Guaranteed Analysis: Protein, Ash, and the NFE Formula

The guaranteed analysis is a legal disclosure, not a nutritional profile. Every number on it is a minimum or a maximum that the manufacturer promises to meet, batch after batch, and crude protein means nitrogen times 6.25 rather than usable protein. Read it as a floor and a ceiling, convert both to dry matter, and most premium claims lose their force in under 5 minutes.

Four numbers do the work: protein, fat, ash and moisture. Subtract all of them, plus fiber, from 100 and you get NFE, the carbohydrate estimate that most labels never print.

What each line legally means

Crude protein is a minimum, usually 26 percent for adult maintenance and 30 percent for growth. Crude fat is a minimum, commonly 9 percent. Crude fiber and moisture are maximums, and ash is a maximum when it appears at all. A bag printing 34 percent protein is promising at least 34, and independent testing of that batch often lands 2 to 5 points higher.

Moisture is the hinge. Dry food sits at 6 to 12 percent, canned food at 75 to 84, and every other percentage is diluted by the water in front of it. That is why the same food reads 34 percent protein dry and 10.5 percent in a can.

Converting to dry matter, step by step

Dry matter percentage equals 100 minus moisture. To move any nutrient onto that basis, divide its as-fed percentage by the dry matter fraction and multiply by 100. A can at 78 percent moisture has 22 percent dry matter, so 10.5 percent protein as fed becomes 47.7 percent protein DM; a dry food at 10 percent moisture has 90 percent dry matter, so 34 percent protein becomes 37.8 percent DM. The can is the higher protein food, and on an as-fed reading it looked like a third of the dry food.

Ash is the number that is usually missing

Ash is what remains after combustion: calcium, phosphorus, magnesium, sodium and trace minerals. It is not an ingredient and it cannot be zero. Dry food typically runs 5 to 8 percent ash DM, canned food 6 to 10 percent DM, and a bone-heavy formula reaches 12 to 14 percent. AAFCO nutrient profiles do not set a maximum ash for adult cats, so a manufacturer can omit it, and many do.

Omitting ash breaks the arithmetic downstream. NFE is a subtraction, so a missing 8 percent ash does not vanish; it reappears inside the carbohydrate number. That is how a food with 30 percent carbohydrate DM can print an NFE near 22 and look lower in carbs than it is.

The NFE formula, with the check built in

Nitrogen-free extract is the carbohydrate fraction estimated by difference: NFE = 100 − moisture − crude protein − crude fat − crude fiber − ash, all in the same units, usually as fed. When fiber is not printed, the shorter version NFE = 100 − moisture − protein − fat − ash gives an upper bound, because crude fiber is part of the carbohydrate fraction and gets counted twice.

Worked example, wet food: 100 − 78 moisture − 10.5 protein − 5.5 fat − 0.5 fiber − 2.0 ash = 3.5 percent NFE as fed, which is 15.9 percent NFE on dry matter. The 5 numbers sum to 96.5, and 3.5 is what is left; if your column does not add to 100 after conversion, one input is wrong.

Nutrient	Dry, as fed %	Dry, % DM	Canned, as fed %	Canned, % DM	Canned, g per 100 kcal
Protein	34.0	37.8	10.5	47.7	11.0
Fat	16.0	17.8	5.5	25.0	5.7
Crude fiber	3.0	3.3	0.5	2.3	0.5
Ash	7.0	7.8	2.0	9.1	2.1
NFE	30.0	33.3	3.5	15.9	3.7
Moisture	10.0	0	78.0	0	0
Energy, kcal per 100 g	360	400	95.8	435	100

The energy row uses modified Atwater factors of 3.5 kcal per g for protein and carbohydrate and 8.5 for fat: 34 × 3.5 + 16 × 8.5 + 30 × 3.5 = 360 kcal per 100 g for the dry food, and 10.5 × 3.5 + 5.5 × 8.5 + 3.5 × 3.5 = 95.8 for the can. Those are estimates, and a manufacturer's measured metabolizable energy can sit 5 to 10 percent below them.

Trap 1: comparing as-fed numbers across formats

This is the most common error and the easiest to catch. A 34 versus 10.5 protein comparison makes the dry food look 3 times stronger; on dry matter it is 37.8 against 47.7, and the can wins. The same flip happens with fat, where the can reads 25 percent DM against 17.8, and with ash, where 9.1 percent DM against 7.8 looks worse than it is once you account for the water.

The fix is mechanical: never compare two foods without writing both columns. If a label or a sales page quotes a single percentage with no basis, assume it is as fed and convert before you believe the ranking.

Trap 2: reading minimums as measured values

A guaranteed minimum is a promise with a margin. Formulators target 34 to 40 percent protein to guarantee 34, so two bags both printing 34 can differ by 6 points inside the bag. The same applies to fat and to fiber maximums, which are often met by adding cellulose or beet pulp rather than by controlling the ingredient mix.

Crude protein also counts non-protein nitrogen, so a food boosted with added amino acids can hit its guarantee with less intact protein than the number implies. That is legal and usually harmless, but it means the guaranteed analysis cannot tell you protein quality, only quantity.

Trap 3: NFE as a marketing number

Because NFE is a subtraction, it absorbs every gap and every rounding: missing ash, unprinted fiber, and the difference between a minimum and an actual. A grain-free food can carry 30 percent NFE DM and be higher in carbohydrate than a food containing rice, since potato, pea and tapioca starch are starches too.

Use NFE as a range, not a verdict. Adult cats tolerate 10 to 30 percent carbohydrate DM without a documented problem, and the number that matters more for a cat with diabetes or obesity is the energy density: 360 kcal per 100 g is a different feeding problem from 430.

The 4-number worksheet

Write down moisture, protein, fat and ash from the label, then fill fiber if printed. Compute dry matter, divide each nutrient by it, subtract, and check the column sums to 100 ± 1. Then compute energy and compare foods per 100 kcal rather than per 100 g, because that is the unit a cat actually eats in.

Two things the worksheet cannot tell you: whether the food is complete, which is the AAFCO statement's job, and whether the cat will eat it. The label reading method and the statement check are separate skills, and both are covered in label reading and AAFCO statements.

When to stop reading labels

Vomiting more than 2 times in 24 hours, or any blood in stool, is a vet call regardless of what the bag says.
A cat that refuses food for 24 hours needs a clinical eye more than a better formula, especially if overweight.
Weight loss of 5 percent in 2 weeks, or a cat drinking more than 100 ml per kg per day, points to disease rather than diet.
Anything the cat ate off the floor goes to the toxicity checker before the ingredient list.

Terms mentioned in this article

DM AAFCO

Frequently asked

What does NFE mean on a cat food label?

NFE is nitrogen-free extract, a carbohydrate estimate obtained by subtraction: 100 minus moisture, protein, fat, fiber and ash. It is not printed on most labels, and because it is a difference it absorbs any missing value, including ash the manufacturer chose to omit.

Why is canned food 10 percent protein and dry food 34?

Because moisture dilutes every other number. A can at 78 percent moisture has 22 percent dry matter, so 10.5 percent protein as fed is 47.7 percent on dry matter, higher than the 37.8 percent in a dry food printing 34. Always convert before comparing formats.

Is a low ash food better for my cat?

Lower ash means a lower mineral load, which helps cats over 10 years old or with rising kidney values where phosphorus should stay under 1.2 percent DM. Ash is not a toxin, though, and a food with no ash figure printed cannot be checked at all, so ask the manufacturer for it.