Physiology is a complex thing - and now I'm over 40, I find building any muscle very hard - I think I'm going to try the protein shake tip
dar917, I suspect however that being female, I'm guessing you're rather more interested in increasing your covering of subcutaneous fat (i.e the fat that is deposited just below the skin, and gives our bodies a smoothed appearance) rather than muscle per se
question? do you feel well? It is possible that your low body weight may be caused by an underlying illness. For instance hyperthyroidism can cause low body weight. Any doubts get checked out by your doctor.
Assuming that you are well, the first point to understand is that our bodies have a genetically determined set point. This is believed by most researchers to account for around 70% of our total weight. we have control over the remaining 30% - that is we can through diet and exercise move our weight up or down by around 15% of our natural set point weight (most people achieve much less than this)
It is likely that you are one of those individuals whose set point is naturally low. Probably in your case you're looking to gain a maximum of around 10lbs before your body decides that it's too heavy and starts to adapt to the extra calories you are eating (contrary to popular opinion most humans do have compensatory mechanisms to stop them gaining too much weight through excess food consumption)
I don't know much about the effects of protein on the body, but the body treats carbs and fat almost interchangeably - if you eat more carbs and less fat then the body will convert the excess carbs into fat (there is some evidence that eating like this will help you lay down fat more easily) - if you do the reverse and eat more fat and less carbs, the body stops converting carbs into fat and starts to prefer to burn fats over than carbs, thereby preserving your limited carb supply.
In your case - I'm wondering about the "healthy" foods you are eating. I'm thinking that this means wholefoods - i.e. wholegrains, fruits, vegetables etc.
One of the reasons that wholefoods are recommended is because of their fibre content (which is believed to improve digestive tract health - now disproven), and because of the belief that wholefoods are higher in vitamins minerals etc (I have no idea whether this is true or not)
The issue with wholefoods is that they are very hard to break down - it is almost impossible for the body to extract all the available energy from them, so....
you might be eating 1800 calories a day, but your body is likely to be extracting much less.
I suggest that you eat slightly less healthily - include some processed food in your diet - i.e. white flours. If you can increase your carb consumption (without making your control worse), then (in theory) this should help you gain weight more easily than increasing the amount of fat. Although realistically if you increase your carb consumption you fat consumption is also likely rise. (most western peoples eat fat and carbs in equal amounts)
