I personally feed carrots once or twice a week and I believe this is the general recommendation?
As a root vegetable, they have a high sugar content.
Also, they are high in vitamin A. Vitamin A is a fat soluble vitamin, so it's much harder for the body to get rid of (compared to vitamin C for example, which is water soluble, so when there's too much in the body it's excreted in the urine).
Too much vitamin A can cause toxicity, particularly affecting the liver.