Introduction

This section contains the percentages of phosphorus, protein, sodium and fat contained in many UK and European cat foods, all of which should be taken into consideration when choosing a food for a CRF cat (see Food Composition and Nutritional Requirements for reasons why). These data are calculated on the basis of dry matter analysis. 

1. What is Dry Matter Analysis?

Cat foods vary in how much moisture they contain, which makes it difficult to compare them to each other. It is very hard, for example, to compare a tinned cat food to a dry cat food because the former naturally contains much more water; and this affects all the percentages of the different nutrients. Dry matter analysis is a way of comparing foods assuming all the moisture content has been removed: this makes it easier to compare different products. Whenever this site mentions levels of the various components of foods, it is talking about them on the basis of dry matter analysis, which is not necessarily the same as what it says on the tin. 

Let's take an example. Let's say:

• you give your cat a food with 80% moisture, a typical level for many tinned foods;

• the food apparently has phosphorus of 0.25%;

• your cat eats 100g of the food.

It therefore appears that your cat is eating 0.25g of phosphorus (100g x 0.25%).

However, the food is 80% water. So of the 100g your cat has just eaten, 80g (80%) of it was simply water, and only 20% was actual food, or dry matter. So the amount of phosphorus is actually higher - in percentage terms - than it first appeared, i.e. your cat has eaten 0.25% divided by (100%-80%) or 1.25% phosphorus. 

Another way of looking at it is to say that your cat food initially had 1.25% phosphorus. Then the manufacturers added 80% water. There is still the same total amount of phosphorus in the food, but at first the percentage appears lower because of the diluting effect of the water. So in order to understand exactly how much phosphorus your cat is eating, you need to discount the water in the food.

Calculating Dry Matter Analysis Yourself

To calculate dry matter yourself, you need to know the amount of moisture in the food and the amount of whatever you are measuring (often this will be phosphorus), and then you need to crunch the numbers a little. This is the formula:

• The dry matter in a food is always 100 - (% moisture in the food). So if, for example, a food contains 76% moisture, that leaves 24% dry matter.

• Next, look at the phosphorus content of the food in percentage terms. Let's say you have a food containing 0.2% of phosphorus on an as fed basis.

• To do the sum, you need to divide the phosphorus content by the dry matter, so in this case you would do 0.2/24%, which gives you 0.833% phosphorus content of this food on a Dry Matter Analysis basis.

The Pet Center has information about calculating dry matter analysis.

The US Food & Drug Administration also explains about dry matter analysis (scroll down to Guaranteed Analysis).

Since many cats like to eat either tinned food or dry food (biscuits), I have divided the dry matter analysis into two sections, tinned food (below) and Dry Food. 

2. Choosing a Food

A CRF cat needs food which is low in phosphorus, ideally under 0.5% according to Dr Scott Brown in Management of feline chronic renal failure (1998) Waltham Focus 8 (3). However, you also need your cat to eat, so if necessary you may have to have a less ambitious goal, at least to start with, of, say, less than 1% phosphorus.

In advanced CRF it can also be beneficial to restrict protein intake, although there is some debate as to whether this is necessary in early CRF.

If your cat is particularly fond of one food, I suggest you check that food in the appropriate table and decide whether:

1. you consider it to have an acceptable level of phosphorus (preferably under 1%; as you can see, the prescription foods tend to have a much lower level, under 0.5%), protein etc.; or

2. it would be worth trying a different food; or

3. to leave your cat on his/her favourite food but also use a phosphorus binder if your vet agrees. 

In advanced cases, you may need to use a phosphorus binder even if you are feeding food with a phosphorus level of under 1%. 

Many CRF cats are finicky eaters who can lead you to have a whole cupboard full of different foods in a desperate attempt to find a food they will eat. There are suggestions on persuading your cat to eat in Persuading Your Cat To Eat, but if all else fails and you cannot persuade your cat to eat low phosphorus foods, the tables should at least enable you to balance foods with low levels of phosphorus with those with higher levels of phosphorus in your cat's diet; this can also be useful if you have a CRF cat and one or more healthy cats or cats with other health problems.

3. Sources of Data

These analyses have been compiled in good faith from information provided by the manufacturers (last updated: July 2003; precise details for individual manufacturers can be found in Food Manufacturers' Contact Details) and are subject to change without notice; no responsibility can be accepted. Several of the manufacturers have asked me to emphasise that their non-prescription foods are not intended for CRF cats.  

4. Equivalent Information for US Foods

Please note that the information provided relates to cat food available in the UK; those in the USA should not use these calculations and should instead visit Kat Karma for analyses of the foods available in the USA.

5. Food Data Tables

Please check the Cat Food Manufacturers' Contact Details page to see when details for a particular food were last obtained.

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