Obtaining a healthy weight is one of the most important targets for anyone who is underweight. Yet it isn't the only reason individuals want to gain weight, as you might want to build muscle mass for sports or might be recovering from an illness.
Whatever the reason is, you should always contact your GP for advice. After visiting your GP, for extra support and a diet plan, a nutritionist will be able to help.
Listed below are a few ways you can gain weight safely, tailored for people who are underweight:
- Eat regularly. Eating five or six meals smaller meals per day when you are underweight can be more manageable compared with the standard three. This is because you typically feel full faster than a person at a healthy weight.
- Don’t drink before meals. Fluids before meals can affect your appetite. Try limiting drinking until 30 minutes after you’ve eaten.
- Add nutritious drinks to your diet. Avoid soft drinks and caffeine-heavy coffee that adds little nutritional value to your diet. Try milk, fruit juices, healthy shakes and fruit smoothies instead. The will help increase the energy in your diet and also add important nutrients for your health too.
- Eat nutritious foods. When underweight, sticking to a healthy diet can help you safely gain weight. Choose fruits, vegetables, nuts, seeds, whole grain cereals, bread and pastas, lean meats and dairy products.
- Snack healthy. Snacking on avocados, nuts, cheese, peanut butter or dried fruits between mealtimes can be a good way to gain weight.
- Treat yourself. Even when underweight it’s fine to have an unhealthy treat every now and then. But be mindful of excess fat and sugar - try to keep most treats nutritious and healthy.
- Work out. Strength training (building muscle) can help you gain weight. Exercise may also help you get your appetite back.
On this page we will explore how to gain weight safely for people who are underweight. We will highlight the risks and causes of being underweight, give you the tools to find out your BMI and explain how a nutritionist can help with your weight gain.
On this page
- Being underweight
- Am I underweight?
- Is my weight already affecting my health?
- Causes of being underweight
The term 'underweight' is typically used to describe an individual who is not within a healthy weight range and has less body fat than is required for maximum well-being.
Body fat is estimated using a measurement known as Body Mass Index (BMI), which classifies a person as underweight if their BMI is below 18.5.
Whilst there are some people who have a naturally slender body type, those who are underweight as a result of poor nutrition or a health issue stand a significant risk of developing certain health problems. These include, but are not limited to:
- heart rhythm irregularities
- thinning bones
- vitamin deficiencies
- weakened immune system function
- psychological issues.
If you are underweight, aiming to gain weight safely until you are within a healthy range for your age and height will help to reduce any health risks. If you are considering weight gain, visit your GP or practice nurse beforehand so they can rule out any underlying medical causes for low weight as well as providing you with some useful advice.
Am I underweight?
Household scales can be helpful to track your weight, but they rarely tell the whole story. They aren’t sophisticated enough to determine if the fat percentage in your body is appropriate for your height. If you are concerned that you are underweight then the best way to measure this is to calculate your BMI.
Anything between 18.5 and 24.9 is a healthy weight, and anything above or below poses significant health risks. BMI reference ranges were developed for use with adults only, with separate charts available for children. Be aware that these markers could be inaccurate for women that are breastfeeding or pregnant, people that are frail or for those who have a high muscle percentage.
If you have a set of scales and know your height then you can use the following formula to work out your BMI:
Working out your BMI
- Firstly, find your weight in kilograms and your height in metres - e.g. weight = 50 kg height = 1.8
- Multiply your height by itself - e.g. 1.8 x 1.8 = 3.24
- Divide your weight by that figure - e.g. 50 ÷ 3.24 = 15.4
- 15.4 is a BMI that is below the healthy body weight range.
Is my weight already affecting my health?
If you determine that you are underweight then as mentioned previously, pay a visit to your GP so they can ensure your low weight is not caused by any underlying medical conditions.
At this point your GP may also ask you questions about your energy levels, your menstrual cycle if you are a woman and any other aspects of your health that can be visibly affected by low weight. Tests may also be needed to determine the presence of other associated health risks such as nutritional deficiencies, anaemia, type 1 diabetes, osteoporosis or heart irregularities.
In addition to the physical issues that may be caused by low weight, some individuals may also be affected on a psychological level. Our self-image plays an important part in both our confidence and self-esteem and feeling unhappy with weight and image can lead to the development of a mental health condition. On the other hand, if an individual is living with a mental health condition such as depression, they may experience a loss of appetite which can lead to sudden weight-loss. In order to determine whether or not there are any signs of a mental health concern, your GP may ask you questions about your current moods, feelings, relationships, work stress etc. so that appropriate treatment can be sought.
If it is found that you are underweight because your diet is not providing you with enough calories, then this is something that can be rectified by adopting a balanced diet.
Causes of being underweight
A person’s body weight is determined by the amount of calories consumed in relation to the amount of energy used by the body. If we eat the right amount of calories this means consumption and energy use will be balanced and our weight will remain the same. In contrast, if we consume too many calories the body will take any extra and store them as fat (resulting in weight gain). If we consume too little, the body will have to dip into its own stores in order to obtain the energy it needs (resulting in weight-loss). There are, however, additional factors which need to be taken into consideration when it comes to determining why someone is underweight, including the following:
- elderly difficulties (mobility, swallowing difficulties, motivation to cook for one etc.)
- emotional issues (stress, anxiety, depression etc.)
- loss of taste
- overtraining (excessive physical activity)
- underlying illnesses (such as an overactive thyroid, inflammatory bowel disease and coeliac disease may cause a loss of appetite).
Risks of being underweight
If you are underweight then you are at risk of developing a range of weight related conditions including the following:
Iron deficiency anaemia is caused by a lack of red blood cells because the body does not have enough iron to produce them. This makes it very difficult for your body to carry oxygen, usually resulting in tiredness and fatigue.
If a person is not consuming enough calories and is underweight, their body will begin to conserve energy by adjusting its normal process and slowing the heartbeat. A slower heartbeat can cause the blood pressure to lower, which over a prolonged period of time can damage the heart.
Women who are underweight may have irregular periods or may stop regular menstruation altogether. When menstruation ceases this is known as amenorrhea, which is a sign that the body's calorie intake is not high enough to maintain optimum function.
Osteoporosis is a condition which causes bones to become weak, brittle and more likely to break. Underweight people who do not consume enough bone strengthening nutrients such as calcium and vitamin D will be more susceptible osteoporosis. Woman who stop menstruating as a result of low weight will also stand a higher risk of developing osteoporosis.
Gaining weight for sport
High protein diets are prevalent in sports men and women who are aiming to gain weight and build muscle. The relationship between diet and muscle gain is the subject of many studies. In fact, research into nutrition programs for weight gain and building muscle mass far outweigh the research into workout routines. Focusing on your intake of fat, carbohydrates and protein can have a dramatic impact on your results.
For more information on sports nutrition, please visit our dedicated fact sheet.
Treatment options to gain weight
How to gain weight safely
If you are underweight and wish to reduce your health risks and reach a healthy weight range, then it is important this is done gradually and safely. It goes without saying that you need to increase the number of calories consumed, but this does not mean eating calorific junk food and zero physical activity.
If you are intending to gain weight, then it is advisable you consult your GP or practice nurse before starting out. They will be able to record your current weight, advise you on whether they consider it necessary and provide you with advice about healthy eating and changes to your diet.
If you are unsure of how to put on weight and may be concerned it could lead to excess calorie consumption, you might want to seek the guidance of a qualified nutritionist.
How can a nutritionist help me achieve a healthy weight?
A qualified nutritionist can help you gain weight safely. Once you have reached your goal, they will help you to ensure you eat the right amount of food to meet your needs and remain within a healthy weight range.
Prior to your first consultation your nutritionist may ask you to compile a food diary to help them establish your eating habits. This will allow them to build a picture of what you eat, when you eat and how much you eat. They may also ask you to include other details such as whether you feel stressed or anxious as this may help to establish moods which trigger a loss of appetite.
After a detailed history has been taken, your nutritionist can then move onto formulating a weight gain diet/programme that’s specifically tailored to you and your target.
When putting together your programme a nutritionist may take into consideration some or all of the following points:
Physical activity: Eating more calories than you burn is essential for weight gain, but ceasing physical activity is not the answer. Remaining active is essential for the maintenance of optimal well-being. Many nutrition programmes will include exercise regimes or physical activity recommendations. Be sure to discuss with your nutritionist what kind of exercise you enjoy. Also notify them if there are certain activities you are unable to participate in for health reasons.
Food enjoyment: If you do decide to consult a nutritionist, they will provide you with a programme with the intention of you learning to adopt a healthier way of eating indefinitely. The initial adjustment to a different way of eating may be difficult at first, but it is important that the foods included on the list are ones that you enjoy and could commit to eating regularly. If there are some suggestions you really dislike then it's unlikely you will stick to the plan. If this is the case, talk to your nutritionist and see if there are any suitable substitutes or alternative choices.
Your final programme should be achievable and realistic. It may include suggested supplements, physical activity recommendations, meal plans and a list of suitable foods.
Regular consultations to monitor your progress will also provide opportunities to seek advice, support and motivation. These can help you focus on your goals so you do not stray from your programme and continue to improve your diet and fitness.
An issue which commonly crops up in 'fad' dieting is individuals reaching their target weight, resuming old eating habits and seeing the weight return. In a similar fashion, if an underweight person reaches a healthy weight and then begins to eat as they did before, they may experience weight loss once again. In order to avoid this, your nutritionist will work with you to ensure any changes made can be integrated into your daily lifestyle and are permanent.
Once you have achieved a healthy weight, your nutritionist may suggest continuing with regular consultations until you feel confident and comfortable with continuing your new lifestyle and weight management independently.
Content reviewed by dietitian, Dawn Shotton. All content displayed on Nutritionist Resource is provided for general information purposes only, and should not be treated as a substitute for advice given by your GP or any other healthcare professional.
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