Could my cat be malnourished since going on a diet.

As of August of this year my cat had a full check up. And he weighed 20 lbs. My vet suggested to put him on a diet. I did some research and found a calculator that said he needed 340 cals per day to maintain his weight. So I gave him about 250 cals of food.  He had elevated blood sugar but the vet felt that it wasn't that high.. 270 and was likely due to stress and Predlisone. He had no glucose in his urine.

I got rid of his dry food (which had a ton of calories and also was prescription) and started him on Earthborn wet. He gets about 250 cals of that.  He gets no more dry.   I suspect before August he was getting about 400 cals per day. With dry food given throughout the day and two cans of wet Earthborn.

Since August he has lost 1.2 lbs. Currently topping the scale at 18.8 lbs. So that seems like a reasonable weight loss in 4 months.  But recently, my cat is ravenous. But also, seems to have some thinning hair on the back of him. I wouldn't say it is bad.... just noticeable in the light. 

Consulting Dr. Google, Diabetes could be a cause of it.. but I find that hard to believe as this seems to have happened fast. And since August I got him off Predlisone, and dry food. Two things bound to be of assistance in lowering blood sugar.

I keep coming back to the idea that my cat could be malnourished. Either because the earthborn I am giving him doesn't give him the proper nutrients.  I am not giving him enough food. And or, my cat enjoys almost exclusively fish (though typically give him some chicken each week).

Has anyone had any experience with a malnourished cat due to a diet?

With all due respect to my vet, anything that is not an emergency, I am 100% more knowledgeable and sensitive to my cat that she is. So I feel like asking her will get a big nothing and I also feel like she gets annoyed with too many questions. So please... don't ask me to ask the vet.

The general rule of thumb is to feed 20 to 25 caloires per pound of idea body weight daily, 30 calories per pound of ideal body weight if you want to be super cautious. An 18.8 pound cat needs roughly 470 calories to maintain body weight. 250 calories daily is a huge reduction. While there has been some weight loss, your cat isn't getting enough calories to meet his needs so that's why he's always hungry. Cats who don't get enough calories are at risk for developing fatty liver disease. Earthborn is a complete balanced diet so your cat is getting enough nutrients.

How many meals a day do you feed? You'll need to add in an extra meal or two and / or increase the amount of canned food you are feeding.

How much is your cat supposed to weight ideally?

There's some great weight loss tips here: http://catinfo.org/?link=felineobesity

LTS3 said:

 
The general rule of thumb is to feed 20 to 25 caloires per pound of idea body weight daily, 30 calories per pound of ideal body weight if you want to be super cautious. An 18.8 pound cat needs roughly 470 calories to maintain body weight.

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But I am not sure that is right. My cat lost 1.2 lbs since early august... so that indicates to me that is just about spot on in terms of what he is getting.  Except for his complaining and fur loss.

I give him either 2 five ounce cans of food or 3+ 3oz cans of food.  The vet would be fine with him at 16 lbs. But I suspect he should be about 14.  He is a big boy (muscular)

I decided to up his food by 1 3oz can and give him a vitamin supplement.

Ideally he should be losing weight from activity + diet.  This sounds like too drastic a caloric reduction to me.