A person will lose weight when they expend more calories than they take in. There are a multitude of factors that can result in weight loss, and we can distinguish between intentional weight loss, or weight loss from changes a person is actively making, and unintentional weight loss, or weight loss that is happening without any intent to do so. Below are just a few of the diverse variety of factors and conditions contributing to weight loss:

1. Intentional Weight Loss

Intentional weight loss is typically the result of someone wanting to lose weight through established approaches such as changing their diet, exercise, or other medical interventions. Some common causes of weight loss are:

• Caloric Deficit: In order to lose weight, one must expend more calories than they take in. In nearly all cases, weight loss from caloric deficit is a two-step process, decreasing caloric intake and increasing levels of physical activity.

• Changes in Diet: Eating in accordance to a specific eating plan such as: low-carb, keto, or intermittent fasting, can be a significant influence on weight loss.

• Exercise: Increasing activity levels through aerobic exercise, strength training, or high-intensity interval training (HIIT) can increase calorie expenditure.

• Behavioral Change: A behavioral change, such as changing eating behaviors, reducing portion sizes, or simply thinking more carefully about food choices, can help you achieve your weight-loss goal.

2. Unintentional Weight Loss

Unintentional weight loss is when someone loses weight without trying to lose weight. Unintentional weight loss can occur for many reasons, including medical reasons or other reasons:

Medical Explanations

• Hyperthyroidism: Increased thyroid hormones fastens the metabolism making the individual lose weight rapidly, even if they have an increased appetite.

• Diabetes: In poorly controlled diabetes, some individuals may be breaking down muscle and fat to use for energy resulting in weight loss.

• Cancer: Certain types of cancer can result in notable weight loss because of effects such as a higher metabolism rate, lack of appetite, or the conversion of muscle and fat to energy by the body.

• Gastrointestinal Disorders: Disorders ranging from Crohn’s disease to celiac disease or ulcers may lead to nutrient malabsorption resulting in weight loss.

• Infections: Chronic infections like tuberculosis or HIV/AIDS can cause weight loss due to increased energy needs of the body, along with a lack of appetite.

• Nutrition: The lack of nutrition or not being able to absorb nutrients properly can lead to weight loss.

Psychological Causes

• Stress and Anxiety: Long-term chronic stress or anxiety can also cause weight loss and a decrease in appetite, and sometimes due to increased energy expenditure from stress, combined with reduced intake.
• Depression: Certain individuals diagnosed with depression lose interest in food which can lead to weight loss.
• Eating Disorders: Anorexia and bulimia are examples of eating disorders that can cause severe and intentional weight loss due to a poor body image or poor eating habits.

Medications and Treatments

• Chemotherapy: Chemotherapy for cancer frequently results in nausea and a loss of appetite, which may cause unwanted weight loss.

• Medications: Weight loss can be a side effect of certain medications, such as those given for hyperthyroidism, or certain types of diuretics.

Age-Related Causes

• Sarcopenia: With aging there is decrease in muscle mass without replacement, and this may lead to weight loss and/or changes in body composition.

• Decreased Appetite: Older adults may experience change in their sense of taste and smell or loss of appetite, which can lead to less food consumed and unwanted weight loss.