A new study published in October 2026 in the Journal of Animal Physiology and Animal Nutrition examined how weight loss affects body composition, energy expenditure, and protein metabolism in older dogs.

The researchers followed 20 client-owned obese dogs, averaging 8 years of age, through a supervised weight-loss program. All dogs were otherwise healthy and received the same specially formulated, high-protein, high-fiber diet.

Calories were initially restricted by approximately 30–40%, with feeding amounts reassessed every two weeks. The researchers measured changes after approximately 10% and 20% weight loss.

How Much Fat and Lean Tissue Did the Dogs Lose?

The dogs started with an average body fat percentage of 40.6%, which decreased to 31.1% by the end of the program.

Their absolute lean mass also decreased, from an average of 19.81 kg to 18.31 kg. Most of that reduction occurred during the first phase of weight loss.

Interestingly, the composition of the weight lost differed considerably between the two phases. During the first approximately 10% weight loss, 69% of the weight lost was fat and 31% was lean tissue. During the second phase, those proportions shifted to 91% fat and just 9% lean tissue.

However, because individual dogs responded differently, the difference in absolute lean-mass loss between phases was not statistically significant.

Lean tissue includes water and other nonfat tissues, so these measurements can’t tell us exactly how much skeletal muscle was lost.

For senior dogs, preserving muscle is particularly important. Age-related muscle loss can already affect strength and mobility, and a weight-loss program should account for that risk.

Weight Loss Slowed, but Energy Expenditure Did Not Decline

The dogs lost an average of 0.85% of their body weight per week during the first phase, slowing to 0.70% during the second. This happened even though their calorie allowances were reduced further.

Interestingly, the researchers did not find the expected decline in energy expenditure. When adjusted for metabolic body weight, energy expenditure actually increased. When adjusted for lean mass, it remained essentially unchanged.

Both protein synthesis and protein breakdown decreased during weight loss, although their relative balance remained stable. Voluntary physical activity did not change significantly.

These observations suggest that changes in body composition and protein metabolism deserve consideration when evaluating how older dogs respond to prolonged calorie restriction.

Nutritional Adequacy Was Built Into the Diet

An important detail is how the researchers formulated the experimental food.

Rather than simply reducing portions of a maintenance diet, they adjusted nutrient concentrations to account for the lower calorie intake. Protein and essential amino acid intake remained at or above the FEDIAF recommendations used in the study.

The diet supplied 10.4 grams of protein per 100 kcal, and the researchers monitored food intake throughout the program.

A weight-loss diet has to provide fewer calories without simply providing proportionately less nutrition.

I’ve discussed the potential problems with excessive portion reduction in my earlier article, Even Modest Weight Loss Can Help Your Dog.

What makes this new research particularly relevant is that the investigators deliberately accounted for nutrient adequacy while studying changes in fat and lean tissue.

There are limitations. Only 20 dogs completed the study, and there was no comparison group receiving a different diet. We therefore cannot conclude that this particular protein concentration was optimal or responsible for the relative preservation of lean tissue during the later phase. The dogs were also free of other diagnosed health conditions, which limits how broadly the findings can be applied.

Still, the results provide useful information for managing weight loss in older dogs. Regular assessment of body condition, muscle condition, calorie requirements, and nutrient intake remains important, particularly as the rate and composition of weight loss change.

Research Reference

Freire, T. G. B., et al. (2026). Metabolic Adaptations to Weight Loss in Obese Senior Dogs: Effects on Energy Expenditure, Protein Turnover, and Voluntary Physical Activity. Journal of Animal Physiology and Animal Nutrition.

Read the full open-access study.

Funding disclosure: The authors declared no conflicts of interest. The research laboratory acknowledged financial support from BRF Ingredients, BRF Pet Food, and Adimax Pet. CAPES provided scholarship and publication support.