Causes of Weight Loss

Weight loss can be intentional (through diet and exercise) or unintentional. Unintentional or unexplained weight loss—especially when significant—can signal an underlying health issue and should be evaluated by a healthcare professional. The causes are diverse and can be grouped into several major categories:

Medical Conditions

• Cancer: Many types of cancer can cause weight loss, sometimes as an early symptom. Cancers of the pancreas, stomach, esophagus, bowel, lung, and blood (like lymphoma) are particularly associated with unexplained weight loss1579.

• Endocrine Disorders:

  • Hyperthyroidism (overactive thyroid): Increases metabolism, leading to rapid calorie burn even with normal or increased appetite12568.

  • Diabetes (especially Type 1): Uncontrolled diabetes can cause the body to burn fat and muscle for energy, resulting in weight loss1567.

  • Adrenal insufficiency (Addison’s disease): Leads to reduced appetite and weight loss due to low hormone levels56.

• Digestive Disorders:

  • Inflammatory Bowel Disease (IBD): Includes Crohn’s disease and ulcerative colitis, causing malabsorption and reduced appetite56.

  • Celiac Disease: An autoimmune reaction to gluten that impairs nutrient absorption56.

  • Chronic pancreatitis, peptic ulcer disease, and other GI disorders can also cause weight loss due to malabsorption or pain6.

• Chronic Infections:

  • Tuberculosis (TB), HIV/AIDS, endocarditis, and other chronic infections can result in weight loss through increased metabolism, decreased appetite, or malabsorption567.

• Chronic Heart and Lung Diseases:

  • Heart failure and chronic obstructive pulmonary disease (COPD) can cause weight loss, especially in advanced stages, due to increased energy expenditure and reduced intake256.

• Neurological Disorders:

  • Dementia, Parkinson’s disease, stroke, multiple sclerosis, and amyotrophic lateral sclerosis (ALS) can all contribute to weight loss through a combination of reduced appetite, difficulty eating, and increased energy needs168.

Mental Health and Social Factors

• Depression and Anxiety: These can reduce appetite and interest in food, leading to weight loss. In some cases, anxiety may increase metabolism or cause gastrointestinal symptoms that further reduce intake15679.

• Social Factors: Loneliness, poverty, social isolation, and difficulty obtaining or preparing food can result in weight loss, particularly in older adults69.

Medications and Substance Use

• Medication Side Effects: Many drugs can cause nausea, vomiting, dry mouth, altered taste, or other effects that reduce food intake. Examples include chemotherapy, certain antidepressants, and some diabetes medications69.

• Substance Abuse: Alcohol, amphetamines, cocaine, and other drugs can suppress appetite or increase metabolism, leading to weight loss467.

Other Causes

• Dental Problems: Poor dentition, gum disease, or pain with chewing can make eating difficult, resulting in weight loss169.

• Muscle Loss (Atrophy): Inactivity, aging (sarcopenia), neurological diseases, or malnutrition can cause muscle wasting and weight loss568.

• Stressful Life Events: Major stressors such as bereavement, divorce, or job loss can lead to reduced appetite and unintentional weight loss38.

Summary Table: Common Causes of Unintentional Weight Loss

Category	Examples
Cancer	Pancreatic, stomach, esophageal, bowel, lung, lymphoma
Endocrine Disorders	Hyperthyroidism, diabetes, Addison’s disease
Digestive Disorders	IBD, celiac disease, chronic pancreatitis
Infections	Tuberculosis, HIV/AIDS, endocarditis
Heart/Lung Diseases	Heart failure, COPD
Mental Health	Depression, anxiety
Neurological	Dementia, Parkinson’s, ALS, stroke
Medications/Substance	Chemotherapy, antidepressants, amphetamines, cocaine, alcohol
Social/Dental	Poverty, isolation, dental problems
Muscle Loss	Sarcopenia, inactivity, neurological disease
Stressful Events	Bereavement, divorce, job loss

Unintentional weight loss—especially if it exceeds 5% of body weight over 6–12 months—should prompt medical evaluation to identify and address any underlying cause7810.

1. https://www.mayoclinic.org/symptoms/unexplained-weight-loss/basics/causes/sym-20050700
2. https://my.clevelandclinic.org/health/symptoms/unexplained-weight-loss
3. https://www.nhs.uk/conditions/unintentional-weight-loss/
4. https://www.mountsinai.org/health-library/symptoms/weight-loss-unintentional
5. https://www.healthline.com/health/unexplained-weight-loss
6. https://www.merckmanuals.com/en-ca/professional/special-subjects/nonspecific-symptoms/involuntary-weight-loss
7. https://patient.info/signs-symptoms/weight-loss-unintentional
8. https://www.medicalnewstoday.com/articles/326417
9. https://www.uclahealth.org/news/article/unexplained-weight-loss-what-it-could-mean
10. https://www.mayoclinic.org/healthy-lifestyle/adult-health/in-depth/symptoms-not-to-ignore/art-20045276