I had a few ideas until I read no dairy. What about going to your public health office or dietician and asking for an sheet on how to gain weight in a healthy manner. We got one when Ty was diagnosed, but I do believe most of it was adding skim milk powder to milk and stuff like that. Here are I couple of things I found while surfing. Of course they'll need to be adapted to gluten-free and any other dietary restrictions you have.
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From U. Michigan website:
What can I do to gain weight?
The average person wanting to gain weight should try to eat foods with high calorie content and larger portions of food. Mild exercise can help to increase your appetite and help you add muscle rather than fat.
The following recommendations can be used by anyone wanting to gain or regain weight. It may be necessary, however, to change them to meet your specific needs. Check with your health care provider or dietitian about this.
To start, eat about 500 calories more a day than you have been eating. Some people will need to add up to a 1,000 extra calories. Look at the top section of the Nutrition Facts label of packaged foods to see how many calories are in a serving.
Eat snacks throughout the day.
Choose higher calorie foods often.
Try to always eat larger-than-normal portions at meals. If you have a poor appetite, it may work better to eat smaller, very high calorie meals and to eat more often.
Fat contains more calories than any other food group: 45 calories in just 1 teaspoon. Adding healthy fats, such as plant oils (canola, olive, or peanut oil), soft margarines (look for those with no trans fats), old-fashioned peanut butter (the kind that needs stirring before you eat it), and avocado, is an easy way to add a lot of calories without having to eat a lot more food.
Avoid saturated fats. Although all fats contain the same amount of calories, saturated fats can increase your cholesterol and other harmful blood fats. Foods high in saturated fat include: whole-milk dairy products, chicken skin, bacon, sausage, sour cream, butter, high-fat cuts of meat, and many processed snack foods.
Even though you are eating a high-calorie diet, you should try to keep it healthy by including a lot of unprocessed (unrefined) carbohydrates, such as whole grains and fruits, as well as vegetables and lean protein foods such as skim milk products, skinless poultry, fish, egg whites, beans, and low-fat cuts of pork and beef.
What are some ways to add calories and protein to foods?
Sauté meats and vegetables in canola or olive oil or wine sauces, or add breadcrumb toppings. If you are an athlete or are recovering from surgery, you need more protein, so add extra portions from the protein group.
To mashed potatoes add extra powdered milk and margarine.
To soups, add skim milk instead of water. Top with croutons and Parmesan cheese. Bean and minestrone soups have more calories than broth types of soups.
To cold cereals, add dried or fresh fruits, nuts, and seeds. Try to choose the higher calorie cereals such as muesli and granola.
Prepare hot cereals with milk instead of water, and add powdered milk, margarine, honey, syrup, or dried fruits to the cereal.
When you drink juice, you can add the most calories by drinking grape, cranberry, pineapple, or apple juice.
To salads, include different types of lettuce and add tomatoes, avocado, garbanzo beans, raisins, and sunflower seeds. Chopped nuts and lean meats will add extra protein as well as calories. Generously use dressings made with heart-healthy oils, such as canola or olive oil.
To 1% milk add a quarter cup of powdered milk to add both calories and protein. Carnation instant breakfast, commercial milk flavorings, and Ovaltine can also boost the calories in milk.
Dietary supplements such as Boost and Ensure are good between-meal snacks. There are specialty liquid supplements available for people recovering from illness or with a chronic health condition. Products for diabetics include Glucerna and Glytrol. If you have an intestinal problem, Enlive may be a good choice. Ask your health care provider about these products.
Beans and legumes, such as kidney, navy, pinto, and garbanzo beans; peas; lentils; and peanuts are a great source of protein and fiber.
Eat dessert after dinner. Look for desserts made with healthy oils or low-fat milk.
Snacking is a big part of successful weight gain. Try yogurt with crunchy cereal added, fruit wafers, graham crackers, crackers with lean cheese, mini sandwiches and burritos, milkshakes made with frozen yogurt or low-fat ice cream, dried fruits, trail mix, vegetables dipped in salad dressing, and nuts.
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http://nutrition.about.com/od/dietsformedi.../GainWeight.htmTools to Help You Gain Weight
Use these tips and tools to help gain weight:
Start with this Calorie and Nutrition Guide to find out how many calories you will need to reach the weight you desire.
Use a food diary to track your calorie intake to be sure you are getting all of the calories and foods that you need.
You can also use an online program such as Calorie-Count to track your calorie intake, your exercise and your changes in weight. Calorie-Count has a large data base of nutrition information for almost any food you like to eat.
Add healthy calories to breakfast with an extra slice of whole-grain toast and peanut butter.
Mid morning and afternoon snacks should be nutritious, not junk. Trail mixes contain healthy nuts, dried fruits and can add healthy calories to your diet.
Don't add calories to your meals by choosing unhealthy fried foods such as french fries, chicken nuggets and fish sticks. Choose healthy foods such as baked potatoes, baked chicken and fish, just make your portions a bit bigger.
Keep your meals balanced. One meal should have a healthy protein source such as fish, low fat meat, chicken or legumes plus two or three servings of vegetables. Green vegetables add lots of vitamins and phytonutrients while starchy potatoes and corn add extra calories.
Drink healthy beverages such as milk and fruit juices to add calories. Don't choose sugary sodas, which only add calories and no nutrition.
Choose a weekly weight lifting exercise program to gain muscle mass.