Garden worms v. mealworms

[Mary]

Whilst searching the compost bin for some garden worms for the blackbirds, it occured to me that they eat the same kind of 'food' as mealworms e.g vegetable material. Is there a scientist out there who can tell me why mealworms have a high fat content, making them not so good in large quantities? If they only eat bran and carrot, where do they get the fat from? :eat:
Just wondering! :h?:
Mary