Calories are what's most important but (IMO)I think weight loss can be achieved best with lower calories and lower carbs. High protein does not *cause* kidney problems so I would'nt worry about that unless your cat already has existing kidney problems.
Here is some more info. for you on obesity in cats and carbs:http://home.earthlink.net/~jacm2/id1.html
Timely Topics in Nutrition
The carnivore connection to nutrition in cats
Debra L. Zoran, DVM, PhD, DACVIM
"Obesity in Cats
Although figures vary, it is estimated that 25 to 33% of cats are overweight or substantially obese.38 In fact, obesity is the most common nutritional disorder in dogs and cats in the United States. There are a large number of factors that contribute to this problem, including sex (sexually intact vs neutered; male vs female), age, activity (indoor vs outdoor), and feeding style (meal feeding vs free choice).39 Neutered male and female animals require fewer calories (estimates of 25 to 30%) for maintenance than sexually intact animals.40 It has also been suggested that neutering may increase food intake, especially in male cats, and result in disordered leptin control of body fat mass.c Furthermore, many people prefer to feed their cats dry food that is available free choice. Active cats with a thin body condition that effectively self-regulate their intake may be fed food free choice. However, many inactive cats cannot be fed this way, because they tend to overeat as a result of the increased amount of fat and palatability of commercially available foods. There are a variety of possible explanations for obesity in pet cats, including hormonal changes (eg, neutering), boredom (eg, indoor cats), type of diet (eg, dry CHO-based food), inactivity (eg, decreased energy expenditure), or simple overfeeding. However, although a combination of these factors is likely to be important in the development of obesity, the role of diet in this problem is increasingly being scrutinized. Regardless of the cause, obese cats have many health issues, such as development of diabetes mellitus, joint disturbances or lameness, development of feline lower urinary tract disease, IHL, and nonallergic skin conditions.
One dietary factor that is receiving increased attention in obese cats is the role of CHO-dense diets. Cats housed exclusively indoors and consuming energy-dense, high-starch, dry foods are provided with more energy than they can effectively use. Any dietary CHO not used for energy is converted and stored as fat. Diets that are severely restricted for energy (eg, traditional low-fat, high-fiber, weight-loss diets) may result in weight loss, but it is often to the detriment of lean body mass.41,d Many of these diets contain high concentrations (> 15%) of insoluble fiber, which increases fecal bulk and volume, potentially increases fecal water loss (eg, increase risk of dehydration in cats not consuming an adequate quantity of water), and has detrimental effects on nutrient (eg, protein) digestibility.42,43 Ultimately, successful weight loss requires maintenance of lean body mass, because lean body mass is the major determinant of basal energy metabolism and is a major influence on whether weight is regained.44
Several investigators have evaluated the use of a high-protein, low-CHO diet (protein, 45% or higher; nitrogen free extract [NFE], < 10%; energy, 3,030 kcal of metabolizable energy [ME]/kg of food on an as-fed basis) for weight loss in cats. In 1 study,d weight reduction in cats on a high-protein, low-CHO diet was compared with that for cats fed a commercial hypoenergetic diet (protein, 34%; NFE, 45%; energy, 2,600 kcal of ME/kg of food on an as-fed basis). Cats in both groups lost weight, but cats consuming the high-protein, low-CHO diet maintained lean body mass during weight loss. Additional studies are necessary, but this approach to inducing weight loss in cats makes metabolic and nutritional sense providing that they are fed appropriate amounts of food (ie, food is not available free choice).
Canned foods generally are best to provide a high-protein, low-CHO dietary combination. Most dry foods are energy dense and have greater CHO concentrations (CHO > 25% on a dry-matter [DM] basis), because starch is necessary to make the kibble. The typical nutrient characteristics of canned foods formulated for kittens are 45 to 55% protein (DM basis), 8 to 15% starch (DM basis), and 15 to 25% fat (DM basis) with little dietary fiber (< 1% [DM basis]). These characteristics are not far removed from that of the natural diet of cats (Appendix 1).
One aspect of weight loss that has received a great deal of attention in recent years is the use of carnitine supplementation in an attempt to enhance weight loss. In 1 study,20 investigators revealed that supplemental amounts of carnitine in diets formulated for cats increased lipid metabolism despite an apparent lack of evidence of a carnitine deficiency in those cats. Furthermore, it decreased the amount of time required to achieve safe weight loss in those cats. Oral administration of carnitine (250 mg/d) is recommended for obese cats undergoing weight loss.20,21,23 Underscoring the increased interest in the use of carnitine for weight loss, pet food companies are adding carnitine to their weight-reduction diet formulas, and this should be taken into account when considering the provision of additional amounts of carnitine."