While most people would be happy to suddenly lose weight, unexplained weight loss that occurs without any change in diet or activity level can be a sign that something is wrong. Dieting, loss of appetite, diarrhea and some medications are all easily identifiable causes of weight loss, but if you suddenly lose 10 pounds, 5 percent of your body weight, or if your weight loss persists and you can't explain why, it's time to look at other possibilities.
It makes sense that sudden weight loss can be caused by problems with your gastrointestinal tract. Some conditions cause you to lose your appetite, which will cause you to eat less and lose weight, while others will block your ability to absorb the nutrients that you are eating, causing your body to react as if you were not eating at all. Parasite infections due to drinking tainted water can also result in your body being robbed of the nutrients you eat.
Your endocrine system regulates your hormones, which regulate many of your bodily functions. When these bodily functions become overactive, your body burns more calories than usual, causing you to lose weight (and possibly feel inexplicably tired) even when eating the same diet. Hyperthyroidism, or having an overactive thyroid gland, increases your body's metabolism and not only causes extra calories to be burned, but also speeds up how quickly your body processes food, meaning you absorb less.
Though parasites fall under this category, many other infections can cause weight loss as well, such as HIV, tuberculosis and fungal disease. When your body is infected, much energy is required to fight it. Calories are burned and often your body heats up (fever) and you become fatigued because so much energy is being expended on the infection. It is no surprise that someone suffering from an underlying infection would lose weight even without changing their diet.
Cardiovascular, Lung and Kidney Disease
Cardiovascular and lung diseases are a strain on the body and also require excess amounts of energy for a person to perform even everyday tasks and for the body to function. Congestive heart failure and chronic obstructive pulmonary disease (COPD) are two examples of this. Kidney disease can cause nausea, vomiting and loss of protein through urine, leading to weight loss as well. Chronic illnesses in general tax the body and can cause weight loss.
Perhaps the most feared possible cause of unexplained weight loss is cancer. A tumor requires nutrients to grow, and it takes these nutrients from your body via your blood stream. In addition, cancer can demand excess energy from your body just like other illnesses, often compounding the effect. If you have been eating the same diet and haven't changed your activity level but have lost a significant amount of weight, it would be prudent to have a talk with your doctor.
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