Kelp

Laminaria · Brown Seaweed · Ascophyllum nodosum · Bladderwrack

Snapshot

Indications	Contraindications
Iodine repletion¹ Postprandial glucose support² HbA1c reduction² Blood pressure support³ Androgenetic alopecia^{4, 5} Metabolic syndrome support⁶	Existing thyroid disease⁷ Iodine excess risk¹

Kelp is a large brown macroalga belonging to the order Laminariales, consumed as food and taken as a supplement primarily for its iodine content, which supports thyroid hormone synthesis.⁸ Supplementation is used for iodine repletion in deficient populations, and its bioactive compounds, including fucoidan, alginate, and fucoxanthin, drive additional uses in glycemic support, cardiometabolic risk reduction, and, more recently, androgenetic alopecia. Patients most likely to benefit from the iodine fraction include those avoiding dairy and white fish, pregnant or lactating women with low intake, and individuals on plant-based diets; those seeking the metabolic fraction are typically overweight adults with insulin resistance or early metabolic syndrome.

A 2023 meta-analysis found brown seaweed supplementation associated with significant reductions in postprandial blood glucose, glycated hemoglobin (HbA1c), and homeostatic model assessment of insulin resistance (HOMA-IR) compared with control, with higher doses more effective than lower ones.² A 2025 systematic review and meta-analysis of randomized controlled trials (RCTs) found edible algae intake significantly reduced both systolic and diastolic blood pressure, with effects most pronounced in individuals with cardiometabolic risk and at intakes above three grams per day.³ A 2025 network meta-analysis of dietary supplements for androgenetic alopecia found a compound extract of Cistanche and Laminaria significantly improved hair density compared with placebo, a finding not yet replicated for kelp as a single ingredient.⁵ Over 15,000 products listing kelp are available, primarily as capsules, tablets, and powders; see Form comparison for delivery considerations.

The most consequential concern is iodine excess: commercial kelp products vary widely in iodine content, and one serving can exceed the tolerable upper intake level for iodine, which carries risk of inducing or worsening both hypothyroidism and hyperthyroidism.¹ This is not a reason to avoid kelp categorically but is a reason that per-product iodine quantification and thyroid function monitoring are necessary when use is ongoing, particularly in patients with existing thyroid disease.¹

Last updated October 8, 2026. Read how we make monographs.

In the full monograph

58 sources, 44 studies

When to useThe situations practitioners use it for, each with the reason and the study behind it.
SafetyWho should avoid it, who needs extra care, and the side effects reported in studies.
InteractionsThe medicines and supplements it interacts with, the effect of each, and what labeling says to watch.
Protocol notesThe doses used in studies, how and when they were taken, and how the forms on the market compare.
How it worksWhat it is and how it works in the body, in plain terms.
Published studiesThe studies behind each use, popular claims checked against them, and trials still under way.
Word of mouthWhat patients and practitioners report online, kept separate from the clinical evidence.
Clinical perspectiveA closing read of where the evidence is solid, where it is thin, and what has not been studied.

Snapshot

In the full monograph

Sources