Why is it important?
Cats, unlike dogs or people, are obligate carnivores - this means that they have a biological requirement for a meat diet. It is neither possible nor safe to formulate a vegan or vegetarian diet for a cat! However, just feeding them meat alone isn't sufficient either - they need a properly balanced diet, suited to their specific needs.
OK, let's look at the details:
Like all animals, cats have specific requirements for water, the three major nutrients (protein, fat and carbohydrate), and micronutrients (such as iron, salts and vitamins). Creating a balanced diet can be tricky, because cats are adapted not to eat meat and vegetables (like us), but whole animals - meat, bone and internal organs. Of course, you could probably feed a cat perfectly healthily on a diet of whole mice, but most people are unwilling to try this! So in this guide, we'll look at each of the major nutrient groups, and briefly outline a cat's requirements.
All animals need water - a loss of only 15% of body water is usually fatal. Cats often don't seem to drink much, but that's usually because they are getting all the water they need from their food (wet food, for example, is usually 75% water!). In general, we assume that a cat needs roughly 50mls of water per day per kg, but a healthy intake may be a little lower, especially in cooler weather.
All animals need energy to allow them to move and do things, and also to keep their basic metabolic systems running. Inadequate energy in the diet results in weight loss (as the cat uses its reserves to stay alive) and eventually starvation. Cats can get energy from protein, fat or carbohydrate, and usually need between 65 and 70 kcal (calories) per kg of body weight. However, most diets actually have too much energy in them, which is why so many cats are overweight or obese! The energy requirement will also vary according to a number of factors, such as age (growing kittens need more, old cats usually less), activity (the more they do, the more energy they need), gender (male animals usually need more than females, and neutered animals need less than entire ones). Pregnancy and lactation (milk production) also mean higher energy requirements. Overall, the best solution is usually to work out the rough amount the cat needs, feed them, and then adjust it depending on whether they are gaining or losing weight.
Cats require much more protein in their diet than dogs, or humans - an adult cat's daily diet should usually include at least 26% protein. However, the total amount of protein isn't the only factor you need to take into account - you also need to note the protein quality. While dogs and humans can make some amino acids (the building blocks of proteins) relatively easily from other (which is why although there are 23 amino acids, only 11 are deemed "essential"), cats find this much harder. They require much higher levels of arginine, tyrosine and methionine than we do, for example, and unlike us cannot manufacture taurine at all. These amino acids are found predominantly (and in the case of taurine, exclusively) in animals - not in plants.
Fat provides energy, but also certain vitamins (the "fat soluble" vitamins, A, D, E and K). In addition, it's the part of the diet that has the most impact on palatability and tastiness - and we all know how fussy cats can be! In general, a diet that is too low in fat will be rejected, unless extra carbohydrates are used to bulk it out and increase flavour.
Cats do not actually need sugars and starches in their diet (there's very little sugar in a mouse!) - we add them to incorporate a cheap and easy source of energy. That said, there's nothing wrong with feeding carbs, as long as you remember that all they're doing is providing calories. However, adult cats cannot digest lactose (milk sugar) and should NOT be fed milk - it can give them diarrhoea. Likewise, unlike humans, cats do not need much (if any) dietary fibre, but about 5% is probably beneficial.
(6) Minerals and Vitamins
A cat's mineral requirements (for iron, phosphorus etc.) are similar to ours. However, in keeping with their strict meat-only lifestyle, cats require certain vitamins that are only found in animals, such as A, D and B3. These need to be in exactly the right ratio though, as an excess can cause disease. Growing kittens in particular need large amounts of calcium to form their bones - this is not found in meat alone, which is why it needs to be supplemented in the diet unless you're feeding whole animals.
Does the form matter?
The formulation of the diet is almost as important as its composition; in general, we can feed wetter diets or drier ones. There are advantages and disadvantages each way. A dry diet is better for their teeth (as it tends to scrape them clean, reducing dental problems compared with wet foods) but a wet food provides more water and is usually tastier, which is better in cats who are prone to bladder stones, or those with fussy appetites.
Do all cats have these requirements?
Of course, cats are individuals, and at certain times in their life, they may require a different balance of nutrients. For example, a pregnant cat or growing kitten requires more energy, and more protein, than a healthy adult. Likewise, a cat whose kittens are still drinking her milk benefits from more carbohydrate, to make the milk sugars. You also need to factor in any disease conditions - cats with kidney disease, for example, need a very different diet to healthy cats. Overall, however, these guidelines are pretty accurate for 90% of cats, 90% of the time.
The simplest and easiest way to feed a cat a fully balanced and healthy diet is to feed a reputable commercial diet (wet or dry). However, if you want to make up a homemade diet, that's fine - but we STRONGLY advise that you get advice from a fully qualified feline nutritionist. Talk to your vet, and they'll be able to direct you to someone!