There are many reasons why you might like to gain weight. If you've been unwell or receiving treatment for a condition like cancro, it's not uncommon to lose weight, especially if you are unable to eat or you have no appetite. You might choose to put on weight for aesthetic reasons, such as someone who goes to the gym and wants to build more muscle.

Whatever your motive, it's important to make sure you manage your weight gain healthily.

Healthy ways to gain weight

Create an energy surplus - eating more calories

Kirsty Barrett, a registered dietitian says: "To gain weight you need to create an energy surplus." This means taking in more energy than you burn off.

If you are trying to lose weight through your diet - one way is to consume fewer calories than your recommended daily intake which is around 2,000 calories for women and 2,500 calories for men. If you are trying to gain weight you should eat more, to get extra energy.

"This can be done by generally just increasing the quantities that you eat, but not everyone is able to do this," she explains. "The easiest way to increase energy intake is to increase the fat content of the diet."

According to Barrett, fat has 9 calories per gram, whereas carbohydrates and protein have 4 calories per gram.

"Obviously, eating high-fat foods on their own - think oils, butter, spreads and cream - isn't particularly enjoyable. As dietitians we tend to advise adding these things to meals to increase their calorie content. For example, adding a teaspoon of oil into pasta sauces, cream and cheese to soups, or cream to fruit-based puddings."

However, it's important to continue to eat healthy foods, along with high-fat foods to your diet. These include fruits, vegetables and wholegrains, which provide nutrients and fibre.

Build muscle

There are different ways to approach weight gain. You may be medically underweight or need to gain weight for health reasons. Or you may be of a healthy weight but still want to gain weight - for example, to develop a more muscular build.

The main difference between dietary changes for these scenarios is the amount of fat that you should be eating.

"In both of these situations you want to gain muscle," says Barrett. "If you have been unwell, you likely also want to promote some restoration of body fat as well.

If you have been unwell in hospital and have lost weight, focus more on overall energy intake, ideally looking at things that are both high in fat and also high in protein - cheese, full-fat yoghurt, nuts and seeds.

"If you are at the gym, focus more on higher-protein foods. Choose lower-fat varieties but ensuring your energy intake is enough to support muscle growth. Protein, especially when paired with resistance exercise, promotes muscle synthesis."

Consume healthy fats

If you're looking to put on weight, you might be thrilled by the idea of loading up on cakes and pastries. But there is a big difference between saturated and unsaturated fats.

"Health-wise, saturated fats are linked with increased cholesterol levels and therefore an increased chance of heart disease," says Barrett. "They are generally from animal sources and are solid at room temperature. Unsaturated fats generally reduce your cholesterol levels and protect your heart."

It's important not to throw all of the healthy eating rules out the window. Bingeing on foods packed full of saturated fats can increase your likelihood of health problems - particularly in those who already have high chances, or are younger and of normal weight.

"The gamble would generally be bigger if you have a higher chance of malattie cardiache," says Barrett. "For example, if you who have already had a attacco di cuore oppure ictus, have a strong family history of these conditions, or have an already raised colesterolo level.

"A combination of unsaturated fats and exercise is the healthiest way to up your calories", says Barrett. "Unsaturated fats such as nuts and seeds are good as they contain both protein and fat. Ideally, to gain weight, you should gain muscle rather than fat. Esercizio fisico is crucial in signalling the muscles to build and repair themselves, so is key if you want to gain weight."

If you are going to include saturated fats in your diet, go for dairy, she explains:

"There is some emerging research that suggests some saturated fats may have a protective effect. This has been found in dairy fats, whereby these types of fats actually seem to be protective and reduce your chance of heart disease, and especially stroke.

"Although it contains saturated fat, dairy is a great way to try and increase weight due its protective effect on your heart. It's also a good source of easily absorbable protein. We would generally promote trying to increase unsaturated fats first line, but - as mentioned - increasing dairy fat may be less problematic than increasing fats from cakes, pastry, meat and so on."

Increasing your intake

Increasing your portion sizes and the amount you eat seems like the simplest way to start gaining weight. However, this might not be possible if you find it difficult to keep food down or to stomach large amounts of food.

For this, Barrett recommends food fortification - increasing the content of nutrients in food.

"Food fortification is a great way to increase energy intake without increasing the volume you have," she explains.

Some examples of this might be:

• Adding cream into porridge in the morning.

• Being more liberal with butter on toast.

• Melting butter on to vegetables.

• Topping pasta meals with cheese.

• Making a milky drink.

"Nutrients don't just have to come from food either," she says. "If you are trying to gain weight use nourishing fluids such as milk, fruit juices or smoothies to ensure you are also getting some energy from drinks as well. They all still count towards fluid intake."

If you lose weight through illness you might not be able to enjoy foods that you once loved. If you who have gone through cancro treatment you may experience taste changes which make certain foods bland or nauseating.

"Taste changes can be really difficult," says Barrett. "Sometimes you may prefer cooler foods such as ice cream or yoghurts. If you want stronger flavours, using lemon or lime, spices, gravies or sauces can be helpful. If you are struggling with gaining weight after being unwell, you may referred to see a dietitian about this through your doctor."