A peer-reviewed study analyzing 41 commercial dog foods in the United Kingdom found that cooking temperature — not price or ingredient quality — is the primary driver of advanced glycation end (AGE) product formation across pet food formats.
The study, published open access in the Journal of Animal Physiology and Animal Nutrition, was conducted by Dr. Tolgahan Kocadağlı and Professor Vural Gökmen of the Food Quality and Safety Research Group and co-authored by Dr. Ciara Clarke of Butternut Box. Researchers measured five key Maillard reaction compounds across fresh, kibble, wet/canned and freeze-dried products, all purchased in the UK in 2024.
Study findings
Wet and canned foods showed the highest AGE levels across all formats tested. The AGE compound CEL (Nε-carboxyethyllysine) was recorded in wet/canned foods at more than four times the level found in fresh food. On a protein basis, wet samples also showed significantly greater CML (Nε-carboxymethyllysine) per gram of protein than fresh food, indicating greater heat damage to protein during processing.
The findings map to the three stages of the Maillard reaction — the chemical process between amino acids and reducing sugars that produces browned food's characteristic flavor, aroma and color. Fresh food, cooked at approximately 90°C, remained at the early stage of the reaction. Kibble, typically processed at 120°C to 200°C, showed intermediate and late-stage markers. Wet and canned products, which undergo retort sterilization, showed the highest late-stage AGE concentrations.
The study also found 53% greater lysine blockage in kibble compared to fresh food. Lysine, an essential amino acid, becomes chemically bound to sugars during early-stage Maillard reactions, reducing its bioavailability. Kibble showed 4.9% lysine blockage compared to 3.2% in fresh food — a gap the researchers note is compounded by kibble's lower baseline lysine content of 33 grams per kilogram less than fresh formats.
No correlation was found between product price and Maillard reaction product levels across any of the 41 products tested, including economy, premium and veterinary/prescription diets.
The research is also the first published scientific study on commercial dog food to report glyoxal and methylglyoxal levels, which act as precursors to AGEs.
Should labeling reflect cooking temperatures?
Clarke, in-house vet at Butternut Box and co-author of the study, said current labeling practices do not reflect thermal processing conditions.
"A kibble label might declare 25% protein, but this study shows that nearly 5% of the lysine in protein can be chemically blocked and unavailable to the dog, a function of both how kibble is processed and what goes into it," Clarke said. "For patients with elevated protein requirements, that gap matters. Current labelling tells us what goes into the food, not what the animal can actually use. The processing method is a clinical variable, and it belongs in the conversation."
Butternut Box is calling on pet food manufacturers to label products with maximum cooking temperatures and to incorporate process-related considerations alongside ingredient lists in product communications.
Butternut Box is a UK-based, direct-to-consumer subscription service providing fresh, personalized dog food made from human-quality meat and vegetables.
