Daily Calorie Estimates
“The maximum lifespan of the domestic dog is estimated to be about 27 years” …”However, very few dogs live beyond 18 years” 
“Lean dogs live, on average, 1.8 years longer than obese dogs.”  
Lean dogs have longer lives
Median life span was significantly longer for dogs in which food was restricted. 
Lean dogs have healthier lives
delayed onset of chronic diseases
“diet restriction also was associated with a longer median time to first treatment of osteoarthritis (the most common chronic disease among dogs in this study) and a longer median time to first treatment of any chronic condition.” 
Overweight dogs and cats have an increased risk for urinary stone formations. 
“Weight gain could indicate medical conditions such as hypothyroidism and Cushing’s disease, or that you are overfeeding him.” 
Where to start?
“Before placing your dog on a diet, be sure your veterinarian examines him for an underlying medical condition that could be causing his weight gain, such as hypothyroidism (as we discussed in The Canine Thyroid Epidemic) or Cushing’s disease. Failing to do this can result in unsuccessful weight loss for your dog and a lot of frustration for you. Once your dog has received health clearance, your veterinarian will determine his ideal weight and will serve as an important resource for periodic health checks, weigh-ins and motivational support along the way.” 
Successful weight loss requires exercise, calorie control, and the right balance of protein, fat, and carbs.
How many calories a day does a dog need? A good starting point to establish your dog’s calories needs is to determine the total calories they are currently getting each day in the form of food and treats. Then determine their body condition score (BCS) and muscle condition score (MCS) to evaluate whether your dog needs the same (maintenance), or more or less calories each day. You should work with your veterinarian to determine your dog’s BCS and MCS and his/her ideal weight.
The Canine Daily Calorie Estimator will do all the math to help you determine daily calories needs. Calorie requirements can vary greatly depending on the dog’s breed, size, age, metabolism, activity level, environment, temperature, and humidity. Two dogs of the same breed and size living in the same environment can have completely differing energy (calorie) requirements. (See MER and DER below) “Individual pet needs can vary by as much as 50% from calculated values however, so these are only starting points for estimating the amount of food to be provided daily.” 
“Calculate the resting energy requirement (RER) using the pet’s estimated ideal weight, then feed a percentage of that amount. Although there is no established standard reduction, feeding 80% of ideal-weight RER is effective and well tolerated.”
“High-protein diets are recommended during the weight loss period.” 
“Animals have well defined amino acid requirements; providing additional protein can prevent possible deficiencies of taurine or other amino acids. In addition, high-protein diets preserve lean body mass during weight loss.” 
“Protein restriction for healthy older dogs is not only unnecessary, it can be detrimental. Protein requirements actually increase by about 50% in older dogs, while their energy requirements tend to decrease. When insufficient protein is provided, it can aggravate the age-associated loss of lean body mass and may contribute to earlier mortality. Older dogs should receive at least 25% of their calories from protein, typically provided by diets containing at least 7g protein/100 Kcal ME.“ 
“Dietary protein: If your dog is overweight, you want her to lose fat while preserving muscle mass. Protein is critical to the maintenance and growth of skeletal muscle, which is key for mobility. Studies have found that increased dietary protein may help preserve muscle tissue in dogs on weight-loss plans. "Protein also requires more energy to break down in the dog's body than fat or carbs, so protein calories may be more beneficial for weight loss too," says Shmalberg. As for how much protein, clinical studies suggest that 75 grams per 1,000 calories of food is the minimal amount needed to preserve lean body mass—but even more may be beneficial.” https://www.nomnomnow.com/obese-dog-weight-loss
“More essential vitamins, minerals, and fatty acids: The whole purpose of a weight-loss dog food is that you can restrict the number of calories. But, you don't want to restrict the essential nutrients your pet needs. A food with elevated amounts of vitamins, minerals, and fatty acids ensures your pet gets what he needs even when he's eating less overall.” https://www.nomnomnow.com/obese-dog-weight-loss
"Calorie restriction—but not protein restriction should be pursued in overweight animals to achieve a slow rate of weight loss of 0.5% to 1% weekly." ... "Diets high in protein appear superior to those with moderate amounts of protein"
“foods producing low glycemic responses should be recommended for obese or diabetic patients” 
“Carbohydrate sources that result in a lower glycemic index are more desirable for metabolic weight control.“ 
“Limiting dietary carbohydrate is an important component of metabolic control for weight loss. There are three key advantages to limiting dietary carbohydrate to 20% (DM) or less: 1) lower glycemic index, 2) metabolic shift from energy storage to energy usage and 3) increased satiety.” … “Increasing the dietary protein level during weight loss spares lean body mass.” 
In 2018, an estimated 60% of cats and 56% of dogs in the United States were overweight or obese.
Pet Obesity Remains a Serious Health Threat, Pet Owners and Veterinary Professionals Confront Conflicting Nutritional and Weight Loss Advice - https://petobesityprevention.org/
Are you feeding a diet high in carbohydrates? Pet Food Math Cheat Sheet
One challenge when searching for dog food for a dog with a moderate activity level is finding one that has a percentage of fat that is no more than 50% of the protein listed. Higher fat diets are a great energy source for active dogs but most dogs will get more calories than they can burn on a high fat diet. For dogs that are overweight I would avoid foods marketed for the “less active” dogs. If you checked the label you may find that this food has the same calories or more than the food you are feeding now.
Many of these foods contain a high fiber content which can reduce the overall digestibility and insoluble fiber like cellulose can reduce mineral adsorption. High fiber can lead to increased need to defecate and a larger volume of feces. Instead I would limit treats and slightly reduce the amount of food feed, and increase their daily exercise. There’s an old saying, if your dog is fat you’re not getting enough exercise!
Using the gold standard pet food formulator Steve Brown recommends carbohydrates would be 6% of calories. (Dry kibble ranges from 20% to 60%+) The ideal remaining calorie breakdown would be 49% of the calories would come from protein, and 44% from fat. Remember that the percentage of calories is not the same as the "guaranteed analysis" %. We will show you how to determine the percentage of calories later with the Pet Food Math Cheat Sheet.
“Median life span was significantly longer for dogs in which food was restricted. The onset of clinical signs of chronic disease generally was delayed for food-restricted dogs.”
RER in kcal / day = 70 X (ideal BW kg [kg] 0.75
 Dodds, W. Jean. CANINE NUTRIGENOMICS: THE NEW SCIENCE OF FEEDING YOUR DOG FOR OPTIMUM HEALTH Dogwise Publishing