Why supplement batches differ: the case of algae

The same kelp product can carry very different iodine from one batch to the next. Here is why natural supplements vary — and how to feed from the batch's declared number.

This text is about balancing the diet of a healthy adult dog or cat during maintenance. It does not cover growth, pregnancy, lactation or diets used in illness — those need separate values and an individual feeding plan drawn up with a veterinarian.

A calculator only knows what you type into it. When a supplement is a natural raw material, the amount of a nutrient in one gram can shift from batch to batch — so the number that was right last month may be wrong for the tub you just opened. Iodine in kelp is the clearest example, and it is worth understanding why this happens before you trust a single figure.

Why a natural raw material varies between batches

A supplement made from a whole natural material is not a standardised chemical. Its composition reflects where and when it grew, and that changes from harvest to harvest:

  • Species and part used. Different seaweed species — and even different parts of the same plant — accumulate minerals differently.
  • Harvest site and season. Sea and soil chemistry, water temperature and time of year all shift how much a plant takes up.
  • Environmental accumulation. Seaweeds concentrate elements from the water, so the same species from two coastlines can differ several-fold.
  • Drying and processing. How the material is dried, milled and blended changes the final concentration per gram.

None of this is a defect — it is the nature of a biological raw material. The problem only starts when the recipe treats every batch as if it carried the same amount.

Algae and iodine: the clearest example

Kelp (usually Ascophyllum nodosum or Laminaria species) is the most practical iodine source in a raw diet, and also the one where batch variation bites hardest. A chemical characterisation of one Ascophyllum nodosum material reported a specific mineral content for that sample, and the authors are explicit that such a value cannot be transferred to another product or batch without its own analysis [7]. In practice the iodine in dried seaweed can swing widely between batches of the very same product.

That matters in both directions. A documented case describes goitrous hypothyroidism in a dog caused by iodine deficiency in a home-prepared diet [6]. In the other direction, a commercial diet with a high iodine level — from a seaweed-based source — has been linked to thyroid disturbance in dogs [8]. The gap between the useful dose and the excessive one is narrower than for most minerals, so a random bag of dried kelp with no declared iodine per gram can deliver a fraction of the target one week and a multiple of it the next.

Standardised or not: what to actually buy

The way out is standardisation, not guesswork:

  • A standardised supplement states a certified nutrient content per gram and controls it batch to batch. That declared figure is what you enter into the recipe.
  • Random dried seaweed with no declared iodine per gram cannot be balanced. It is not that it is unsafe by definition — it is that you have no number to feed.
  • A synthetic salt such as potassium iodide carries a consistent, known amount, which is why it is used where precision matters — at the cost of careful dosing.

What this means for your recipe

  • Enter the declared content per gram for the amount you actually use, not a rule of thumb.
  • Mind where the number sits on the label. The FEDIAF labelling code explains that a figure under "additives" is the amount added, while a value among analytical constituents is the total measured in the finished product [5]. For balancing you want the total, or a reliable total-content analysis from the maker.
  • Do not expect iodine on the mandatory panel. EU feed law lists crude protein, fibre, oils/fats and ash in the compulsory analytical declaration — iodine is not there, so its absence does not mean the product contains none [4].
  • Re-check when the product or batch changes. A new tub or a new brand means confirming the declared value again — do not assume the last batch's number still holds. Ingredients with high iodine content can be reviewed in the ranking of products rich in iodine.

The numbers you compare against

These references are for healthy adults during maintenance. Do not apply them to growth, pregnancy, lactation or clinical diets:

  • Dogs. NRC gives a recommended allowance of about 0.22 mg iodine per 1000 kcal [1]. The AAFCO maintenance profile sets a minimum of 0.25 mg and a maximum of 2.75 mg per 1000 kcal [3]. FEDIAF 2025 gives roughly 0.26–0.30 mg per 1000 kcal depending on the assumed energy intake [2].
  • Cats. The AAFCO maintenance profile sets a minimum of 0.15 mg per 1000 kcal [3], while FEDIAF 2025 gives roughly 0.33–0.43 mg per 1000 kcal depending on the assumed energy intake [2].

These values assess a recipe. They are not a diagnosis of deficiency or excess, and not an individual treatment recommendation for a particular animal.

What to remember

A natural supplement is only as reliable as the number on its batch. Kelp iodine is the textbook case: it varies with species, harvest, season and processing, so the same spoon can carry very different amounts. Feed only a product with a declared content per gram, enter that figure, compare it with a clear reference, and confirm it again whenever the product or batch changes.

Sources

  1. National Research Council (2006). Nutrient Requirements of Dogs and Cats. The National Academies Press. https://nap.nationalacademies.org/catalog/10668/nutrient-requirements-of-dogs-and-cats
  2. FEDIAF (2025). Nutritional Guidelines for Complete and Complementary Pet Food for Cats and Dogs. https://europeanpetfood.org/wp-content/uploads/2025/09/FEDIAF-Nutritional-Guidelines_2025-ONLINE.pdf
  3. AAFCO (2014). Revised AAFCO Dog and Cat Food Nutrient Profiles. https://www.aafco.org/wp-content/uploads/2023/01/Pet_Food_Report_Annual_2014-Appendix_A-Revised_AAFCO_Nutrient_Profiles-Final_092214.pdf
  4. Regulation (EC) No 767/2009 on the placing on the market and use of feed, Annex VII. https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32009R0767
  5. FEDIAF (2019). Code of Good Labelling Practice for Pet Food. https://europeanpetfood.org/wp-content/uploads/2022/02/FEDIAF_labeling_code_2019_onlineOctober2019.pdf
  6. Frontiers in Veterinary Science (2022). Case Report: Technetium-99m pertechnetate scintigraphy findings in a dog with iodine deficiency-induced goitrous hypothyroidism, 9, 922456. https://doi.org/10.3389/fvets.2022.922456
  7. Frazzini, S. et al. (2024). Chemical-functional characterization of Ascophyllum nodosum and Phymatolithon calcareum and dietary supplementation in post-weaning pigs. Frontiers in Veterinary Science, 11, 1431091. https://doi.org/10.3389/fvets.2024.1431091
  8. Castillo, V. A. et al. (2001). Commercial diet induced hypothyroidism due to high iodine. A histological and radiological analysis. Veterinary Quarterly, 23(4), 218–223. https://pubmed.ncbi.nlm.nih.gov/11765244/