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Consume an additional 500 calories per day. Consume more healthy fats. Eat more protein. Cook using oil or butter. Drink more calories.
An extra 500 calories per day is enough to help you gain weight, but should not leave you feeling sluggish, bloated or sick.  Just by gaining an extra 500 calories a day (which is pretty easy if you follow the instructions below) you can aim to gain between 1lb and 1.5 lbs per week. However, it is important to be aware that these extra 500 calories should be gained in a healthy way, by eating more calorie-rich foods that are still packed with vitamins and nutrients. Gaining weight by eating more junk food is not a good idea, as it will leave you feeling unwell and sapped of energy, and could lead to further health problems down the line. You might also boost your calorie intake by adding a protein powder to your diet. Protein powders are rich in lean protein and extra calories and can be blended into foods like smoothies, yogurt, hot cereals, and others. You should always consult with your doctor or nutritionist before embarking on a weight gain plan. Foods containing healthy fats are full of nutrients and also high in calories, making them a great option for weight gain.  Foods containing plant fats should be your first option - this includes items such as nuts, seeds, peanut butter, avocados and olive oil. Spread peanut butter (or almond butter) on toast, eat half an avocado with every meals, eat handfuls of nuts or seeds as a snack and sprinkle olive oil over salads and veggies. You can also gain some healthy fats from animal sources, however these foods also contain saturated fats (the unhealthy kind) so you should only eat in moderation. Foods containing healthy animals fats include lean meats and full fat dairy products - although if you have high cholesterol, you should probably stick to low-fat options. Protein-rich foods are your best friend when you're trying to gain weight healthily. They help to build lean muscles, rather than adding much fat. Eating protein is particularly important if you plan on doing strength gaining exercises.  Good sources of protein include lean meat, fish and poultry, in addition to eggs, whole grains, dairy products and legumes. You should aim to eat about 5 oz of protein per day, from a combination of these sources. You can also increase your protein intake by drinking protein shakes, or adding a protein supplement to juices and smoothies. One easy way to increase your calorie intake with each meal without the necessity of actually eating more food is to cook using oil or butter.  Try stir frying your vegetables in a knob of butter, or drizzling a little olive oil over salads and cooked veggies. Simply by adding a tablespoon of these fats to each dish, you could be adding 100 calories! However, it's important not to go overboard when cooking in fat, as too much can be unhealthy. When possible, gravitate to healthier fats like olive, canola or safflower oil and avoid using unhealthy ones like lard or margarine. Another good trick for increasing your calorie intake is to simply to consume more calorie-laden drinks. This will help you to gain weight without ruining your appetite or causing you to feel bloated.  Try drinking a large glass of orange juice in the mornings (along with your normal breakfast), it's high in calories and is tasty and refreshing too! Consider drinking a glass or two of milk throughout the day - the full fat option is high in calories but will also provide lots of protein and calcium - which is ideal for thinner people who are more prone to lower bone densities. Protein shakes will help you to gain muscle mass, particularly if you're working out, while tasty milkshakes are perfect for the occasional treat.