Lots of people believe that inactivity and weight gain follow ovariohyslerectomy in the bitch. These changes, however, are difficult to distinguish from changes due to normal maturation. Recent studies have tried to uncover the truth about these supposed effects of spaying.
One report on gonadectomy (removal of the ovaries or testes) in immature dogs' determined that neutered dogs were actually more active than sexually intact dogs. Gonadectomy was found to neither affect food intake nor weight gain. But another group of researchers, comparing food intake in sexually intact and neutered Beagle bitches fed ad libitum, concluded that neutered bitches ate much more than their sexually intact counterparts.
In yet another study5, activity level and weight fluctuations in spayed bitches were compared to those of intact females. The spayed bitches did not gain weight on a diet of a fixed amount of commercial dog food. No data on the effect of orchiectomy on food intake and weight gain in male dogs, other than the study on immature dogs, has been published to date.
The bottom line is that the available scientific evidence about whether neutering influences weight gain and activity in dogs shows that the question is, as yet, unresolved. In other words, it's too early for breeders to jump to either conclusion and therefore, one can't assume the worst.