You didn't specify, but usually issues with loosing weight is due to having a hypothyroid. Do a search on "Foods hypothyroid" and you'll find a billion results (or it feels that way), or "hypothyroid foods to avoid".

http://www.umm.edu/altmed/articles/hypothyroidism-000093.htm
Following these nutritional tips may help reduce symptoms:
•Eat foods high in B-vitamins and iron, such as whole grains (if no allergy), fresh vegetables, and sea vegetables.
•Avoid foods that interfere with thyroid function, including broccoli, cabbage, brussels sprouts, cauliflower, kale, spinach, turnips, soybeans, peanuts, linseed, pine nuts, millet, cassava, and mustard greens.
•If you take thyroid hormone medication, talk to your doctor before eating soy products. There is some evidence soy may interfere with absorption of thyroid hormone.
•Taking iron supplements may interfere with the absorption of thyroid hormone medication, so ask your doctor before taking iron.
•Eat foods high in antioxidants, including fruits (such as blueberries, cherries, and tomatoes) and vegetables (such as squash and bell pepper).
•Avoid alcohol and tobacco. Talk to your doctor before increasing your caffeine intake, as caffeine impacts several conditions and medications.

These supplements may also help:
•Omega-3 fatty acids, such as fish oil, 1 - 2 capsules or 1 - 2 tbs. of oil daily, to help decrease inflammation and help with immunity. Omega-3 fatty acids may increase the risk of bleeding, especially if you already take blood-thinning medication. Ask your doctor before taking omega-3 fatty acids if you take blood thinners such as warfarin (Coumadin) or if you have a bleeding disorder.
•L-tyrosine, 500 mg two to three times daily. The thyroid gland combines tyrosine and to make thyroid hormone. If you are taking prescription thyroid hormone medication, you should never take L-tyrosine without direction from your doctor. Do not take L-tyrosine if you have high blood pressure or have symptoms of mania.
•Do not take an iodine supplement unless your doctor tells you to. Iodine is only effective when hypothyroidism is caused by iodine deficiency, which is rare in the developed world. And too much iodine can actually cause hypothyroidism.