Seaweed supplements have become very popular for both dogs and cats. And for some good reasons.
Ascophyllum nodosum, a brown algae, has been shown to significantly reduce plaque formation and calculus deposition in both dogs and cats.
We don't yet know exactly how it works, however it may change the composition of saliva by inhibition of some pathways that could increase plaque or calculus development.
Other seaweeds like kelp are a rich source of many vitamins and minerals, like potassium and magnesium. Lithothamnium calcareum, a red marine algae, accumulates minerals in its fronds, which then break off and are sustainably harvested and used as a source of calcium. It has been shown to support normal bone mineral density and structure and is currently used in both human and pet supplements.
What's the concern?
Seaweed-derived supplements and products can be very high in iodine, and the amount of iodine in these products can vary significantly. Brown algae like kelp contain the highest amounts.
The absorption of minerals by seaweed also depends on a range of factors, such as availability of nutrients in the sea, oceanic currents, pH, salinity, temperature and solar irradiance. These factors vary with geographical location and conditions throughout the year, leading to geographical and seasonal variations in mineral content in the algae.
This means that the iodine content of seaweeds can vary from season to season, and can sometimes be more than 10 times higher than at other times.
Seaweed as a source of iodine in balanced diets
Obviously this variation may be a concern if you are relying on seaweed to ensure your pet's homemade diet contains adequate iodine - an essential nutrient required in very small amounts for the synthesis of thyroid hormones. At times, the diet may become deficient in iodine, and at other times, excessive. Is this likely to cause harm, though?
Iodine and thyroid disease
In one human study, the iodine content in one portion of seaweed-containing products ranged from 128 to 62400 mcg. In seaweed-containing foods, the iodine content ranged from 30 to 25300 mcg per portion, and in supplements it ranged from 5 to 5600 mcg per daily dose.
The species with the highest analysed iodine content were oarweed, sugarkelp and kombu, with mean iodine levels of 7800, 4469 and 2276 mcg/g, respectively. For 54 products, the intake of one portion or dose would exceed the tolerable upper intake level for iodine.
Often this is transient, with the patient making a full recovery after stopping the seaweed supplement or product. In some cases, hyperthyroidism does not resolve spontaneously, and antithyroid medication is needed.
Why does this happen? Most individuals can tolerate chronic excess iodine without clinical symptoms; the normal thyroid gland can adapt to excess iodine. Sometimes these mechanisms fail, and excess iodine leads to overt clinical hyperthyroidism or hypothyroidism. In some instances, the excess iodine can trigger hyperproduction of thyroid hormones leading to clinical signs of hyperthyroidism.
Can excess iodine cause thyroid disease in dogs and cats?
For cats, dietary iodine may play a role in the development of hyperthyroidism - low or fluctuating iodine levels in commercial cat foods may predispose to thyroid hyperplasia and hyperthyroidism, however this has not been definitively shown. Diets with excessive iodine have been shown to cause a depression in thyroid hormone production, however one study by Kyle et al. (1994) showed that cats can be fed a high iodine diet for 5 months, with no negative effects on thyroid function occurring.
For dogs, there is research demonstrating that consumption of treats or foods containing thyroid gland can cause thyrotoxicosis, however there are no studies demonstrating thyrotoxicosis due to seaweed supplements or products. One study in dogs showed that excessive iodine caused depressed thyroid function and bone abnormalities.
So, where does this leave us? Clearly there is a lack of research in dogs and cats regarding the risk of thyrotoxicosis and seaweed supplements. This may be because recommended doses are safe and cause no adverse effects, or because adverse effects are not being detected, or it may be because many of these seaweed supplements have only increased in popularity over the last few years. As pet food manufacturers seek food-derived additives as a source of iodine (rather than compounds like potassium iodide), kelp in commercial diets has also become much more common over the last 2-3 years.
How much iodine is in commonly used seaweed products for pets?
Here are some figures for supplements and foods that are popular for both dogs and cats in Australia and New Zealand:
Simply Seaweed (dried Ascophyllum nodosum):
Manufacturer's figures: Per 100 grams: 20.2 - 100 mg
However, research papers on Ascophyllum nodosum have shown some conflicting results:
102 mg per gram (dried product)
482-712 mcg per gram (dried product)
Tasmanian kelp meal:
There are varied figures quoted online; for example:
180 mcg per 3.6 grams
500 mcg per gram
Seaweed calcium (from Lithothamnium calcareum; 34% calcium)
3.3 mg per 100 grams
Sardines (fresh, including bones):
30 mcg per 100 grams
Whole egg, including white and yolk (no shell):
22 mcg per 100 grams
What can I do?
Given that research in other species has shown that the consequences of excess iodine consumption are not necessarily benign, it is worth being cautious when it comes to seaweed supplements for dogs and cats.
This is especially the case because many of these supplements are intended to be fed daily, for long periods of time - they are not just supplements used for short periods.
So, here are my recommendations:
Don't feed multiple seaweed-derived supplements every day. Be especially careful of supplements made from kelp, oarweed or kombu, as these species are very high in iodine.
If you are using a kelp supplement as an iodine source in your pet's homemade diet, be sure to use a kelp supplement with a standardized amount of iodine. A good option is Now Foods organic kelp powder. Clear labelling of iodine content for seaweed supplements should be essential.
Don't be afraid to ask pet food manufacturers if the kelp in their diets contains a standardized amount of iodine, and if so, how much. It is not good enough for manufacturers to just "add kelp for iodine".
Be careful to not overdose seaweed-derived supplements; keep to the lower dose recommended by the manufacturer, if a range is provided.
For pets with thyroid disease (hypothyroidism or hyperthyroidism), be careful to avoid giving any treats, diets or supplements that contain added seaweed of any kind. This is especially vital for cats on a low iodine diet (for example, Hill's Prescription Diet y/d Thyroid care).
If you have any questions, please don't hesitate to get in touch: info@vngpets.com
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