Your pet looks fine, but their body might be running short

Why these nutrients fail, and what breaks down when they do

Taurine sits at the top of my list for cats. Here’s the short version of why: cats simply cannot make enough of it themselves. Their bodies keep shunting the building blocks down the wrong chemical path, so very little taurine ever gets produced. On top of that, plant-based ingredients contain no taurine whatsoever, so any cat eating a diet that skimps on meat and fish is already running low before you’ve even thought about cooking losses.

When taurine runs short, two serious things happen. First, the heart muscle starts to fail, studies using cardiac ultrasound have shown that taurine-deficient cats develop a specific form of heart disease called dilated cardiomyopathy, where the heart enlarges and loses its pumping strength. Second, the retina at the back of the eye degrades, which can lead to blindness. The encouraging part is that supplementing with taurine at 500-1,000 mg/day produced clear improvement in 59% of affected cats, and one-year survival rates climbed from 13% to 58%. Taurine also plays a quieter but important role in helping the body digest fats and absorb fat-soluble vitamins, when it’s depleted, that process is compromised too.

Omega-3 fatty acids (EPA and DHA): dogs and cats cannot meaningfully convert the plant-based version of omega-3 (alpha-linolenic acid) into the forms the body actually uses. The conversion rate is simply too low in both species to count on. That means EPA and DHA need to come from fish oil, krill oil, or algae oil, sources that supply them ready-made. It’s also worth knowing the downsides: omega-3 supplementation at high doses can affect how blood clots, upset the stomach, slow wound healing, and, in poor-quality products, bring unwanted exposure to contaminants like mercury.

Vitamin A: cats can’t convert the orange plant pigment beta-carotene into vitamin A at all, they’re missing the gut enzyme that does that job. Both dogs and cats need preformed vitamin A from liver, fish oil, or synthetic sources. Get too little and you’re looking at night blindness and a painfully dry, damaged eye surface.

Vitamin D3: dogs can’t make useful amounts of vitamin D from sunlight the way humans can, they simply don’t have enough of the right precursor in their skin. So they rely entirely on getting vitamin D3 from food. In puppies fed poorly balanced home-cooked diets, deficiency has led to serious bone problems including fractures. But the upper limit matters just as much: too much vitamin D3, as little as 0.1 mg/kg, can cause dangerous calcium build-up in the blood, with potentially fatal damage to multiple organs.

Thiamine (vitamin B1): in cats, a shortage causes neurological signs, wobbly movement, head tremors, depression, and in serious cases, seizures or death. In dogs, it tends to show as loss of appetite, weakness in the hindquarters, seizures, and collapse. Raw fish diets are a particular risk here because raw fish contains an enzyme called thiaminase that actively destroys thiamine.

Zinc: deficiency in dogs shows up on the skin, scaling, crusting, thickened skin, and secondary infections. Alaskan Malamutes and Siberian Huskies are especially prone to a zinc-related skin condition that responds well to supplemental zinc at 2-3 mg/kg/day. Some inherited forms, such as the condition seen in white Bull Terriers, don’t respond to oral zinc at all.

Iron: in dogs and cats, iron-deficiency anaemia is usually caused by ongoing blood loss rather than a simple lack of iron in the diet. Internal parasites, hookworms in particular, are a common culprit. Young animals are most at risk because mother’s milk is very low in iron and newborns have almost no reserves to draw on.