If you have neurasthenia, it may causes include
Gas: If you have a problem with gas due to any reason or any it affects your brain or brain more, due to which the memory of the person starts becoming weak, and the person gets this disease.
Due to lack of nutrition: If there is a lack of nutrition in your body or any person, that person starts having many problems. Due to lack of nutrition, nutrition does not reach the brain, due to which a person can get this disease.
I have also covered Nutrition Value of Food: Importance, Benefits, and Diet . You should definitely read.
Due to physical labor: When a person does a lot of work in a day which is much more than his physical capacity, then his body, as well as his mind, is affected, due to which the person can also get this disease.