Most of us would be happy to lose a few pounds, with seemingly no effort on our part.
But, losing weight without cutting down calories or working out more in the gym, could be a sign of something sinister.
Rapid weight loss can be a symptom of a host of potential serious conditions from diabetes, stress, serious illnesses and even cancer.
In fact, if you unintentionally lose more than five per cent of your normal body weight, or more than 10lbs (4.5 kg) in less than a year, you should consult your doctor, experts advise.
Here, MailOnline explores why you should never ignore this critical health sign.
If you unintentionally lose more than 5 per cent of your normal body weight, or more than 10 lbs (4.5 kg) in less than a year, you should consult your doctor
An overactive thyroid could be to blame…
An overreactive thyroid, medically known as hyperthyroidism, can cause an array of symptoms including mood swings, muscle weakness and persistent thirst.
However, an overactive thyroid also causes a spike in the metabolism, which leads to rapid weight loss. This can also increase your appetite, the NHS explains.
That’s because the thyroid is responsible for producing hormones that help regulate the body’s metabolism. These hormones are called triiodothyronine (T3) and thyroxine (T4).
Medicines called thionamides can be used to stop the thyroid from producing excess hormones, but other treatments including radiotherapy to destroy the cells in the thyroid gland and surgery to remove all or part of your thyroid may be recommended.
An overactive thyroid also causes a very good metabolism, which leads to rapid weight loss
Rapid weight loss could be Type 1 diabetes…
Losing weight despite eating a high number of calories could also be a symptom of diabetes.
That’s because insufficient insulin prevents the body from getting glucose from blood to body cells to use as energy.
This causes the body to burn fat and muscle as energy instead which can trigger rapid weight loss.
‘Rapid weight loss can be a symptom of type 1 diabetes because one of insulin’s main jobs is to move sugar from the blood into the cells where it can be used for energy or stored for use later,’ Esther Walden, senior clinical advisor at Diabetes UK, told MailOnline.
‘When there is no insulin the cells become starved of energy, the body works harder to try to get rid of the excess sugar in the blood, and the excess sugar can’t be stored in the fat cells,’ she explained.
‘Additionally, when there is no insulin, the body starts to break down fat to produce a form of energy the body can use,’ she said.
This weight loss can happen quite quickly in the lead-up to a diagnosis of type 1 diabetes, she warns.
It’s a symptoms of Inflammatory bowel disease…
Crohn’s disease and ulcerative colitis are both types of inflammatory bowel disease, which cause tummy pain and diarrhoea.
The conditions also cause bleeding from your bottom, blood or mucus in your poo, constant tiredness and unintentional weight loss.
Although it is not clear what causes the conditions, which affect about 10million people globally, the NHS says it happens when your immune system attacks your bowel, which causes it to get inflamed.
It’s this inflammation in the gut which stops your body from fully absorbing nutrients from the food you eat, as a result leading to weight loss and malnutrition, according to Crohn’s & Colitis UK.
It happens in 80 per cent of people with certain cancers and other serious health condition…
A loss of appetite and unintentionally losing a significant amount of weight is a symptom which hits 80 per cent of people with upper gastrointestinal cancer, such as stomach cancer or liver cancer.
‘Sudden and unplanned weight loss can happen when you are going through stressful events in your life, or it can be linked to other underlying health conditions,’ Cancer Research UK’s health information manager, Maxine Lenza told this website.
‘Unexplained weight loss can also be a symptom of several types of cancer, including lung, bowel, and pancreatic cancer,’ she added.
Cachexia, also known as wasting syndrome, is a potential cancer symptom and causes extreme loss of muscle and fat, according to Cancer Research UK.
People with early-stage cancers don’t usually get cachexia. But up to four in five of people with advanced cancer develop some degree of cachexia, the charity explained.
But cachexia isn’t just a symptom of cancer.
This particular cause of rapid weight loss in cancer can also happen with HIV, kidney disease and heart disease.
In cancer, it’s thought to be driven by inflammation.
That’s because inflammation can promote muscle wastage and a tumour can take energy away from the rest of the body.
Cachexia also known as wasting syndrome causes extreme loss of muscle and fat, according to Cancer Research UK
Inflammation has also been linked to muscle loss because it causes protein to be broken down faster than it is being made. That’s because cancer can damage the mitochondria, the part of the cell responsible for converting nutrients into energy.
But catching these cancer symptoms early could make a big difference.
‘If you notice any unusual changes like losing a noticeable amount of weight without trying to, it’s important to speak to your doctor. In most cases it won’t be cancer, but if it is, finding it early can make a real difference,’ Ms Lenza said.
Stress and mental health could be a trigger…
Unintentional weight loss isn’t just a warning sign of a serious physical illness.
Going through a stressful event like losing a loved one, a divorce, breakup or losing a job can all contribute to weight loss, the NHS explains.
That’s because in some cases stress can cause a loss of appetite, indigestion, or repetitive nervous movements.
When you are stressed over an extended period it can cause stress hormones such as cortisol and adrenaline, to build up in your body which can have a variety of physical effects from disrupted sleep to a suppressed appetite.
Adrenaline, the ‘fight or flight’ hormone, usually gives us a boost to motivate us. But too much of it can affect our mood and even our bowels causing stomach cramps and constipation, the NHS warns.
Going through a stressful event like losing a loved one, a divorce, breakup or losing a job can all contribute to weight loss, the NHS explains
This can make us not want to eat, and as a result, cause weight loss.
Weight loss can also be due to nervous fidgeting and foot tapping which if done constantly can burn extra calories.
Stress isn’t the only mental condition that that can cause rapid weight loss, depression, anxiety and eating disorders can also be a reason for someone losing a significant amount of weight.
For example, the main symptom of anorexia is deliberately losing weight or keeping your body weight much lower than is healthy for your age and height.
Losing control of how much you eat and then taking drastic action to not put the weight on, is also a sign of bulimia.
It’s estimated that 1.25million people in the UK have an eating disorder, according to charity Beat.
Signs to watch out for include if someone is worrying about their weight or body shape, avoiding social occasions where food is involved, exercising too much and having strict habit around food.