Nutrition basics for dogs and cats

Feeding a dog or a cat is one of those daily decisions that quietly shapes the rest of their health. It is also one of the noisiest corners of the pet-care internet, thanks largely to marketing budgets. This guide steps back from the noise and lays out what is actually known, what is reasonable, and what to ignore.

What "complete and balanced" actually means

In the UK and EU, the industry body that publishes nutritional standards for pet food is FEDIAF (the European Pet Food Industry Federation). In the United States, the equivalent role is filled by AAFCO (the Association of American Feed Control Officials). Both publish detailed nutrient profiles for different species and life stages, and both are the reference points that reputable manufacturers formulate against.

When a food's label states that it is "complete and balanced" for a given life stage, it is claiming that a normal daily portion supplies every nutrient the animal is known to require, at levels within the safe range set by the relevant standard. In practice this means an owner does not need to add anything else — no vitamin sprinkles, no supplements, no rotation across brands to "cover the bases." The single food is the meal plan.

What the label does not promise is that the food is high quality in the way most owners intuitively use that phrase. Two foods can both be "complete and balanced" and be very different animals: one formulated by a board-certified veterinary nutritionist and cooked to a stable recipe; another assembled from whatever protein was cheapest that quarter, meeting the minimums on paper and no more. The label is a floor, not a ceiling.

What to look for on the packaging:

• A statement referencing FEDIAF (UK/EU) or AAFCO (US), naming the life stage — for example, "complete and balanced for adult maintenance."
• The species and life stage explicitly named.
• A named manufacturer with a contact address and a customer service line that will answer questions about formulation.

How dogs and cats differ

Dogs and cats are grouped together in every pet-food aisle, but nutritionally they are not the same animal.

Cats are obligate carnivores

Cats have evolved on a diet of small prey and have lost the metabolic flexibility to make some nutrients from plant precursors. They require:

• Taurine, an amino acid found in animal tissue. Cats cannot synthesise enough of it themselves, and deficiency causes serious heart and eye disease.
• Arachidonic acid, an omega-6 fatty acid; cats cannot make it from plant-based linoleic acid.
• Preformed vitamin A; cats cannot efficiently convert beta-carotene from plants.
• A higher proportion of protein in the overall diet than dogs of a similar size.

They also drink less water than dogs relative to their size, having evolved to get most of their moisture from prey. This is the single strongest argument for including wet food in a cat's diet, and something we come back to below.

Dogs are omnivores

Dogs, by contrast, are omnivorous scavengers. They digest starch far better than cats, tolerate a wider range of ingredients, and thrive on diets that include vegetables and grains as well as meat. This is a genuine biological difference, not a marketing framing: it is why dogs can eat a "grain-inclusive" diet without issue and why the current wave of grain-free enthusiasm is not well supported by evidence.

Working out how much to feed

There are two tools worth knowing about, and one to be suspicious of.

The tool to be suspicious of is the feeding guide printed on the packaging. It is a starting point, not an answer. Manufacturers err on the generous side because customers who feed more finish the bag faster, and because "average" pets don't exist — the guide is calibrated for a hypothetical animal that may or may not resemble yours.

The first useful tool is body condition scoring (BCS). WSAVA (the World Small Animal Veterinary Association) publishes free BCS charts for both dogs and cats, using a 9-point scale where 4–5 is ideal. The chart teaches you to look and feel for three things:

1. Ribs: you should be able to feel them under a thin layer of fat, without pressing.
2. Waist: viewed from above, there should be a visible narrowing behind the ribs.
3. Abdominal tuck: viewed from the side, the belly should rise from the ribcage to the hind legs, not sag parallel to the ground.

A pet scoring above 5 is overweight; above 7, obese. Below 4 is underweight. The BCS is more useful than the number on the scale because it accounts for the animal's frame and muscle.

The second useful tool is trend. Weigh your pet, note the score, adjust the portion, wait two to four weeks, and re-check. Change is slow and cumulative; a single weigh-in tells you very little.

A rough map, not a prescription

The table below sketches the categories your vet or the food's feeding guide will use as a reference point. It is deliberately not a gram-per-kilogram table — those numbers vary too much by individual, activity level and food density to publish responsibly. Use it to sanity-check the packaging's guide, then adjust from body condition.

The right column is the point. There is no single number; there are categories, and within each category, an animal.

Wet, dry, raw, fresh

Each format has genuine trade-offs. None is inherently correct.

Dry (kibble) is convenient, energy-dense, cheap per calorie, and stores well. Its main limitation for cats is that it contributes almost no moisture; for dogs, moisture matters less because most drink readily. Claims about kibble cleaning teeth are largely overstated — most kibble shatters on the first bite and does little abrasive work.

Wet (tinned or pouched) is the most reliable way to increase a cat's daily water intake. It is more expensive per calorie and has a shorter shelf life once opened. Many cats prefer it, which matters for older cats whose appetite has narrowed.

Raw diets — whether commercial or home-prepared — are a more contested area. The nutritional case (raw meat is what the ancestor ate) is easily overstated, since ancestral diets included whole prey rather than muscle meat alone. The food-safety case against raw is well documented: the US FDA and UK RSPCA both note the elevated risk of salmonella, listeria and campylobacter contamination for both the pet and the household, and the risk is higher in homes with immunocompromised people, young children, or elderly members. If you feed raw, source from a manufacturer that publishes its pathogen testing and follow strict handling protocols.

Fresh cooked foods, delivered as portioned meals, sit somewhere in between. The good ones are formulated by veterinary nutritionists and cooked at food-safe temperatures; the less good ones are effectively wet food with better marketing. As with any food, the important question is who formulated the recipe and to what standard.

Treats and the 10% rule

The widely cited veterinary rule of thumb, endorsed by WSAVA, is that treats should make up no more than 10% of daily calorie intake. Above that, they begin to unbalance the diet, because treats are almost never formulated to the same nutrient standard as complete meals. If you use training treats heavily, subtract the treat calories from the main meal, or use a portion of the animal's normal food as the reward.

Some human foods are actively unsafe and should never be given: grapes and raisins, onions and garlic, chocolate, xylitol (found in some peanut butters and sugar-free sweets), macadamia nuts, and alcohol are the common ones. When in doubt, look it up or ask a vet before offering.

Life-stage transitions

The two transitions that matter most are into adulthood and into old age.

Puppy and kitten to adult. Growth diets are more energy-dense and higher in some minerals than adult food. Continuing to feed a growth diet after skeletal maturity is a common route into adult obesity. As a rough guide, most small and medium dogs finish growing around 12 months; large and giant breeds around 18–24 months; cats around 12 months. Your vet is the best judge for a specific animal.

Adult to senior. There is no fixed age at which a pet becomes "senior" — it depends on breed and size — but the questions to start asking are: is activity dropping? Is the animal maintaining muscle, or losing it? Are joints stiffening? A senior formulation is usually lower in energy and often higher in specific supportive nutrients (joint support, omega-3s), but there is real variation between products; not every "senior" food does anything meaningfully different.

When to talk to your vet

A short list of nutritional signals that warrant a call rather than a wait:

• Rapid weight loss or gain of more than about 10% of body weight, unplanned.
• Increased thirst or urination that persists for more than a few days.
• A cat that has not eaten in 24 hours. This is not the same threshold as for a dog — anorexic cats can develop hepatic lipidosis quickly.
• Repeated vomiting after meals, or diarrhoea lasting more than 48 hours.
• Sudden change in appetite, thirst, or interest in food in an older animal.

None of these mean disaster. They mean something has changed, and change in an animal that cannot tell you what is different is the strongest signal you have.

Bottom line

Pick a food that is complete and balanced for your pet's species and life stage, formulated by someone qualified to do so, from a manufacturer that will answer questions about how it was made. Feed the amount the animal's body says is right, checked with a body condition score, not the amount the bag suggests. Treat calories are calories. And when something changes — appetite, weight, thirst, energy — treat that as information worth a phone call to your vet, not something to Google harder.

Most of the nutrition arguments online will not matter to your particular animal. Getting the boring things right almost always will.