A significant loss of total body weight is a serious, chronic illness. Substantial, unintentional weight loss is a symptom of acute or chronic illness, especially if other evidence is present.
Weight loss, for example, accompanied by early satiety, bilious vomiting of partially undigested food, postprandial epigastric pain and eructation may indicate Superior Mesenteric Artery Syndrome. Weight loss accompanied by insatiable thirst and hunger and fatigue may indicate diabetes mellitus, a chronic disease characterized by an abnormal accumulation of carbohydrates in the bloodstream due to insufficient production of insulin, a hormone produced in the pancreas that, when secreted into the bloodstream, permits cellular metabolism and utilization of glucose.
Poor management of type 1 diabetes mellitus, also known as insulin-dependent diabetes mellitus (IDDM), leads to an excessive amount of glucose and an insufficient amount of insulin in the bloodstream. This triggers the release of triglycerides from adipose (fat) tissue and catabolism (breakdown) of amino acids in muscle tissue. This results in a loss of both fat and lean mass, leading to a significant reduction in total body weight. Note that untreated type 1 diabetes mellitus will usually not produce weight loss, as these patients get acutely ill before they would have had time to lose weight.
Myriad of additional scientific considerations are applicable to weight loss, including but not limited to: physiological and exercise sciences, nutrition science, behavioral sciences, and other sciences.
One area involves the science of bioenergetics including biochemical and physiological energy production and utilization systems, that is frequently evidence of diabetes, and ketone bodies, acetone particles occurring in body fluids and tissues involved in acidosis, also known as ketosis, somewhat common in severe diabetes.
In addition to weight loss due to a reduction in fat and lean mass, illnesses such as diabetes, certain medications, lack of fluid intake and other factors can trigger fluid loss. And fluid loss in addition to a reduction in fat and lean mass exacerbates the risk for cachexia.
Infections such as HIV may alter metabolism, leading to weight loss.
Hormonal disruptions, such as an overactive thyroid (hyperthyroidism) , may also exhibit as weight loss.
 Intentional weight loss
Weight loss may refer to the loss of total body mass in an effort to improve fitness, health, and/or appearance.
Therapeutic weight loss, in individuals who are overweight, can decrease the likelihood of developing diseases such as diabetes. , heart disease, high blood pressure, stroke, osteoarthritis and certain types of cancer. For healthy weight loss, a physician should be consulted to develop a weight loss plan that is tailored to the individual.
Weight loss occurs when an individual is in a state of negative energy balance. When the human body is spending more energy in work and heat than it is gaining from food or other nutritional supplements, it will use stored reserves of fat or muscle.
Although weight loss may involve loss of fat, muscle or fluid, weight loss for the purposes of maintaining health should aim to lose fat while conserving muscle and fluid.
It is not uncommon for people who are already at a medically healthy weight to intentionally lose weight. In some cases it is with the goal of improving athletic performance or to meet weight classifications in a sport. In other cases, the goal is to attain a more attractively shaped body. Being underweight is associated with health risks such as difficulty fighting off infection, osteoporosis, decreased muscle strength, trouble regulating body temperature and even increased risk of death.
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 Therapeutic weight loss techniques
Main article: Bariatrics
The least intrusive weight loss methods, and those most often recommended by physicians, are adjustments to eating patterns and increased physical exercise.
Usually, health professionals will recommend that their overweight patients combine a reduction of the caloric content of the diet with an increase in physical activity. A good approach to do this might be to choose calorie-poor food and exercising at night (to avoid eating while watching TV, ...)  If however, you exercise a lot during the course of the day (or all day for that matter), it may be better to not try to reduce your calories in your food during the day at all (you will usually burn all of it anyway during the day), and just eat when hungry, and, in addition, only try to reduce your calories at night. This method may be better for those people exercising all day to avoid to lose energy too quickly, which often translates in eating a lot more at certain moments, usually to this degree/speed of eating that you can no longer assess you've had enough. A good indicator to tell you that you didn't eat too much is again the hunger feeling; exercise until you feel tired and/or hungry and then go to bed only slightly hungry. However, even if you do not watch your calorie-density of your food (by the method explained just above), it is always best that you eat slowly, or eat a bit and then wait a little. Often, your hunger disappears if you leave a little time as the stomach needs time to digest and tell your body you've had enough.  In addition, a much proposed method to hold oneself to a decreased calorie-diet is to increase the amount of water (or otherwise tea, coffee, mate, Kombucha, Near-beers, ...) you drink. When water or low-calorie drinks are not a working alternative/option, vegetables may be used instead. However, whenever possible (not hungry), (overweight) people are advocated of not eating/drinking at all, to allow the body/stomach to shrink, so that the person will automatically reduce the amount he eats (as the hunger feeling disappears), hereby normally (although not always) also decreasing the amounts of calories he consumes. Yet another method maybe the increasing of ambient temperature. Increasing the ambient temperature makes it much easier for certain people to endure the hunger feeling (as its intensity weakens with higher temperature  Increasing the temperature may be done by either heating up the house (trough radiators, convectors, ...) -or- by putting on more clothes (eg several layers thereof and making sure the ends of the clothes overlap). This latter method is recommended as it allows continuous high temperature (even if you go outside), is cheaper and also puts up less stress for the environment. Also, the implementation of a new hobby (eg listening music, ...)   Most of these methods have been proposed by nutritionists as BCM and other organisations involved in weight loss Also, nutritionists as BCM advocate reducing the amounts you eat entirely (to only 3 meals a day).
Other methods of losing weight include use of drugs and supplements that decrease appetite, block fat absorption, or reduce stomach volume. Especially medicine with herbs as Fucus vesiculosus are popular . Also, there are also some mental tricks (eg brushing your teeth early to avoid snacking at night, avoiding to buy high-calorie foods at the supermarket, ...) that -although they may be low-tech-, have return big results . Finally, a more radical (and usually less recommended, unless really necessairy) method is Surgery. Bariatric surgery artificially reduces the size of the stomach, limiting the intake of food energy. Some of these treatments may have serious side-effects.
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