Underweight issues - the other end of the obesity spectrum

The media is full of scare stories about how many of us are obese and how much damage excess weight does to our health. But what happens when our bodies have less fat or essential nutrients than we need for good health?

The media is full of scare stories about how many of us are obese and how much damage excess weight does to our health. But what happens when our bodies have less fat or essential nutrients than we need for good health?

How do I know if I'm underweight?

Your body mass index, or BMI, is the relationship between your weight and your height. A BMI of 20-25 is ideal; 25-30 is overweight and over 30 is obese. If your BMI is under 18.5, you're considered underweight. If your BMI is 18.5-20, you're a bit underweight and can't afford to lose more. Your GP or practice nurse will be happy to work out your BMI or try this online calculator.

Some people naturally find it hard to put on weight. If your weight is constant and you have no long- term medical problems and a good diet, you probably don't need to worry. If you're malnourished, on the other hand, you definitely need to do something about it.

What's the difference between underweight and malnourished?

Scarily, over three million people in the UK are malnourished or at risk of malnutrition. Most of them are older and have long-term health problems - one in three people going into hospital or care homes have malnutrition. You don't have to be underweight to be malnourished - if, for instance, you've stopped eating healthily because of illness, you can lose weight and become malnourished even if your BMI isn't in the underweight range. If you've lost weight and you're now in the underweight range, you're at very high risk of malnutrition

How do I know if I'm malnourished?

If you've been ill and not eating properly, or if you're underweight, ask your doctor about screening you with a simple questionnaire called the MUST tool. In my practice, I often find relatives of elderly loved ones are the people who flag this up as a possible issue. If you're concerned about a loved one, try the MUST score for them and speak to their doctor if you think they're at risk.

The MUST score

Step 1 - what's your BMI? (over 20 scores 0, 18.5-20 scores 1 and under 18.5 scores 2)

Step 2 - have you lost weight without meaning to in the last 3-6 months? (under 5% weight loss scores 0, 5-10% scores 1, over 10% scores 2)

Step 3 - have you been seriously unwell so that you haven't eaten any proper food for at least 5 days? (this rarely happens unless you're unwell enough to be in hospital, but scores 2 if the answer is yes)

Step 4 - if your total score from steps 1-3 is 0, you're at low risk of malnutrition; if it's 1 you're at medium risk; 2 or more means high risk

What are the risks of being malnourished?

Possible problems include:

  • Getting more infections
  • Taking longer to recover from illness
  • Slow wound healing (eg, after surgery)
  • Thinning of the bones
  • Irregular heart rhythms
  • Higher risk of heart attack
  • Lack of periods and infertility

Could loss of weight mean something more?

There's a world of difference between people who lose weight because they're actively trying to become more healthy (congratulations!) and losing weight without meaning to. The most common reason for this that I see in my practice is overactive thyroid. However, a host of other conditions ranging from digestive problems such as Crohn's disease and inflammatory bowel disease to more serious conditions like cancer could be to blame. If you have lost weight without meaning to, please don't panic - cancer is not the most likely cause - but do see your GP to get it looked into.

How can I help myself?

Many people who are malnourished because of illness have simply been unable to eat enough calories. Here's the good news! You may be able to take up all those unhealthy foods, like full-fat milk and cream, on doctor's orders! Full-fat foods provide more calories in less volume.

  • Ask your doctor for a referral to a dietician to assess what you need
  • Eat small quantities frequently, especially if you get full quickly
  • Avoid filling up on fluids before a meal
  • Don't fill up on 'empty' calories like sugary sweets - if you're not eating much, getting enough vitamins and minerals is crucial
  • Ask about oral nutritional supplements. These are carefully designed to provide the protein, vitamins and minerals you need in small volumes. They come in a variety of sweet and savoury flavours. The National Institute for Health and Clinical Excellence (NICE) has recommended they should be considered for anyone who's malnourished, as they may help you recover more quickly and avoid hospital admissions. You may only need them for a few weeks
  • Do exercise when you can (see below).

Exercise for good health

We all know exercise can help you lose weight, so you should avoid it if you're underweight, right? Wrong! Regular exercise can counter the side effects of malnutrition and help you build muscle tone.

  • Weight-bearing exercise guards against thinning of the bones. Swimming is great gentle exercise for your heart and muscles, but doesn't help your bones
  • Yoga and Pilates can help you to tone up and keep supple without burning too many calories. They also help if you have joint problems like arthritis
  • Many councils run courses for older or less fit people, so you can start gently
  • Exercise improves balance and muscle strength. This cuts the risk of falls, which are a major problem in older patients and can lead to a loss of confidence and independence even if you don't suffer a major injury.


With thanks to 'My Weekly' magazine where this article was originally published.

Dr Sarah is unable to provide medical advice or respond directly to questions concerning your health. If you have health concerns we recommend contacting your GP.