You have to eat at a surplus of 3500 cals above the amount of calories it takes to maintain your current weight to gain 1 lb of permanent weight.
So if it takes around 2000 cals to maintain your current weight and you ate 500 cals + per day (2500 cals), you should gain 1 lb per week.
The other posters have given you some suggestions on how to do that with diet. Healthy fats are an easy way to increase your calories without feeling too stuffed (they are calorie dense and have twice the amount of calories per gram as carbs and protein). Cooking with extra virgin olive oil will add a ton of extra cals to you day without filling you up too much. Eat more frequently.
You can also increase your calorie intake slowly so your body has time to adapt. Just add 200-300 cals per day each week until you reach your target.
I would highly recommend you strength train while gaining. This will create new muscle weight (LBM) so that your gain is not all fat. Do full body, compound exercises with a challenging weight (one that you reach muscle fatigue in 8-10 reps before increasing weight) or you can use resistance bands; 3x a week on alternating days to give your muscles a chance to heal and grow (on recovery days).
Limit the amount of cardio you do and keep it to 30 mins 3x a week of mod-high intensity cardio for the health benefits. The more cardio you do, the more you'll have to eat to gain.
Make sure you eat healthy while gaining and get adequate protein for muscle repair.
Edited by: JENNILACEY at: 8/31/2013 (07:21)
Take your focus off the Marshmallow. www.leangains.com/2010/01/marshmallo
"Toning" is marketing muscles to women who are afraid if they pick up a barbell, they'll leave the gym looking like She-Hulk. If you don't want to get bulky, lift heavy!
I can bench Nicole Richie, eat more than she does in a day before noon, I have a good 20 lbs on her but could still wear her pants.
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