Are you medically underweight?

If you think you might be underweight you should check your Body Mass Index or BMI (use the interactive tool below); any number less than 18.5 generally means you are underweight - although there can be exceptions, for example if you're of Chinese or Indian descent you could expect to be slighter. Your doctor (GP) can advise you on this.

The right food for weight gain

To put on weight you need to consume more calories than your body is burning off. The Government recommends that women eat about 2000 calories a day and men consume about 2500 per day, so if you want to put on weight you should try to eat more than that. But there are good ways and not-so-good ways of doing this.

Junk food is highly calorific, but stuffing yourself isn't the best way of putting on weight. Fast food and snacks such as crisps are high in saturated fat, leaving you with greasy skin, lank hair and high cholesterol - which in severe cases can cause strokes and heart attacks.

On the other hand unsaturated fats, such as oily fish, nuts and seeds, avocados, olive oils and vegetable fats, are powerful sources of energy but still healthy. They can lower cholesterol and provide you with the essential fatty acids you need.

A balanced diet containing all of your main food groups is the healthiest way to put on weight. Ideally every meal should contain some protein and some carbohydrates. Proteins can be found in foods like nuts, fish, dairy products and meats. These food groups will help you build muscle mass, rather than adding body fat.  Proteins are essential for growth and repairing the body - about 15% of your daily calories should come from protein.

Carbohydrates are an ideal source of energy for the body as they are the easiest to convert into glucose which the body uses to power itself. Basically, if you eat lots of carbs you are more likely to be consuming more energy than you're burning off. Pulses, bread, potatoes and rice are all good sources of carbohydrates.

How much to eat?

If you are used to eating small portions and infrequently you may want to think about eating slightly more than you normally would - you will find your appetite beginning to increase over time.  But you can't just be expected to double your food intake - there is a limit to what you can eat.

Be clever with your calories: every time you cook, think how you can add extra calories without just eating more food. Consider mashing your potatoes with olive oil (which is full of healthy unsaturated fats), drizzle dressing on your salad or make creamy sauces for your fish.

When to eat

If you can, try and eat every three hours or so, which means you'll be eating three main meals a day and three snacks. Evenly pace how often you eat to make sure you get a chance to digest your food.

Make it easy to snack by keeping your fridge and cupboards stocked with easy foods - breakfast cereal with full-fat milk is quick to prepare and can be eaten any time of day.  If  you're too busy to eat regularly try and think of ways to cheat. Cook more food than you need for dinner and save half for lunch the next day or take snacks, like bananas, to eat while you're out.

Exercise for the underweight

It's a myth to think if you want to put on weight you should cut down on exercise. Different types of exercise can help you lose or gain weight. Resistance exercises, like weight training, will help to increase your weight through building muscle mass, while aerobic exercising, such as running, burn off calories. Focus on activities that will build you up.

Keep your GP informed

Putting on weight isn't any easier than losing it - you'll need will power. But if you commit to eating more over a period of time you will achieve your goal.

If you're consuming more food and not putting on any weight there might be an underlying medical reason. For example, undiagnosed diabetes or an under-active thyroid can be reasons for weight loss. Also if you're really struggling to put on weight, there are food supplements your doctor (GP) can prescribe for you that can help increase your daily calorie intake. These include high calorie juice drinks or milkshakes which you drink as an addition to meals, rather than as a replacement.

Being underweight can be just as bad for you as being overweight, so don't feel you can't discuss it with your doctor (GP) - they will be interested and will want to help.

Updated: 06/06/2011

Sharon Brennan