Reading Food Labels

The phosphorus, protein, and other percentages printed on a cat food can or bag are not the numbers you need for a CKD cat. They are calculated on an “as fed” basis, meaning they include all the water in the food, and in the US they are usually minimums or maximums rather than actual values. To compare foods properly, those numbers need to be converted to dry matter analysis (DMA), which strips out moisture and shows you the real concentration of each nutrient. Once you understand DMA, you can judge any food on its own terms, not just the ones on a pre-built list.

If you have spent any time comparing cat foods since your cat’s CKD diagnosis, you have probably noticed something frustrating: the phosphorus number on one can looks low, but a vet or a forum tells you it’s actually high in phosphorus. This is not a contradiction. It’s the result of two foods being measured two different ways, or one food’s label number being a minimum rather than an actual amount.

There are four common methods used to describe food content: guaranteed analysis, typical analysis, metabolizable energy, and dry matter analysis. Only one of these, dry matter analysis, lets you compare a canned food to a dry food to a raw food on equal terms. The other three share the same underlying flaw: they don’t account for how much water dilutes the numbers.

Nearly every cat food sold in the United States lists its nutrient content as Guaranteed Analysis (GA), because this is what AAFCO (the Association of American Feed Control Officials) requires for labeling. GA is given on an as-fed basis, meaning the percentages include moisture.

GA has two problems for a CKD cat owner. First, the only values manufacturers are required to guarantee are minimum protein, minimum fat, maximum fiber, and maximum moisture. Phosphorus is optional, and when it does appear, it’s almost always listed as a minimum, not an actual figure. A label that reads “phosphorus min 0.20%” could mean the food contains close to 0.20%, or it could mean the food contains considerably more. A 2020 evaluation of phosphorus, calcium, and magnesium content in commercial cat foods, published in the Journal of Veterinary Internal Medicine, found that minimum values on packaging cannot be used to reliably estimate actual phosphorus or calcium content, which means the label alone cannot confirm whether a food is genuinely low in phosphorus.

Second, GA does not adjust for moisture. A wet food with 80% moisture and a dry food with 10% moisture cannot be compared side by side using their as-fed percentages, because the wet food’s numbers are diluted by water in a way the dry food’s are not.

Small shifts in the moisture or phosphorus figure can change the picture significantly. A food listed as 80% moisture with a 0.20% minimum phosphorus works out to roughly 1.0% phosphorus on a dry matter basis. If the true figures are 79% moisture and 0.25% phosphorus instead, the dry matter figure comes out closer to 1.19%, around 20% higher. That gap is the difference between a food that looks acceptable and one that isn’t, and it’s invisible if you only read the can.

Most UK and European manufacturers label food using Typical Analysis instead of Guaranteed Analysis, in line with EU feed hygiene regulation. Typical Analysis tends to be more reliable than GA because it reflects actual measured values rather than minimums, so there’s less guesswork about whether the real phosphorus content is higher than what’s printed.

Typical Analysis still shares GA’s other weakness, though: it’s given as fed, with moisture included. That means the same cross-comparison problem applies. A typical analysis figure for a canned food still can’t be compared directly to a typical analysis figure for a dry food without adjusting for water content first.

Some manufacturers, in various countries, describe their foods in terms of metabolizable energy (ME), the calories actually available to the cat after digestion. ME is built around protein, fat, and carbohydrate content rather than mineral content, and it’s often expressed as grams of a nutrient per 1,000 kilocalories rather than as a percentage.

ME has real uses, particularly for managing a diabetic cat’s carbohydrate intake. But because phosphorus control is the priority for a CKD cat, and ME doesn’t translate cleanly into a phosphorus percentage, it isn’t the tool to reach for here.

Dry matter analysis (DMA) solves the moisture problem by assuming all the water has been removed from the food. What’s left is the “dry matter,” and every nutrient is expressed as a percentage of that dry matter rather than as a percentage of the whole, wet food. This is what lets you compare a 78% moisture pate to a 10% moisture kibble on equal footing, and it’s the format leading veterinary references use when setting target nutrient levels for CKD cats.

Here’s the intuition. Say a canned food is 80% moisture and lists 0.25% phosphorus on the label. Your cat eats 100 grams of it. It looks like your cat just ate 0.25 grams of phosphorus. But 80 of those 100 grams were just water, contributing nothing. Only 20 grams was actual food. So that same 0.25% phosphorus, measured against the 20% that’s actually food rather than the 100% that’s food plus water, comes out to 1.25% on a dry matter basis. The total amount of phosphorus in the can didn’t change. What changed is whether you’re measuring it against the whole can or against just the food in it.

This is also why figures on this site, on the FKMB forum, or on any other DMA-based food list will consistently look higher than what’s printed on the can.

You shouldn’t often need to do this yourself, since most published food comparison lists already do this conversion. But if you want to check a specific food, here’s the formula. Say a food is 76% moisture with 0.2% phosphorus listed as fed:

1. Dry matter is always 100 minus the moisture percentage. Here, 100 – 76 = 24% dry matter.
2. Divide the as-fed phosphorus by the dry matter percentage: 0.2 ÷ 24 = 0.833% phosphorus on a dry matter basis.

One caution: US labels typically report phosphorus as a minimum rather than an actual figure, so a DMA calculation built on a GA minimum is still only an estimate, not a guarantee. If a manufacturer can provide an actual analysis rather than a label minimum, that number will give you a more accurate result.

Phosphorus. This is the priority. AAFCO requires a minimum of 0.50% phosphorus on a dry matter basis for any food labeled complete and balanced for adult cats, which means no standard commercial adult food will ever fall below that line. Veterinary nutrition references generally put the target range for CKD management around 0.3 to 0.6% DM, with under 0.5% DM cited as an ideal goal in older veterinary literature. In practice, that means therapeutic kidney diets are formulated below the AAFCO adult minimum specifically for this reason, and if you’re not feeding a therapeutic diet, your realistic goal with a regular food is simply to find the lowest phosphorus option your cat will actually eat, often starting around 0.75 to 1% DM and working down from there.

Protein. The case for restricting protein in CKD cats is debated and depends heavily on disease stage, but many cats feel better with moderate protein, particularly as BUN rises. Most therapeutic kidney diets run 28 to 35% DM protein. When two foods have comparable phosphorus and your cat will eat either, the lower protein option is generally the better pick.

Sodium. CKD cats are prone to high blood pressure, so lower sodium is generally preferable, though the evidence connecting sodium directly to blood pressure outcomes in CKD cats is mixed. AAFCO’s minimum is 0.2% DM, and there’s little reason to go much higher than that.

Fat. Fat doesn’t generate the nitrogenous waste that protein does, so it places less burden on the kidneys, and many therapeutic diets increase fat specifically to compensate for reduced protein. If your cat needs to maintain or gain weight, a higher fat option is usually the better tool, all else being equal.

None of these numbers exist in isolation. A food with great phosphorus and terrible protein, or vice versa, still needs to be weighed against your cat’s actual stage of disease and your vet’s guidance, not picked off a sorted list in isolation.

Rather than maintain a separate phosphorus database on this site, the FKMB forum keeps an actively updated CKD food list thread covering North American, UK/European, and international foods, including starter lists, a main list generally sorted low to high by phosphorus, and prescription renal diets. Because food formulations change without notice, a community-maintained list that gets flagged and corrected when something is discontinued or reformulated is more reliable long term than a static table.

Why does this site’s phosphorus number not match what’s on the can? Because the can shows an as-fed minimum, and DMA figures show actual phosphorus as a percentage of the food with moisture removed. The DMA number will almost always be higher.

Can I compare a wet food and a dry food using the numbers on their labels? Not reliably. As-fed numbers are diluted by however much moisture each food contains, so a wet food and a dry food need to be converted to dry matter analysis before they can be compared.

Is there a food with 0% phosphorus I can feed instead? No. AAFCO requires a minimum of 0.50% phosphorus DM in any food labeled complete and balanced for adult cats, and phosphorus is a necessary nutrient. The goal is the lowest amount your cat will tolerate and eat, not zero.

Should I trust the phosphorus number a manufacturer gives me over the phone or by email? Generally yes, more than the can, since it’s more likely to be an actual figure rather than a label minimum. But ask whether the figure is “as fed” or “dry matter,” since you’ll need to convert it yourself if it’s as fed.

• Summers SC, Stockman J, Larsen JA, Zhang L, Rodriguez AS. Evaluation of phosphorus, calcium and magnesium content in commercially available foods formulated for healthy cats. Journal of Veterinary Internal Medicine. 2020;34(1):266-273.
• Davenport D, Kirpensteijn J (Eds.). Small Animal Clinical Nutrition, 6th Edition, Chapter 37: Chronic Kidney Disease. Mark Morris Institute; 2019.
• Brown SA. Management of feline chronic renal failure. Waltham Focus. 1998;8(3).
• Kirk CA, Jewell DE, Lowry SR. Effects of sodium chloride on selected parameters in cats. Veterinary Therapeutics: Research in Applied Veterinary Medicine. 2006;7(4):333-346.
• Roman G. Building a strong recommendation for a veterinary therapeutic diet. Vetted. 2020;115(1).