaweed rather than a kelp; see our sea moss guide.
Who should be careful with kelp
- Thyroid conditions. ODS notes that people with autoimmune thyroid disease may react to iodine intakes considered safe for others. Talk to your clinician before eating kelp often.
- Antithyroid medicines. ODS lists an additive effect between iodine and medicines such as methimazole that could cause hypothyroidism.
- Warfarin. Kelp is a meaningful vitamin K source in USDA data. People on warfarin are usually advised to keep vitamin K intake steady; see our vitamin K foods guide and ask your clinician.
- Pregnancy and breastfeeding. Iodine needs rise, but so does the importance of avoiding excess. Ask your OB-GYN rather than relying on kelp.
- Supplements and powders. ODS notes that iodine-only supplements often provide high amounts, sometimes above the upper limit. Read the iodine line and look for third-party testing.
- Sodium. Plain kelp is moderate in sodium; seasoned kelp products and broths can be much higher. Our low sodium foods guide explains label terms.
Upper limit and excess iodine
According to the ODS fact sheet, the Food and Nutrition Board set the Tolerable Upper Intake Level for iodine at 1,100 micrograms a day for adults, counting food and supplements together. It is a regulatory reference, not a target. ODS lists goiter, raised TSH, hypothyroidism, iodine-induced hyperthyroidism and thyroiditis among the effects of excess iodine. At the high end of the seaweed range ODS cites, one gram of seaweed would exceed that adult upper limit, so frequent kelp eating or kelp powders deserve a conversation with a clinician.
Heavy metals
Concern about arsenic in seaweed centers on hijiki, a different brown seaweed. A 2012 review in Regulatory Toxicology and Pharmacology notes that the UK Food Standards Agency and similar agencies warned consumers not to eat hijiki because of high inorganic arsenic, and a 2019 Japanese study found higher inorganic arsenic intake in children and pregnant women who ate hijiki often. We did not find comparable agency warnings for kelp in the sources we checked, but contamination depends on where seaweed grows, so testing information from the seller is worth asking for.
Frequently Asked Questions
What is kelp?
Kelp is a group of large brown seaweeds. Kombu is a dried kelp used for broth.
Is kelp good for you?
In normal food amounts it is a low-calorie source of vitamin K, folate, magnesium and iodine. People with thyroid conditions or on warfarin should ask a clinician first.
How much iodine is in kelp?
There is no single answer. ODS cites seaweed iodine from 16 to 2,984 micrograms per gram, and food labels rarely list it.
Can kelp affect your thyroid?
Yes. ODS says excess iodine can cause hypothyroidism, hyperthyroidism or thyroiditis, and people with autoimmune thyroid disease may react at lower intakes.
Is kombu the same as kelp?
Kombu is a type of kelp, usually sold dried for making dashi broth.
Does kelp help with weight loss?
There are no good human trials showing that eating kelp causes weight loss.
Questions about a label? Contact us or read our Buyer’s Guide. Food comes first; follow the label and ask a healthcare provider before adding seaweed products or supplements, especially if you have a thyroid condition, take medicines or are pregnant. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
Last reviewed October 2026.
Related guides: Nori and Seaweed: USDA Nutrition, Benefits, Iodine and Safety, Iodine Supplement Guide: Deficiency, Kelp and Thyroid Safety, Sea Moss: Benefits, Iodine, Heavy Metals and Sea Moss Gel Safety.