While we often hear about the health risks of obesity or being overweight, there are serious health risks associated with being underweight. Being underweight can weaken your immune system, leave you feeling tired or lethargic, and put you at higher risk of some serious health conditions, such as osteoporosis.
What does being underweight mean?
Being underweight simply means your weight is lower than recommended for your height and age. Healthcare professionals use body mass index (BMI) to work out if you are underweight. This is calculated by dividing your weight in kilograms (kg) by your height in metres squared (m2). Try this BMI calculator.
The number obtained is then compared on a chart to see which part of the range you are in. If the number is less than 18.5 this is called being underweight, around 18.5 to 20 might be a warning not to go lower, 20 to 24.9 is said to be a healthy weight.
However, there are exceptions. BMI does not take into account the different proportions of muscle mass we have and so on. Some people may have been slender all their life, having eaten a healthy diet and exercised normally and not have health problems. However, there may be more cause for concern if a person’s BMI changes and they become underweight.
Common causes of being underweight
Being underweight is more common in women than in men.
Common causes of being underweight include:
Uncontrolled diabetes can lead to severe, fairly rapid weight loss.
Medication that affects appetite or metabolism can trigger weight loss.
Thyroid problems – an overactive thyroid, that is producing an excess of the thyroid hormones can lead to weight loss.
Cancer or malignancy can cause a state called cachexia where after there is loss of fat on the body there will also be loss of muscle and wasting generally.
Coeliac disease – this proven intolerance of gluten causes malabsorption at the small bowel thus nutrients will not be absorbed.
Older age can be accompanied by unintentional weight loss. If someone is isolated or has lost interest in preparing food the changes can be gradual and not noticed which is why doctors are careful to assess nutritional status at consultations with older patients. There is a scoring system called MUST that clinicians sometimes use for assessing this.
Muscle loss in older age, partly due to age, partly less activity (called sarcopenia) will also show as weight loss. As we age we naturally lose what we call motor units (areas where a nerve supplies an area of muscle). By keeping active we can maintain more motor units and keep muscle healthy.
Health consequences of being underweight
Here are the main problems that can occur if you are not eating enough of the right nutrients:
• Vitamin and mineral deficiencies
These are essential in the healthy diet for the functioning of all systems of the body. The nervous system, cardiovascular, skin, gut and more all rely on certain nutrients. Deficiencies can lead to specific problems. Some may not lead to symptoms but long term unwanted effects on health can creep in.
• Bone thinning
Calcium deficiency in the diet can lead to thinner bones and this can go on to create osteopenia (mild bone thinning) or osteoporosis (more severe bone thinning). Weight loss also means there is less adipose or fatty tissue in the body. In women, this is one of the major sources of oestrogen post-menopausally and this usually helps protect the bones. We know osteoporosis goes on to increase risk of fractures. Similarly lack of dietary vitamin D will affect bone integrity and many other systems in the body.
Studies have also shown that weight loss in middle aged women (such as those from crash or yoyo dieting) can be accompanied by bone loss and even if weight is regained this may not improve again.
• Cardiovascular problems
Being very underweight has been suggested as a risk factor for cardiovascular disease. More studies are needed in this area but it seems the risk may not be confined to those with obesity.
• Immunity reduced and harder to recover from infection
You may be more likely to catch infections and find it harder to shrug off colds, flu and other infections if underweight. This could be because protein, fat and nutrients are needed for the immune system to function. Different studies have shown that providing malnourished people with a diet rich in protein and nutrients, such as zinc and vitamin E may reduce their chances of catching infections.
• Wound healing slows
Inadequate protein levels reduce the synthesis of collagen in the body and this is needed for wound healing. Also enough food is needed to provide energy for wound healing in addition to general functioning of the body. You have to make extra sure you are fuelling your body whilst healing.
• Fertility reduced
Just as young girls need to reach a threshold weight to start menstruating (around 45 kg), similarly if you lose weight your periods may stop and as a woman this will affect your fertility. Preconceptual counselling with your GP may give you simple advice about a healthy diet alongside appropriate recommended pre-pregnancy vitamin supplements including folic acid.
• Dementia risk potentially increased
A study found being underweight as opposed to normal weight carried a 34 per cent higher risk of dementia.
Interestingly, studies looking at the risks of being underweight are compromised by there being differences between patients due to the actual cause of weight loss. So other things may be at play such as amount of protein intake, smoking, diabetes. Not all underweight patients in studies have all the same health risks.
20 expert tips to gain weight safely
First, it is important to establish the reason for any weight loss with your doctor as it is paramount to treat the underlying cause. Remember if an eating disorder is causing your weight loss, it is vital that you have specific specialist care. The following tips can help you gain weight quickly and safely:
1. Admit you may need help
If you think poor food choices are the cause of weight loss, consider asking for referral to a dietician or registered nutritionist. After illness this may be part of rehabilitation programmes after hospital discharge. At different ages there will be different requirements for food groups and this will be personal to you and your activity level.
2. Make healthy choices part of your long term plan
Filling up with unhealthy fatty or sugar laden foods to gain weight fast will not do your body any good in the long-run. This will encourage you to gain visceral fat (fat around your internal organs). Rather you want to gain a healthy amount of muscle and subcutaneous fat slowly. You can be slim and unhealthy, so don’t aim for visceral fat gain. Think about your body composition as healthy building blocks you are adding in. Think of each meal plate as a tool for topping up nutrients.
3. Balance your activity with your intake
Unless exercising excessively, a small tweak in intake of food may help you regain lost weight. You might want to use a calorie counter to help you estimate this or ask your dietician. Slower gain should be healthier than fast. An additional 300 to 500 calories a day may be necessary.
Similarly regular exercise and light weight use may help any additional protein in your diet be put to good use to build muscle – it is literally a case of ‘use it or lose it’. Go gently if you have any bone or muscle or mobility problems and consult your doctor first. Beware that overdoing cardio may causes further weight loss.
4. Make protein a priority
Unless there is a specific medical reason to limit protein (as in kidney problems), most dieticians will explain that protein is the building block for repair and building muscle. It also makes us feel full. Protein-rich foods include meat, fish, dairy, eggs, legumes, nuts. Only in severe cases of malnourishment or hospital confinement is protein added as a supplement but it does have a role in nourishing drinks as food replacement when prescribed to the elderly or those recovering from illness.
5. Maintain your calcium intake
Whole fat milk is a great balanced food but if you are vegan or simply dairy free look at the nutrient content of your replacement milk. Is it fortified? Does it have adequate calcium and no added salt? Does it contain iodine (a component of cow’s milk essential for health and a healthy thyroid)?
6. Follow healthy recommendations
A good example of a balanced diet is the EAT WELL campaign providing correct calories for your age, height and activity.
The Mediterranean diet and its principles have been proven to reduce long term illnesses and promote health. Low salt, plenty fruit and veg, less red meat, good healthy proteins, olive oil and so on.
Avoiding over-processed foods and saturated fats will not promote an unhealthy rise in body fat and unhealthy cholesterol.
7. Routine and snacking
Try aiming for routine – sitting down for meals, eating mindfully and enjoying them, rather than snacking on the go. Think of how each meal can nourish you rather than just eating ‘for the sake of it’. Enjoy the process of preparing food and trying new recipes. It does not need to be rocket science. If you need snacks, plan ahead and make them healthy and energy dense, such as dried fruit, nuts. It might help to eat the nutrient rich foods on your plate first.
8. Reduce empty calories
Avoid fizzy drinks, over processed white carbs. These will not fill you up and will not sustain your energy and you will repeatedly slump in energy during the day. You will need healthy energy dense carbs so aim for wholegrains (oats, brown rice) for sustained energy release. Choose unsaturated fats as spreads, drink plenty fluids. Make sure your diet is full of variety and colour with plenty fruit and veg and pre and probiotics to promote gut health. Use herbs and spices to make food more appetising and make you eat more of it.
9. Sleep well
Sleep is vital for all function of the body, repair and to aid metabolism of what you have taken in during the day and to help protein restore muscle.
10. Don’t smoke, stop excessive alcohol
Smoking and alcohol often replace healthy meals. Being underweight as an adult should not be ignored. Protect your health by aiming for a healthy weight in a sensible way and do not be afraid to ask for help.
11. Eat more calories than you use up
This really is a simple one. If more calories go into your body than you use up in activity, you will gain weight - just make sure they come from a healthy diet rather than junk food or snacking.
12. Increase your carbs and some 'good' fat foods
Eat three good wholesome meals each day with enough high carbohydrate and high-fat foods such as cheese, bananas, oats, quinoa, blueberries, and sweet potatoes.
13. Hit the weights
Regular strength training with weights helps to build quality muscle with any excess calories consumed bulking this up rather than putting on fat.
14. Use a bigger plate
It might seem strange but it is well known that when you use bigger dining plates, you usually end up filling them more than with smaller ones, leading to weight gain.
15. Know when to drink water
Just as we know that drinking water before a meal can dull the appetite by filling the stomach and leaving no room for food, make sure you drink water regularly every day but only after meals.
16. Hit the energy-rich foods
Rather than relying on the quantity of food you eat, make most of your diet energy-dense foods such as nuts, dried fruits, dark chocolate, and high-fat dairy foods including full-fat milk.
17. Eat more eggs
We now know that egg consumption is not linked to higher levels of cholesterol in the body - aim to eat one or two each day. They are almost the perfect food and are eaten by athletes and bodybuilders to help with weight gain as part of their diet.
18. Get sufficient sleep
Sleep plays a vital role in how both our minds and bodies work and is essential to good health. A good night's sleep has also been shown to contribute towards effective digestion and the overall metabolic processes in the body.
19. Eat the right bread
Bread is one of the best sources of carbohydrate out there, and helps in weight gain. If possible go for wholemeal or brown grained bread as these are good low GI foods.
20. Add rice to your diet
Don't forget that a 200-250g bowl of rice (ideally brown) is an excellent source of carbohydrates and should always be included in your diet. If eaten with lean red meat this helps promote muscle growth.
Last updated: 18-06-2021
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