Unexplained weight loss: causes and when to get checked

Your weight can change for all sorts of reasons. You might eat differently, become more active, go through a stressful period or simply notice that your usual routine has changed.

But sometimes the number on the scales starts to fall and you haven't done anything to make it happen.

This is known as unexplained or unintentional weight loss. It can have many possible causes, from changes in appetite and stress to digestive problems, medication or hormonal conditions. In some cases, it can be linked to an underlying health condition.

So when should you pay attention to it, and what can you do if you don't know why you're losing weight?

What is unexplained weight loss?

Unexplained weight loss means losing weight without deliberately changing your diet or exercise habits.

It's normal for your weight to fluctuate. Your weight can vary from day to day depending on what you've eaten, how much fluid you're retaining and how active you've been.

The NHS notes that unintentional weight loss can have many different causes, including stress, mental health conditions, digestive problems, hormonal conditions, poor nutrition and side effects of some medicines.

The important thing is the pattern. A small change over a few weeks may not mean much. Weight that continues to fall without an obvious explanation is more worth investigating, particularly if you've noticed other changes in your health.

Has your routine changed?

Before worrying about what might be causing weight loss, think about what has changed in your life.

You may have:

• Started eating smaller portions
• Become more active
• Cut back on alcohol or snacks
• Been feeling stressed or anxious
• Had an illness or infection
• Started taking a new medicine
• Lost your appetite
• Developed dental problems that make eating uncomfortable
• Been eating differently because of digestive problems Sometimes the reason becomes clearer when you look back over the previous few months.

It's useful to keep track of your weight if you've noticed an ongoing change, along with any changes in appetite, diet, activity or other symptoms. This can help you see whether there's a pattern and gives you useful information to share with a healthcare professional if you need to.

Common causes of unexplained weight loss

There isn't one typical cause of unexplained weight loss. Several different things can affect your appetite, how much you eat or how your body uses and absorbs nutrients.

Stress and mental health

Stress can have a surprisingly noticeable effect on appetite.

Some people eat more when they're stressed, while others lose interest in food altogether. Anxiety and depression can also affect appetite, motivation and eating habits.

If you've been going through a difficult period, this may help explain a change in weight. But if you're continuing to lose weight without trying, it's worth looking at the wider picture rather than automatically putting it down to stress.

Changes in appetite

You don't necessarily have to feel ill to start eating less. Your appetite can change with age, illness, stress, medication or changes in your sense of taste and smell. You might also find that meals become less appealing or that you feel full sooner than you used to.

Over time, consistently eating less can lead to noticeable weight loss.

Medication

Some medicines can affect appetite, taste, digestion or the way your body processes food. If you've noticed weight loss after starting a new medicine or changing your dose, mention it to a GP or pharmacist. They can advise whether your medication could be contributing.

Don't stop taking prescribed medication without medical advice.

Dental problems or difficulty swallowing

Eating can become harder if you have dental pain, poorly fitting dentures or problems chewing.

Difficulty swallowing can have a similar effect. You may gradually start avoiding certain foods or eating less because meals have become uncomfortable.

These problems are worth addressing in their own right. A dentist, GP or another healthcare professional can advise on the appropriate next step.

Digestive problems

Conditions affecting the digestive system can sometimes cause weight loss by affecting appetite or how the body absorbs nutrients.

Ongoing diarrhoea, abdominal discomfort, bloating or changes in bowel habits may provide useful clues.

There are many possible causes of digestive symptoms, and most aren't serious. Persistent symptoms alongside unexplained weight loss are worth discussing so that the underlying cause can be considered.

Could hormones or blood sugar be involved?

Hormones play an important role in regulating appetite, metabolism and energy.

An overactive thyroid, for example, can increase the body's metabolism and cause weight loss even when you haven't deliberately changed your diet. Other symptoms can include feeling unusually warm, sweating more, anxiety, difficulty sleeping or a faster heartbeat.

Blood sugar can also be relevant.

Unexplained weight loss is one of the possible symptoms of type 2 diabetes, alongside increased thirst, needing to urinate more often, tiredness and blurred vision. However, many people with type 2 diabetes don't have obvious symptoms.

A blood test can help assess blood glucose. An HbA1c blood test, for example, gives an indication of average blood glucose levels over the previous two to three months.

This is one reason routine health checks can be useful. They can pick up changes that you might not notice from how you feel day to day.

When should unexplained weight loss be checked?

There's no single amount of weight loss that means the same thing for everyone.

A healthcare professional will consider factors such as:

• How much weight you've lost
• How quickly you've lost it
• Your age and general health
• Whether your appetite has changed
• Whether you're experiencing other symptoms
• Whether you've recently changed your diet or activity levels The NHS recommends speaking to a GP if you keep losing weight without changing your diet or exercise routine.

That doesn't mean every unexplained change needs an urgent appointment. It simply means that ongoing, unexplained weight loss is worth understanding rather than ignoring.

What does the latest NICE guidance say?

NICE updated its guidance on suspected cancer in April 2026, including a new recommendation specifically relating to unexplained weight loss.

For people aged 60 and over who have unexplained weight loss of more than 5% of their body weight within six months, NICE recommends assessing for additional symptoms, signs or findings and offering urgent investigation or an appropriate referral.

This doesn't mean that unexplained weight loss is a sign of cancer.

There are many possible explanations for losing weight without trying. The NICE guidance is about helping healthcare professionals decide when further investigation is appropriate, based on a person's age, weight loss and wider symptoms.

The recommendation is also specific to people aged 60 and over, so it shouldn't be interpreted as a universal threshold for everyone.

What happens when unexplained weight loss is investigated?

There's no single test that explains every case of unexplained weight loss.

A GP will usually want to understand what's been happening over time. They may ask about your appetite, diet, digestion, bowel habits, medication, mental health and any other changes you've noticed.

They may also check your weight and other basic measurements before deciding whether further tests are appropriate.

Depending on your symptoms, this could include blood tests or other investigations. Blood tests may look at areas such as blood count, thyroid function, blood sugar, liver function, kidney function or inflammation.

The tests you need will depend on your individual circumstances. You won't necessarily need every test listed here.

What if weight loss happens alongside bowel changes?

Weight loss can sometimes occur alongside changes in digestion or bowel habits.

A change in bowel habits can have many causes, including diet, stress, medication and infections. If changes persist, it's worth discussing them with a healthcare professional.

NICE recommends faecal immunochemical testing (FIT) in certain circumstances for people with symptoms that could indicate bowel problems. FIT looks for tiny amounts of blood in a stool sample and can help healthcare professionals decide whether further investigation is needed.

The test is not used because every person with bowel symptoms is thought to have a serious condition. It's one way of helping decide who may benefit from further investigation.

Why unexplained weight loss matters more as we get older

Weight naturally changes throughout life, and getting older can bring changes in appetite, activity and body composition.

But losing weight unintentionally can sometimes mean you're eating less than you need. This can affect muscle strength and nutritional health over time.

The NHS notes that losing 5% to 10% or more of your body weight over three to six months can be a sign of malnutrition, although someone can be malnourished at any body weight.

This is particularly worth remembering if an older family member seems to be eating less, has become noticeably weaker or their clothes are becoming looser.

It's not simply about the number on the scales. Maintaining muscle, strength and good nutrition is an important part of staying well as you age.

Should you track your weight?

If you've noticed unexplained weight loss, keeping a simple record can be useful.

You don't need to weigh yourself several times a day. Checking occasionally under similar conditions can give you a better idea of whether your weight is genuinely changing.

You could also make a note of:

• Your appetite
• Any changes to your diet
• Your activity levels
• New medication
• Digestive or bowel changes
• Your energy levels
• Any other symptoms you've noticed This information can be more useful than a single number because it shows what's happening over time.

Can a health assessment help you understand your health?

A health assessment can't diagnose the cause of unexplained weight loss, and if you're continuing to lose weight without trying, it's important to have that investigated appropriately.

But you don't have to wait until something feels wrong to pay attention to your health.

Blood pressure, cholesterol and blood sugar can change without obvious symptoms. A health assessment can provide information about these and other markers, depending on the checks included.

A Bluecrest Health Assessment can include measurements such as BMI and waist circumference alongside blood tests covering areas such as diabetes, cholesterol, liver function, kidney function and iron, depending on the package you choose.

Among surveyed Bluecrest customers, 84% said they would focus on a positive health aim after seeing their results.

That could mean making a change to your lifestyle, keeping an eye on a particular health marker or simply having greater confidence about what you're doing well.

Book a Bluecrest Health Assessment.