Seaweed is packed with a surprisingly broad range of vitamins, including B12, vitamin C, vitamin K, vitamin A precursors, and vitamin E. The exact amounts vary widely depending on the species, whether it’s dried or fresh, and where it was harvested. But as a group, seaweeds deliver a nutrient profile that few other plant-based foods can match.
B Vitamins, Including Bioavailable B12
The standout finding about seaweed’s vitamin content is its B12. Most plant foods either lack B12 entirely or contain inactive forms that your body can’t use. Nori, the dried seaweed sheets used in sushi, is a notable exception. Dried green and purple laver (both sold as nori) contain substantial amounts of biologically active B12, not the inactive analogues found in many other algae. Research published in the Journal of Agricultural and Food Chemistry confirmed that the B12 in dried nori is bioavailable in mammals, meaning your body can absorb and use it.
The amounts are meaningful: dried green laver contains roughly 63 micrograms of B12 per 100 grams of dry weight, while dried purple laver has about 32 micrograms per 100 grams. The recommended daily intake for adults is 2.4 micrograms, so even a few grams of nori can contribute significantly. That said, 100 grams of dried nori is a lot of sheets. A single sushi-size sheet weighs about 2 to 3 grams, so you’d get somewhere around 1 to 2 micrograms per sheet. That’s still a useful amount, especially for people on plant-based diets who struggle to find natural B12 sources.
Seaweed also provides folate and other B vitamins in varying amounts depending on the species, though B12 is the one that gets the most attention because of how rare it is outside animal foods.
Vitamin C
Seaweed contains vitamin C across all three major categories: green, brown, and red algae. A systematic review in Functional Food Science found that the average vitamin C content across seaweed species is about 0.77 milligrams per gram of dry weight. Green algae average 0.78 mg/g, brown algae about 0.82 mg/g, and red algae around 0.72 mg/g.
Some species punch well above that average. Certain red algae reach up to 5 mg per gram dry weight, and a handful of species across all three categories contain more than 3 mg per gram, putting them on par with peas in terms of vitamin C density. For context, though, you’d typically eat far less seaweed by weight than you would peas. A tablespoon of dried seaweed weighs only a few grams, so the vitamin C contribution from a typical serving is modest. It’s a bonus rather than a primary source.
Vitamin A Precursors and Carotenoids
Seaweed is rich in carotenoids, the plant pigments your body can convert into vitamin A. The specific carotenoids vary by species. Brown seaweeds tend to be high in fucoxanthin, a carotenoid that also functions as an antioxidant. Red and green seaweeds contain beta-carotene and other provitamin A compounds. The deep colors of different seaweed types are a visible clue to their carotenoid content.
The most abundant carotenoid-related compounds in seaweed overall are tocopherols (a form of vitamin E) and xanthophylls, which are carotenoid pigments that support eye health and act as antioxidants. So when you eat seaweed, you’re getting a mix of vitamin A precursors alongside other protective plant compounds, not just one isolated nutrient.
Vitamin E
Seaweed provides a good amount of vitamin E in the form of tocopherols. While exact milligram values vary significantly between species and growing conditions, the presence of vitamin E is consistent across seaweed types. Vitamin E acts as a fat-soluble antioxidant, protecting cells from oxidative damage. Because seaweed also contains small amounts of fat, the vitamin E it provides is well-suited for absorption alongside the food itself.
Vitamin K
Seaweed contains vitamin K1, the form involved in blood clotting and bone health. A tablespoon of raw wakame provides about 7 micrograms of vitamin K1. That’s a small fraction of the daily adequate intake (120 micrograms for men, 90 for women), so typical seaweed servings contribute modestly. Larger portions or concentrated seaweed products would naturally deliver more. If you eat seaweed regularly alongside other vitamin K sources like leafy greens, the amounts add up.
How Species and Preparation Affect Vitamin Content
Not all seaweed is nutritionally equal. Nori is the go-to for B12. Kelp and other brown seaweeds tend to be richer in certain carotenoids. Green seaweeds like sea lettuce have slightly higher average vitamin C levels. Drying concentrates vitamins per gram but can also degrade heat-sensitive nutrients like vitamin C during processing. Fresh seaweed retains more vitamin C but contains more water, so you get less of everything else per bite.
The growing environment matters too. Seaweed harvested from colder, nutrient-rich waters often has higher vitamin concentrations than the same species grown in warmer regions. Commercial dried seaweed products can also vary batch to batch, so nutrition labels on packaged seaweed are your most reliable guide for what you’re actually eating.
A Note on Iodine
While iodine isn’t a vitamin, it’s worth mentioning because it’s the nutrient most strongly associated with seaweed, and it directly affects how much you should eat. Dried kelp can contain as much as 180,000 micrograms of iodine per 100 grams. The recommended daily intake for adults is just 150 micrograms. That means even a small piece of dried kelp can push you well past what your thyroid needs, and chronic excess iodine can cause thyroid problems.
Nori is much lower in iodine than kelp, making it a safer choice for regular consumption. If you’re eating seaweed primarily for its vitamins, nori gives you the best balance: high B12, reasonable amounts of other vitamins, and relatively low iodine risk. Kelp and kombu are better limited to occasional use or small quantities unless you’re tracking your iodine intake carefully.