As far as gaining weight is concerned, a safe and easy way to do this is to increase your protein intake. It sounds kind of cliche, I know, but it’s tried and true. You can also drink shakes that were made for weight gain, such as Ensure Plus or Ensure High Protein — http://ensure.com/products/index.aspx. In addition to protein, you should also eat a lot of complex carbohydrates, like pasta, brown rice, potatoes, oats, and whole grains. Since you’re trying to build muscle, you should also definitely cut out sugar, like soda, and simple carbohydrates, like anything with enriched white flour (white bread, cookies, etcetera). A lot of protein and complex carbs might make bowel movements a little harder and less often, but you can alleviate that by upping your fiber intake as well.
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