Why do a dog's nutritional needs change after spaying or neutering?
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After spaying or neutering, a dog's nutritional needs decrease because two processes work together: resting metabolism drops by roughly 20 to 30%, while appetite tends to increase. Weight gain that follows is not automatic; it reflects a failure to adjust the diet in the weeks after surgery.
A dual hormonal mechanism
Castration or ovariectomy stops the body from producing sex hormones (testosterone, oestrogen), which played a role in regulating energy expenditure and eating behaviour. Without these hormones, the dog's basal metabolism slows down: veterinary studies estimate this drop at around 20 to 30% compared to an intact dog of the same weight. At the same time, the loss of oestrogen removes a natural brake on food intake: the spayed or neutered dog naturally tends to eat more, because the brain receives weaker signals of fullness.
The problem is not the surgery itself, but the imbalance that develops if the diet stays the same as before: the dog consumes the same amount, or even more, while the body burns fewer calories. This imbalance develops gradually, often without the owner noticing, as weight gain is slow and spreads over several months.
An adjustment to make in the weeks following surgery
The critical period lies in the weeks after the procedure: this is when the diet needs to be reduced, usually by around 20 to 30%, or replaced with a food lower in energy but higher in protein to preserve muscle mass. Weighing the dog every two to four weeks during the first three months allows any drift to be caught quickly and the amount fed to be adjusted before excess weight becomes established. This regular monitoring, combined with maintaining physical activity, remains the most reliable way to manage this metabolic change without spaying or neutering leading to weight gain.
