Low Appetite in Healthy People

1. Normal aging

Psychic/Emotional Causes

1. Emotional stress: anxiety, depression, grief or loss, relationship problems
2. Physical stress
3. Anorexia nervosa
4. Bipolar disorder
5. Bulimia
6. Social isolation

Infections

1. Childhood infections
2. Viral hepatitis 
3. Hemorrhagic fever
4. Bacterial endocarditis
5. Cyclosporiosis
6. Typhoid fever
7. Tuberculosis
8. Urinary tract infection
9. Other acute or chronic infections

Fever

1. Rheumatic fever
2. Still’s disease – systemic onset juvenile rheumatoid arthritis (JRA)

Nausea

Alcoholism

Smoking

Conditions and Diseases

1. SWALLOWING DIFFICULTIES: mouth pain, sore throat
2. STOMACH/DUODENUM: hiatus hernia, stomach/duodenal cancer or ulcer, pernicious anemia, gastroparesis
3. BOWEL: constipation, malabsorption, celiac disease, intestinal parasites/worms, colorectal cancer, inflammation (Crohn’s disease), infection (food poisoning)
4. APPENDIX: appendicitis
5. LIVER: viral hepatitis, cirrhosis
6. GALLBLADDER
7. HEART: congestive heart failure
8. HORMONES: diabetes mellitus, hypo/hyperthyroidism, hyperparathyroidism, adrenal insufficiency, hypopituitarism, APECED (Autoimmune PolyEndocrinopathy, Candidiasis, and Ectodermal Dysplasia)
9. KIDNEY: uremia
10. LUNGS: chronic pulmonary disease, pneumonia by Pseudomonas aeruginosa, psittacosis, silicosis
11. PANCREAS: pancreatitis
12. NERVES: transverse myelitis

General Diseases Affecting the Whole Body

1. DEHYDRATION
2. CANCER: metastases, lymphoma
3. BLOOD: anemia, leukemia

Nutrient/Vitamin Deficiencies

1. Malnourishment
2. Zinc deficiency
3. Milk-alkali syndrome – hypercalcemia, caused by excessive milk and calcium carbonate antacids consuption
4. Beriberi -vit B1 (thiamine) deficiency – in chronic alcoholics: tingling and numbness in  hands and feet; if heart is affected: shortness of breath on exercise, legs swelling
5. Dilutional hyponatremia. Causes: heavy vomiting or diarrhea, diuretics, heart failure, kidney disease, cirrhosis
6. Hypercalcemia
7. Hypomagnesemia
8. Arginase (enzyme that breaks down amino acid arginin) deficiency – inborn disorder, causes accumulation of arginin and ammonium in the body – muscle stiffness, shows up around age of 3. 
9. N-acetyl glutamate synthetase (NAGS) deficiency – a rare genetic disorder where a lack of enzyme causes ammonia accumulation in the blood

Pregnancy

1. Pregnancy itself
2. Acute fatty liver of pregnancy

Medications

1. Antibiotics
2. Glucocorticoids
3. Iron supplements
4. Antithyroid drugs (methimazole, propylthiouracil)
5. Oral contraceptives
6. Oral hypoglycemics
7. Tamoxifen
8. Cough and cold preparations
9. Codeine and Morphine
10. Demerol
11. Digitalis
12. Chemotherapeutic agents
13. Ephedrine and other sympathomimetics
14. Medications causing liver damage: chlorophorm, cyclopropane, ether, halothane, methoxyflurane, nitrous oxide, chloramphenicol, phenobarbital, 5-Fluorocytosine, metronidazole, ethionamide, vidarabine 
15. Amphetamine intoxication
16. Anabolic C-17

Poisoning

1. Lead
   • Lead pipes, old paints, house dust
   • Cosmetics, or remedies containing some traditional herbs: kohl, surma, bali goli; deshi dewa, ghasard, kandu, cordyceps, Hai Ge Fen, rueda, alacron, liga, Maria Luisa, etc
2. Herbal agent adverse reaction: chaparral, foxglove, henna, kombucha, cottonseed, garlic, germanium
3. Aluminium
4. Arsenic
5. Barium
6. Bismuth
7. Bromide
8. Cadmium
9. Chromium
10. Copper
11. Cresol
12. Cyclopropane
13. Glaze
14. Gold
15. Iron
16. Mercaptans
17. Mercury
18. Nickel
19. Selenium
20. Solder
21. Steroids
22. Thallium
23. Tin
24. Uranium
25. Ecstasy abuse 
26. Heroin withdrawal
27. Marijuana withdrawal
28. Aflatoxin- secreted by fungi on cereal (maize, sorghum, pearl millet, rice, wheat), oilseeds (groundnut, soybean, sunflower, cotton), spices (chillies, black pepper, coriander, turmeric, zinger), tree nuts (almonds, pistachio, walnuts, coconut) and milk
29. Radiation sickness

What to Do?

• Have small frequent meals
• Eat caloric foods you like
• Eat with others
• Have a breakfast
• Consider treating or overcoming the cause of low appetite

References:

1. What to do in low appetite? (cancer.org)