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Most of us would be happy to lose a few pounds without seemingly making any effort on our part.
But losing weight without cutting calories or working out more at the gym could be a sign of something sinister.
Rapid weight loss can be a symptom of a number of potentially serious illnesses, including diabetes, stress, serious illness, and even cancer.
In fact, if you unintentionally lose more than five percent of your normal body weight, or more than 10 pounds (4.5 kg) in less than a year, you should see your doctor, experts advise.
Here, MailOnline explores why you should never ignore this important health sign.
If you unintentionally lose more than 5 percent of your normal body weight, or more than 10 pounds (4.5 kg) in less than a year, you should see your doctor.
An overactive thyroid could be to blame…
An overactive thyroid, medically known as hyperthyroidism, can cause a variety of symptoms including mood swings, muscle weakness, and persistent thirst.
However, an overactive thyroid also causes a sudden increase in metabolism, leading to rapid weight loss. This can also increase appetite, the NHS explains.
This is because the thyroid is responsible for producing hormones that help regulate the body’s metabolism. These hormones are called triiodothyronine (T3) and thyroxine (T4).
Medications called thionamides may be used to stop the thyroid from making too much hormone, but other treatments may be recommended, including radiation therapy to destroy thyroid gland cells and surgery to remove all or part of the thyroid.
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 lot of calories could also be a symptom of diabetes.
This is because insufficient insulin prevents the body from transporting glucose from the blood to the cells to be used as energy.
This causes the body to burn fat and muscle for energy, which can lead to rapid weight loss.
“Rapid weight loss can be a symptom of type 1 diabetes because one of the main functions of insulin is to move sugar from the blood into cells, where it can be used for energy or stored for later use,” Esther Walden, senior clinical adviser at Diabetes UK, told MailOnline.
“When there is no insulin, the cells are left without energy, the body works harder to try to remove excess sugar from the blood and the excess sugar cannot be stored in fat cells,” he explained.
“Also, when there is no insulin, the body begins to break down fat to produce a form of energy that the body can use,” he said.
This weight loss can occur quite rapidly in the period leading up to a diagnosis of type 1 diabetes, she warns.
It is a symptom of inflammatory bowel disease…
Crohn’s disease and ulcerative colitis are two types of inflammatory bowel disease that cause abdominal pain and diarrhea.
These conditions also cause bleeding from the bottom, blood or mucus in the stool, constant fatigue and unintentional weight loss.
Although it is not clear what causes the condition, which affects around 10 million people worldwide, the NHS says it occurs when the immune system attacks the gut, causing it to become inflamed.
It is this inflammation in the gut that prevents the body from fully absorbing nutrients from the foods you eat, leading to weight loss and malnutrition, according to Crohn’s & Colitis UK.
It occurs in 80 percent of people with certain types of cancer and other serious health conditions…
Loss of appetite and unintentional loss of a significant amount of weight is a symptom that affects 80 percent of people with upper gastrointestinal cancer, such as stomach cancer or liver cancer.
“Sudden, 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 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 symptom of cancer and causes extreme loss of muscle and fat mass, according to Cancer Research UK.
People with early-stage cancer do not usually suffer from cachexia, but up to four in five people with advanced cancer develop some degree of cachexia, the charity explained.
But cachexia is not just a symptom of cancer.
This particular cause of rapid weight loss in cancer can also occur with HIV, kidney disease, and heart disease.
In the case of cancer, it is believed to be caused by inflammation.
This is because inflammation can promote muscle wasting, and a tumor can rob the rest of the body of energy.
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 proteins to break down faster than they are produced. This is because cancer can damage mitochondria, the part of the cell responsible for converting nutrients into energy.
But detecting these cancer symptoms early could make a big difference.
“If you notice any unusual changes, such as losing a noticeable amount of weight without trying, it’s important to talk to your doctor. In most cases it won’t be cancer, but if it is, catching it early can make a real difference,” Ms Lenza said.
Stress and mental health could be a trigger…
Unintentional weight loss is not just a warning sign of a serious physical illness.
Going through a stressful event such as losing a loved one, a divorce, a break-up or losing a job can contribute to weight loss, the NHS explains.
This is because in some cases stress can cause loss of appetite, indigestion or repetitive nervous movements.
When you are stressed for a prolonged period of time, 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 sleep disruption to appetite suppression.
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 gut, causing stomach cramps and constipation, the NHS warns.
Going through a stressful event such as losing a loved one, a divorce, a break-up or losing a job can 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 caused by fidgeting and foot tapping, which, if done consistently, can burn extra calories.
Stress is not the only mental condition that can cause rapid weight loss, depression, anxiety and eating disorders can also be a reason for someone to lose a significant amount of weight.
For example, the main symptom of anorexia is deliberately losing weight or keeping body weight much lower than is healthy for age and height.
Losing control of what you eat and then taking drastic measures to avoid gaining weight is also a sign of bulimia.
An estimated 1.25 million people in the UK suffer from an eating disorder, according to the charity Beat.
Signs to look out for include if someone worries about their weight or body shape, avoids social events that involve food, exercises too much, and has strict habits around food.