Composition vs Analytical Constituents: A Label-Decoding Guide
A pet food label carries two regulated blocks that are constantly confused: the composition (the ingredient list) and the analytical constituents (the US "guaranteed analysis"). The first names what the food is made of, ranked by fresh weight before cooking; the second guarantees levels for a few nutrients, expressed as-fed (Regulation (EC) No 767/2009; AAFCO, 2024). Reading one in place of the other is the source of most label-reading errors. This guide decodes each block, explains what crude ash and crude fibre [fiber] actually measure, and shows how to cross-read the two so neither misleads you.
Last updated :General documentary information. For an individual animal, a veterinarian's advice takes precedence over any online content.
What does the composition block actually describe?
Answer capsule. The composition lists ingredients in descending order of their fresh weight at mixing, before cooking (FEDIAF, 2019; AAFCO, 2024). It tells you the nature and inclusion order of materials, but neither their exact proportion, nor their digestibility, nor their quality. A fresh meat at about 70% water loses two-thirds of its mass on drying, so a high position overstates the real share.
The composition answers one question only: what went into the recipe, and in what rough order by raw weight. That is genuinely useful, because it exposes whether the food leans on animal or plant material and reveals two things a casual glance misses. It shows ingredient splitting, where fractions of a single source, pea protein, pea starch and pea fibre, are listed separately to push each one down the ranking (Tufts Petfoodology, 2021). It also shows the vitamin-and-mineral premix that legitimately sits at the end of a complete food.
What the composition cannot do is quantify nutrition. Two recipes with near-identical wording can perform very differently once digestibility and processing are taken into account. The block names the materials; it does not measure what they deliver. For that, the reader has to turn to the second block, and read it with care, because its numbers carry a hidden variable.
What do the analytical constituents guarantee, and what do they ignore?
Answer capsule. The analytical constituents guarantee bounds for a few nutrients: crude protein and crude fat as minima, crude fibre and crude ash as maxima, and moisture above a threshold (Regulation (EC) No 767/2009; AAFCO, 2024). They are guaranteed limits, not exact values, and they are expressed as-fed, so water dilutes every figure. They ignore amino acids, vitamins, individual minerals and omegas.
The constituents put numbers where the composition stays silent, but the numbers are bounds. "Crude protein minimum 28%" means at least 28%, sometimes more; "crude fibre maximum 4%" sets a ceiling. The real value sits inside a range, which protects the buyer without fixing a precise level. Crucially, none of these figures measures digestibility or the quality of the materials behind them (AAFCO, 2024).
The block is also incomplete by design. It describes neither the amino acid profile, nor the vitamins, nor individual minerals, nor the fatty acids, so a food can meet every guarantee while being poorly formulated (WSAVA, 2021). And because every figure is as-fed, a wet food at 78% moisture shows lower percentages than a kibble even when it is the richer food once water is removed. To compare two products, especially across formats, the levels must first be converted to a dry-matter basis.
What does crude ash really measure on a label?
Answer capsule. Crude ash measures the total mineral fraction, what would remain after the organic matter is burned off (Regulation (EC) No 767/2009; AAFCO). A common kibble range is 5 to 9%. It mainly reflects calcium and phosphorus, tied to the share of bone, animal meals and mineral premix. It is not dirty residue and not a quality defect.
The word "ash" misleads many buyers into picturing impurity, but it is simply the standard term for the minerals left after incineration. On a kibble, the figure largely tracks calcium and phosphorus, so it rises with the proportion of animal material that carries bone. A meaty recipe can therefore show a higher ash figure precisely because it is rich in animal content (FEDIAF, 2019).
The total, on its own, hides what matters most: the balance between minerals. The calcium-to-phosphorus ratio (Ca:P) is more telling than the overall value, especially for the growth of large-breed puppies and for renal function (NRC, 2006). A serious maker can supply detailed calcium and phosphorus levels, which the single ash figure never breaks down. Read ash as a mineral-load indicator, then ask for the ratio.
Is a high crude ash figure a sign of poor quality?
Answer capsule. Not in itself. A figure of 8 to 9% often reflects a high share of animal material with bone, that is, a meaty recipe, not a flaw (FEDIAF, 2019). A very low figure, below 5%, is not a mark of superiority and can signal a low animal share. The useful cue is the calcium-to-phosphorus ratio, not the total.
Reading the ash figure as a quality score is a method error. The source of the minerals matters as much as the quantity: a kibble rich in meat and bone naturally lands around 8 to 9% without any quality problem, while an artificially low figure can simply mean less animal material in the recipe (FEDIAF, 2019). The number alone cannot tell a meaty recipe from an imbalance.
The indicator to watch instead depends on the animal. For a large-breed growing puppy, excess calcium is more concerning than the ash total (NRC, 2006); for an animal at renal risk, phosphorus is the variable that counts. Two kibbles with the same ash figure can carry very different Ca:P ratios, one suited to growth, the other not, and the total alone cannot distinguish them. This is why the detail, supplied on request, says far more than the headline percentage.
What is the crude fibre figure useful for?
Answer capsule. Crude fibre is an old regulatory measure that captures mainly the insoluble, poorly digestible fibre fraction (Regulation (EC) No 767/2009; AAFCO). It underestimates total fibre because it misses soluble fibre, so a food's real fibre content can be markedly higher than the printed figure (NRC, 2006). It is mainly useful for light or sensitive-digestion foods.
The crude fibre method dates from an era of simpler assays and measures the structural, insoluble fraction rather than all the fibre present. That makes it a floor, not an exhaustive measure: the total fibre in a recipe can exceed the printed crude fibre value, which explains apparent gaps between the label and the digestive effect an owner observes (NRC, 2006).
Where the figure earns its place is in specific foods. A higher crude fibre level often accompanies weight-control formulas, where fibre adds volume and satiety without calories, and it matters for transit regularity and sensitive digestion (WSAVA, 2021). For a healthy animal with no particular need, the constituent stays a secondary cue, useful context rather than a deciding factor in the choice.
Which constituents are mandatory and which are optional?
Answer capsule. In the EU and UK, Regulation (EC) No 767/2009 makes crude protein, crude fat, crude fibre and crude ash mandatory, with moisture required above a threshold. Carbohydrate and energy density are optional (FEDIAF, 2019). The US guaranteed analysis requires minimum protein and fat and maximum fibre and moisture (AAFCO, 2024).
The mandatory floor is shorter than many buyers assume: four base constituents, plus conditional moisture. Everything else, the carbohydrate level and the energy density in kilocalories, is voluntary on a European label (FEDIAF, 2019). Certain added vitamins and minerals appear instead under the additives declaration rather than the analytical block.
This distinction matters precisely because the optional figures are often the ones a careful comparison needs. A missing energy density hampers ration calculation; a missing carbohydrate figure forces an estimate by difference. A transparent maker supplies these on request, and their spontaneous presence on the pack is a good signal of openness, without being a legal duty (AAFCO, 2024).
Composition vs constituents: the two blocks side by side
The table contrasts the two regulated blocks on purpose, unit and limit, so neither is read in place of the other.
| Criterion | Composition (ingredient list) | Analytical constituents |
|---|---|---|
| Purpose | Nature of the raw materials | Levels of a few nutrients |
| Unit | Order by fresh weight | Guaranteed percentages, as-fed |
| Framework | FEDIAF / AAFCO labelling | Reg. (EC) 767/2009 / guaranteed analysis |
| What it reveals | Splitting, premix, animal vs plant lean | Protein, fat, fibre, ash bounds |
| Key limit | Neither proportion nor quality | Bounds only, as-fed, no micronutrient detail |
The table makes the division of labour explicit: the composition tells you what; the constituents tell you how much of a little. Each is partial, and the reading only becomes reliable when the two are crossed.
A clear method for cross-reading the two blocks
The recommendation is to treat the two blocks as one combined check, never as rivals. Begin with the composition to establish the nature of the recipe: scan the whole list, sum any split fractions of a single source, and note whether the lean is animal or plant (FEDIAF, 2019; Tufts Petfoodology, 2021). This gives you the qualitative picture the numbers cannot.
Then move to the constituents, and read them correctly. Take protein and fat as minima, fibre and ash as maxima, and convert each to a dry-matter basis before comparing any two products, because the as-fed figures are diluted by water (AAFCO, 2024). Read crude ash as a mineral load and ask for the Ca:P ratio rather than judging the total, and treat crude fibre as a partial measure relevant mainly to light or sensitive-digestion foods (NRC, 2006).
Where the figures that matter most for your animal are absent, carbohydrate, energy density, or detailed calcium and phosphorus, request them from the maker. The composition and the constituents together form a faithful but incomplete picture; the missing pieces are obtainable, and a maker's willingness to supply them is itself part of what you are judging.
---
Keep reading
Related questions: What is the difference between the ingredient list and the analytical constituents? | What does the crude ash figure correspond to? | What is the crude fibre figure useful for?
Glossary: crude ash | crude fibre
Hub: Reading and decoding a label
Sources: Regulation (EC) No 767/2009 on the marketing and use of feed (EUR-Lex; retained in UK law); FEDIAF Code of Good Labelling Practice (2019); AAFCO, Understanding Pet Food (2024); WSAVA Global Nutrition Guidelines (2021); NRC, Nutrient Requirements of Dogs and Cats (2006); Tufts Petfoodology (2021).