The Purina site says that the reason is a change in metabolism and "activity levels normally decline with maturity". I don't understand the connection in the second reason. As to the first reason, "metabolism" is defined as "the set of chemical reactions that occur in living organisms in order to maintain life" (Wikipedia). Another website endorses this reason saying that the cat's metabolic rate deceases by about 20%. Obviously food intake needs to be reduced accordingly. A vet's site in the UK says that there is also an increase in appetite so reducing intake of food is going to be hard.
Another site says that neutering causes a hormonal change. So the change is metabolic and appetite caused by an altered hormonal balance. Wikipedia says, "[they] have an increased risk of obesity. Theories for this include reduced metabolism, reduced activity, and eating more due to altered feeding behavior". In short we don't know exactly what is going on.
I would like to know what is actually happening inside the body? Well, I found a proper scientific paper on the subject. These are some summarized notes from this paper (http://jn.nutrition.org/cgi/content/full/132/6/1730S):
- It has been shown that cat neutering significantly increases body weight after neutering.
- Body weight is dependent of energy intake versus energy expenditure
- Neutering decreases the normal concentrations of gonadal (an ovary or testis) hormones altering the regulation of body fat mass.
- Food intake immediately increases significantly after neutering leading to body weights greater than in intact cats by 3 weeks after neutering.
- The weight gain, due to increased body fat mass, continued and then leveled off when it was 28% higher than in intact cats.
- Food intake increase is sufficient to be the cause of weight gain.
- "Circulating leptin and insulin concentrations indicate that the food-intake response is not suppressed by the negative feedback normally imposed by the hormones." - this is quoted for accuracy. In return for this breach of copyright a link to the paper is made here:
Cat Neutering Increases Body Weight -- Photo Feral cats undergoing TNR - by Feral Indeed! and published under a Attribution-NonCommercial-NoDerivs creative commons License -- this site is for charitable purposes in funding cat rescue.