Underweight BMI in women

What counts as underweight

The NHS and WHO both use a BMI of 18.5 as the line. Here's roughly where that falls for a few common heights. For every height and the full healthy range, see our ideal weight guide or the BMI chart.

Plenty of women sit just under 18.5 their whole adult lives, eat well, have regular periods and feel fine. The number is a prompt to check in with yourself, not a diagnosis.

Why a low BMI can matter

According to the NHS, being underweight can mean you're missing out on vitamins and minerals your body needs, which can lead to problems such as a weakened immune system and fragile bones. For women there are a few extra things to know:

• Periods. When body fat drops too low, your body can reduce the hormones that drive your cycle. Periods may become irregular or stop altogether.
• Fertility. For the same reason, a very low BMI can make it harder to get pregnant. Our guide to BMI and pregnancy covers this.
• Bones. Low oestrogen and too few nutrients both weaken bones over time, raising the risk of osteoporosis later in life.
• Energy. Feeling tired, cold or run down more often than you used to can be a sign your body isn't getting enough.

Why some women are underweight

There's no single reason, and often it's harmless. Common ones include:

• a naturally slim build that runs in the family
• being very active, especially in endurance sports (where BMI can also misjudge body fat)
• a health condition that affects appetite or how food is absorbed, such as an overactive thyroid or a bowel condition
• stress, anxiety or depression, which can quietly take away your appetite
• an eating disorder

When to see your GP

The NHS suggests talking to a GP if:

• you've lost weight suddenly
• you've lost weight without changing what you eat or how active you are
• you use controlling food as a way of coping with feelings or situations

It's also worth an appointment if your periods have stopped for three months or more and you're not pregnant. A GP can check for an underlying cause and, if you need it, help you put on weight safely.

Gaining weight in a healthy way

If you and your GP agree it would help to gain some weight, the NHS advice is steady and practical rather than "eat whatever you like":

• Aim for around 300 to 500 extra calories a day. Slow gain is easier on your body and more likely to last.
• Eat smaller meals more often, with snacks in between, especially if big plates feel off-putting.
• Choose foods that pack in nutrients as well as energy: nuts and nut butters, seeds, eggs, oily fish, full-fat yoghurt, beans and lentils.
• Try some strength training or yoga. It helps make sure the weight you gain includes muscle.
• Don't lean on sugary foods and drinks to make up the calories. They add energy without much else.