CUT THE FAT.  Fat always has more calories than carbs.  and no matter how you slice it - ultimately, calories are what causes weight gain.  This is a huge misconception - that somehow carbohydrates are "most fattening".  No, fat is "most fattening"

Hello Cynic, Miss Me?

(there is a human nutritionist out there somewhere - care to comment?)

Well, since we aren't feeding humans here - I think a Veterinary Nutritionist would be a much better source (but they don't have a lot of Nutrition Classes in Vet School, do they!)  YOU didn't download the articles from the NRC - shame on you.

Protein and Carbs are a wash - you can sub one for the other, and not really impact the energy level of the diet.  BUT - Research in pets as well as humans shows that keeping protein levels higher during weight loss keeps muscle mass higher.  This in turn keeps metabolic rate higher, and helps to keep the weight off. 

Nope.  Cats/Dogs have different digestive systems (much shorter) than humans (and also different than Cows/Pigs - I'm sure you are aware of this).  So Cats do not digest carbs the same as Humans.  Ergo - the carbs are stored as fat (they don't even convert to proper energy).  Now Fat, on the other hand, increases the palatability of the Diet, aids in Digestive Enzymes - better simalating the Protien, and is also converted to Energy (while those pesky carbs are still sorta inert gathering as fat cells - HHHHMMM - just exactly how does that happen?  Download those nifty little pamphlets - better yet, buy the over $260 book).

Remember - if you do quick and dirty "proximate" analyses - a diet is only made up of five main things. 

Protein = 4 kcal per gram
Carbs = 4 kcal per gram
Fat = 9 kcal per gram
Mineral = 0 kcal per gram
Fibre = Low to 0 kcal per gram
Moisture = 0 kcal per gram.

So, if you want to do fast eyeball comparisons, if moisture is the same, fibre is similar, and we would hope mineral (ash) would be less than 10 - do the math - diet B, at 16% fat, is going to contain more energy per kilogram. 

The particle size may be different - bigger pieces mean that fewer fit in a cup - but the bottom line is that I have a lot of trouble believing, based on these numbers, that product B is lower in energy on a gram per gram basis - which means if you are an average pet owner, you are more likely to overfeed this diet, and have weight GAIN, not loss, as a result. 

Unfortunately - there you go confusing carbs interchangable with protein.  So you can't gram for gram here.  Its overfeeding the GWAINY CWAP (which the animals do because they are STILL HUNGRY) - that causes them to Gain Weight.  The little, itty bitty book will tell you that.

You should be able to find a diet out there that has MORE protein, lower fat.  (watch it, lots of the "grain free" or low carb products make up the difference with fat - subbing carbs with fat is a dangerous game when it comes to weight loss, diet is more palatable and higher in energy - think cheesecake, and how much it would help you to meet your diet goals..)

Actually, Grain Free, low carb is Great!  However, Indoor Kitties need some Higher Fiber, which is gained from carbs like fruits and Veggies (very low doses).  For Dry Food - the Dry Matter Basis of Carbs should not be over 38% to 40%.  For Wet Food,  Carbs should not be over 18% on a dry matter basis.  Reduced Fat Diets can have up to 50% or more carbs (DMB) in Dry Food.

Diet B is nothing special, and unless you are very careful with your feeding rate, I doubt you will have weight loss success.  You should be able to find something HIGHER in protein, LOWER in fat, and higher in fibre (which has low/no calories, but fills cat up).  Ideally find a High protien, low fat canned diet with an insoluble fiber source (something like cellulose) to give a feeling of fullness - canned diet also adds water - to provide gut fill. 

Also wondering about B - if proximate analysis is prot+fat+carb+moist+fibre+mineral(ash) - then how high is the ash level of their diet?? 

Ash, in not too high a content - is fine.  The latest research shows that Ash is not the Culprit in Kidney/Bladder problems.  What most Vets Recommend (you can google if you like) - is feed regular diet - but less of it (the little Pamphlets I listed showed how to reduce overweight cats Calories by age and weight).  That's it, simple - Pet Diet.

CATKINS DIET

notes from the 2003 avma convention
The 'CatKins' diet: The feline diet: a historical look
(from AVMA.org)
When trying to develop a diet for cats, look no further than what a cat would eat in the wild: mice. Cats shouldn't be eating foods that they have difficulty processing, according to Dr. Deborah Greco, a veterinarian at the Animal Medical Center in New York City.
This internal medicine specialist, who spoke at a Sunday session, isn't arguing that pet owners should be shopping for mice to please Fluffy. Rather, they should be thinking about the nutrition in a mouse—a mouse is 3 percent carbohydrate, 40 percent protein, and 50 percent fat.
Since cats became domesticated, they have been fed various diets, and some of these diets were developed with little thought as to their natural diet in the wild. The result hasn't been good. "Many of the diseases that we treat are a result of the diets we give them," said Dr. Greco, who rattled off several examples.
Before the advent of commercial diets, owners fed cats organ meats that are low in calcium and high in phosphorus. This led to nutritional secondary hyperparathyroidism. In the 1980s, scientists linked a high incidence of struvite stones to high pH levels in food. And in the 1990s, scientists found that feeding acidifying diets to cats with struvite stones lead to an increased incidence of oxalate stones. Diets were reformulated to a more neutral pH. "The ideal pH is 6.5," says Dr Greco, which is the pH found in a mouse.
Also in the late 1980s, scientists traced a high incidence of dilated cardiomyopathy to taurine deficiency, and other researchers found that certain diets low in potassium caused hypokalemic nephropathy. Each time, nutritionists had to go back to the drawing board.
Today, cats are facing a different problem: an epidemic of obesity that comes with a price tag. "Thirty-five to 40 percent of cats are obese," said Dr. Greco, who says the peak years of obesity is between the ages of seven to 12 years.
"Obese cats are four times as likely to develop diabetes mellitus and five times as likely to develop lameness." Fat cats also have a higher incidence of non-allergic skin disease, most likely caused by the cat's inability to clean themselves as effectively, due to their size. This obesity is most likely the cause of diets with too high a carbohydrate content.
"Cats are unique in the way they handle protein, carbohydrates, and fat," Dr. Greco said. Cats are strict carnivores and, because of this, they have a tremendous ability to produce glucose from protein, but have difficulty processing carbohydrates. The feline liver has normal hexokinase activity, but no glucokinase activity. Thus, cats are limited in their ability to mop up excess glucose and store glycogen. "What happens is that glucose is going to hang around for a long period of time," she said, and it eventually becomes fat.
In addition, unlike humans, protein is the stimulus for insulin release in cats. Cats have adapted to high protein diets by being insulin resistant. This maintains blood glucose during periods of fasting, convenient for a cat in the wild, but not so good for pets eating a lot of carbohydrates.
"When you take an individual that is genetically programmed to consume high protein and low carbohydrates, and you put them on a high carbohydrate diet, what happens is their insulin resistance works against them," she said. "Their blood glucose concentrations are too high ... they can't overcome that, and they start to release more and more insulin in an attempt to reduce blood glucose levels." This doesn't work, however, and the cat eventually develops type 2 diabetes mellitus. The cat gets amyloid deposition in the pancreas, exhaustion of the pancreatic cells, and glucose toxicity from consumption of large amounts of carbohydrates.
So what's Dr. Greco's ideal cat food diet? She recommends a wet food, high in protein, high in fat, and low in carbohydrates. It's basically a "CatKins" diet, much like the Atkins diet popular today. This diet is going to keep a cat slim and help it avoid diabetes.
She recommends a wet cat food because if you are trying to mimic what a cat eats in the wild, just think of how much water a mouse contains. Wet cat food is going to give you a pH that is ideal and is, thus, the best way to prevent feline lower urinary tract inflammation. Dr. Greco said. In addition, a cat's jaws and teeth are designed for shearing and tearing meat, and cats that eat dry food grind it in a way that it ends up between their teeth. There it ferments into sugar and acid, thereby causing dental problems.
According to Dr. Greco, it all comes down to common sense. "We must use a cat's natural diet as a guideline."
Denver Colorado, American Veterinary Medical Association
22 July 2003