The first thing to check with a dog who’s struggling to gain weight, is the feeding amount. Some dogs who struggle to reach or maintain a correct body-weight simply need to have their food intake increased to an amount usually served to puppies or more active dogs. However, if the volume of food is already high (and particularly if stool volume is on the high side as well) it may be necessary to look at what types of food are being offered and swap out some for foods that are more calorie dense. Also consider full fat cottage cheese, plain yogurt or an egg cracked into the meal, a few times each week.
Fat and carbohydrates are especially important for weight gain. Consider supplementing with meat such as lamb, which is especially fatty and calorie-dense. If you feed chicken, include the skin, which is the fattiest part. Some grain-free and low carb diets such as Embark cause a pup to become quite trim (just as with low carb diets for humans) so increasing the grain content slightly or adding a vegetable such as sweet potatoes, can help.
Also, consider a veterinary checkup to be sure no underlying health conditions exist. Unexplained weight loss can be a sign of a medical problem that requires veterinary attention. However in most cases, it’s just a matter of establishing the necessary quantity and the right types of food, for the individual animal. If you feed commercial food, a simple change to another recipe may be all that’s needed to meet your pet’s unique nutritional requirements.
Some pets do experience increased stool volume and a slight slimming down when they first change to a new diet, as a natural part of the detox process – especially when transitioning to a higher fiber diet than before. This usually subsides naturally after a few weeks. If it doesn’t, that may be a sign that a different recipe should be tried.