Skin and coat. Why almost every product does the same thing

"For a glossy coat" appears on more packs than any other claim. Behind it sit almost always the same three ingredients, and only one of them can be checked.

Omega-3 carries the category

EPA and DHA from fish or algal oil are the one ingredient here with an amount and a source. Two things about that are uncomfortable:

First, the figure we use for skin and coat comes, in the source, from a row about immune-related complaints rather than from a row marked "glossy coat". The ingredient page says so, because that is the difference between an amount that means something and one that sounds good.

Second, the oils on sale are far apart. Between a salmon oil with a few per cent EPA and DHA and a wild fish oil with thirty, there is a factor of four or five at the same number of spoonfuls. A product can have the same bottle, the same dosing advice, and a fifth of the active ingredient.

Biotin, zinc and vitamin E: declared, but not rated

Biotin, zinc and vitamin E appear on almost every skin-and-coat pack. We show their amounts but do not rate them, and the reason is more technical than it sounds: the requirements for them are stated per 100 g of dietary dry matter, not per kilogram of animal. A figure that refers to the ration cannot sensibly be translated into one per body weight without knowing how much the animal eats and what is already in it.

There is also this: anyone feeding a complete food is already getting all three. A deficiency is then rare, and more does not automatically help more. With zinc in particular, the gap between enough and too much is narrower than for many other nutrients.

A moult is not an emergency, and skin is rarely only skin

Twice a year most animals shed noticeably. That is not a deficiency and not an occasion for a course of supplements. If, on the other hand, skin is reddened, the animal scratches markedly, or bald patches appear, the question is not which oil but what lies behind it. Too many causes are possible for a bottle to hit the right one, and a vet establishes it faster.

Supplements are not medicines and cannot treat any condition.

How to spot a usable oil

• The fatty acid profile is on the bottle, with EPA and DHA separately, in per cent. "Pure salmon oil" on its own is not a figure you can work with.
• The bottle is dark and not too large. Long-chain fatty acids oxidise; a five-litre canister is only cheap if it is used up before it turns rancid.
• Vitamin E is added. It stabilises the oil, and the requirement for it rises with the amount of omega-3.
• The dosing matches the species. Many makers give cats a fixed amount and dogs one by weight. Either is fine, as long as it is on the bottle.