The Light and Sterilized diets have the same objective, to avoid excess weight, but they do so from two different approaches. 

In the first place, it is convenient to clarify that sterilization and being overweight do not always go hand in hand; castration seems to influence the behaviour of the animal more than its metabolism: the animals become calmer and move less, so their energy needs decrease. In this context, an animal only becomes overweight if it is fed more calories than it uses. 

Currently, it is estimated that about 40% of dogs and cats are overweight or obese (Lund et al., 2005; McGreevy et al., 2005). This high prevalence represents an important health problem since obesity has been related to a multitude of pathologies, particularly diabetes and lower urinary tract disease in cats and osteoarthritis and hip dysplasia in dogs (Laflamme, 2006). To find out if your dog or cat is overweight or obese, use body condition assessment questionnaires, in addition to regular weight checks. The assessment questionnaire lists a number of items that must be checked in the animal in order to estimate its body condition, taking into account the possible variations due to the racial pattern of each individual. 

Thus, the main difference between Light and Sterilized diets is that Light diets are intended to address an established overweight or obesity situation, in contrast to Sterilized diets. Opt for a Light diet when you want to reduce the weight of the animal or when it has a great tendency to be overweight. Use a Sterilized diet when you want to moderately reduce caloric intake to prevent a future situation of excess weight. 

When sterilizing a young animal, should you continue with a Junior diet or move on to a Sterilized diet?
Today, it is considered safe to castrate male dogs and cats of both sexes from 2 months of age, while for female dogs, a delay until 3-4 months of age is recommended (Howe, 2015). 
When an animal is castrated at a very young age, you may have concerns about whether to continue using a specific diet for young animals or to switch to a diet for sterilized animals. In this context, it is advisable to continue using the Junior diet until the animal reaches 90% of its adult weight. Although the animal is already neutered, it is still in the growth phase, so its energy and nutrient needs per unit of body weight will be higher than those of a sterilized adult animal.