On the following website I came across the title Prevalence of Overweight and Obesity.
That doesn't sound quite right to me.
Obesity is the state of being obese, but how can overweight be the state of being overweight?
Nevertheless, I've come across this use of the word overweight in scientific literature as well.
Shouldn't the sentence be phrased Prevalence of Overweightness and Obesity?
That is a fascinating question, because at first it appears you are right. Something along the lines of "Prevalence of Overweightness and Obesity" would be correct. However, checking the dictionary, "overweight" can be a noun meaning "weight in excess". All that "overweightness" would be doing is changing an adjective to a noun, and that's apparently not required. Thus, "Prevalence of Overweight and Obesity" is acceptable usage.
Several months ago I was also amazed to discover that overweight is used as a noun - especially in scientific and medical literature. I've composed a poem, "The Battle with Overweight", haha
Fridgeward he pressed, his famishment to sate,
But soon drew back, afraid of Overweight.
O, Overweight! How many gallant gluttons
For fear of you forbear their spicy muttons.
