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What is obesity?

Obesity is a chronic metabolic condition defined by the World Health Organization as “abnormal or excessive fat accumulation that presents a risk to health.” In 2016, there were 1.9 billion overweight adults and 650 million obese adults worldwide. That same year, 41 million children under 5 years old and 340 million between 5 and 18 were classified as overweight.1

In the U.S. alone, approximately 18.5% of children and adolescents, and about 40% of adults over 20 years old, were obese in 2016.2

What are the common symptoms?

Obesity’s main symptom is an increased Body Mass Index (BMI). The BMI is calculated as a person’s weight in kilograms divided by the square of their height in meters (CDC). For an adult, a BMI between 25 and 30 kg/m2 is considered overweight, and a BMI greater than 30 is considered obese. These numbers may change according to the population to which a patient belongs. For example, metabolic problems are observed in Asian people starting at a BMI of 25.2 kg/m2,2,3,4,5 whereas for Europeans (BMI norms were based on this population), problems start at 30 kg/m2.

For children, being overweight is defined as a BMI between the 85th and 95th percentiles, and obesity is a BMI greater than the 95th percentile.

 What are the causes?

Several factors affect the risk of becoming overweight or obese:

Lifestyle risk factors:

  • An unhealthy diet high in calories and rich in fats and sugars
  • Bad sleeping habits
  • Sedentarism (i.e. lack of physical activity)1
  • Smoking, particularly when quitting, since nicotine withdrawal can prompt increased food consumption


  • Obesity can be a consequence of endocrine disorders such as Hypothyroidism and Cushing’s syndrome.6

Genetic risk factors:

  • Genes: some studies suggest that 40-70% of our BMI can be attributed to our genes.7
  • Sex differences: for example, women are more likely to become overweight/obese after pregnancy or during menopause. Men, on the other hand, tend to gain weight after their twenties, and continue to do so until their sixties.

How does this topic relate to my microbiome?

Much research has been conducted on the interaction between the microbiota and obesity. Increased numbers of several taxa inhabiting the colon – including Bacteroides, Fusobacterium, and Oscillospira – correlate with obesity. The abundance of other bacteria – including Akkermansia muciniphila, Bifidobacterium, Blautia, Butyrivibrio crossotus, Butyricimonas, Dialister, and Ruminococcus flavefaciens – is lower in obese patients.8,9,10,11 In addition, low microbial diversity in the gut is associated with obesity.9

Whether changes in microbial composition are a cause or consequence of obesity is an area of active research. It is also not clear if the differences in microbiome composition that was found in the studies mentioned above was associated with BMI or with the consumption of certain types of food items. A meta-analysis from 2016 found large variations between persons and studies that obscured the relationship between obesity and microbial communities.1212

Microbial diversity is a quantitative measure and it has great importance for the study of structure and function of microbial communities. Microbial diversity measures how many different kinds of microorganisms are present in a given sample, and how evenly the number of microorganisms are distributed over these different kinds.

Which diseases/topics are related to obesity?

Being overweight or obese is a common risk factor for many health conditions:

Metabolic conditions:

  • Diabetes mellitus13
  • Dyslipidemia14

Cardiovascular risks:

  • Hypertension13
  • Heart disease15
  • Stroke15
  • Venous thrombosis16


  • Colon
  • Postmenopausal breast
  • Endometrial
  • Kidney
  • Esophageal

Musculoskeletal conditions:

  • Osteoarthritis19
  • Gout20
  • Arthritis13

Gastrointestinal conditions:

  • Gastroesophageal reflux disease (heartburn)21
  • Gallbladder disease22
  • Non-alcoholic fatty liver23
  • Gastric cancer18

Reproductive and genitourinary conditions:

  • Irregular periods in women24
  • Decreased fertility24
  • Compromised renal function25
  • Glomerulopathy26
  • Kidney stones25
  • Urinary incontinence27

Mental conditions:

  • Depression28
  • Dementia29
  • Low self-esteem28

Respiratory conditions:

  • Sleep apnea30
  • Asthma13
Dyslipidemia refers to abnormal levels of lipids in the blood.
Glomerulopathy encompasses health conditions that affect the glomeruli of the nephron, a critical structure of the kidney that helps to filter blood and produce urine.

How can I take action?

It is possible to prevent becoming overweight or obese. Regular physical activity is important; The World Health Organization recommends 60 minutes of physical activity a day for children, and 150 minutes a week for adults.1

Healthy food habits – like a diet low in fats, sugars, and alcohol, and high in fruits, vegetables, nuts, and legumes – are also essential. The Mayo clinic recommends planning your meals, recording your food habits, and checking your weight regularly. This way, you can determine which habits help you maintain a healthy weight.31

After onset, treatment generally includes lifestyle changes, such as reducing caloric intake, modifying diet composition, and increasing time spent exercising. Doctors may recommend the use of certain medications to lose weight. In severe cases of obesity, when other treatments have failed, bariatric surgery is also an option.


1. World Health Organization. Obesity and overweight.

2. The state of Obesity. Obesity Rates & Trends Overview.

3. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. (2004). The Lancet, 363(9403), 157–163.

4. Tillin, T., Sattar, N., Godsland, I. F., Hughes, A. D., Chaturvedi, N., & Forouhi, N. G. (2015). Ethnicity-specific obesity cut-points in the development of Type 2 diabetes – a prospective study including three ethnic groups in the United Kingdom. Diabetic Medicine, 32(2), 226–234.

5. Ford, A. C., Moayyedi, P., & Hanauer, S. B. (2013). Ulcerative colitis. British Medical Journal, 346, f432–f432.

6. Müller, H. L. (2016). Craniopharyngioma and hypothalamic injury: latest insights into consequent eating disorders and obesity. Current opinion in endocrinology, diabetes, and obesity, 23(1), 81.

7. Locke, A. E., et al. (2015). Genetic studies of body mass index yield new insights for obesity biology. Nature, 518(7538), 197.

8. Haro, C., et al. (2016). The gut microbial community in metabolic syndrome patients is modified by diet. The Journal of nutritional biochemistry, 27, 27-31.

9. Le Chatelier, E., et al. (2013). Richness of human gut microbiome correlates with metabolic markers. Nature, 500(7464), 541.

10. Santacruz, A., et al. (2010). Gut microbiota composition is associated with body weight, weight gain and biochemical parameters in pregnant women. British Journal of Nutrition, 104(1), 83-92.

11. Schwiertz, A., Taras, D., Schäfer, K., Beijer, S., Bos, N. A., Donus, C., & Hardt, P. D. (2010). Microbiota and SCFA in lean and overweight healthy subjects. Obesity, 18(1), 190-195.

12. Sze, M. A., & Schloss, P. D. (2016). Looking for a signal in the noise: Revisiting obesity and the microbiome. MBio, 7(4).

13. Mokdad, A. H., Ford, E. S., Bowman, B. A., Dietz, W. H., Vinicor, F., Bales, V. S., & Marks, J. S. (2003). Prevalence of obesity, diabetes, and obesity-related health risk factors, 2001. JAMA, 289(1), 76–79.

14. Lee, C. M. Y., Huxley, R. R., Wildman, R. P., & Woodward, M. (2008). Indices of abdominal obesity are better discriminators of cardiovascular risk factors than BMI: a meta-analysis. Journal of clinical epidemiology, 61(7), 646-653.

15. Lavie, C. J., Milani, R. V., & Ventura, H. O. (2009). Obesity and cardiovascular disease: risk factor, paradox, and impact of weight loss. Journal of the American College of Cardiology, 53(21), 1925-1932.

16. Pomp, E. R., Le Cessie, S., Rosendaal, F. R., & Doggen, C. J. (2007). Risk of venous thrombosis: obesity and its joint effect with oral contraceptive use and prothrombotic mutations. British journal of haematology, 139(2), 289-296.

17. Basen-Engquist, K., & Chang, M. (2011). Obesity and cancer risk: recent review and evidence. Current oncology reports, 13(1), 71-76.

18. Vucenik, I., & Stains, J. P. (2012). Obesity and cancer risk: evidence, mechanisms, and recommendations. Annals of the New York Academy of Sciences, 1271(1), 37-43.

19. Bliddal, H., Leeds, A. R., & Christensen, R. (2014). Osteoarthritis, obesity and weight loss: evidence, hypotheses and horizons–a scoping review. Obesity reviews, 15(7), 578-586.

20. Aune, D., Norat, T., & Vatten, L. J. (2014). Body mass index and the risk of gout: a systematic review and dose–response meta-analysis of prospective studies. European journal of nutrition, 53(8), 1591-1601.

21. Chang, P., & Friedenberg, F. (2014). Obesity and GERD. Gastroenterology Clinics of North America, 43(1), 161–173.

22. Larsson, S. C., & Wolk, A. (2007). Obesity and the risk of gallbladder cancer: a meta-analysis. British Journal of Cancer, 96(9), 1457–1461.

23. Bedogni, G., Miglioli, L., Masutti, F., Tiribelli, C., Marchesini, G., & Bellentani, S. (2005). Prevalence of and risk factors for nonalcoholic fatty liver disease: the Dionysos nutrition and liver study. Hepatology, 42(1), 44-52.

24. Jacobsen, B. K., Knutsen, S. F., Oda, K., & Fraser, G. E. (2012). Obesity at age 20 and the risk of miscarriages, irregular periods and reported problems of becoming pregnant: the Adventist Health Study-2. European journal of epidemiology, 27(12), 923-931.

25. Taylor, E. N., Stampfer, M. J., & Curhan, G. C. (2005). Obesity, weight gain, and the risk of kidney stones. Jama, 293(4), 455-462.

26. Ejerblad, E., Fored, C. M., Lindblad, P., Fryzek, J., McLaughlin, J. K., & Nyrén, O. (2006). Obesity and risk for chronic renal failure. Journal of the American society of nephrology, 17(6), 1695-1702.

27. Hunskaar, S. (2008). A systematic review of overweight and obesity as risk factors and targets for clinical intervention for urinary incontinence in women. Neurourology and urodynamics, 27(8), 749-757.

28. Luppino, F. S., de Wit, L. M., Bouvy, P. F., Stijnen, T., Cuijpers, P., Penninx, B. W., & Zitman, F. G. (2010). Overweight, obesity, and depression: a systematic review and meta-analysis of longitudinal studies. Archives of general psychiatry, 67(3), 220-229.

29. Xu, W. L., Atti, A. R., Gatz, M., Pedersen, N. L., Johansson, B., & Fratiglioni, L. (2011). Midlife overweight and obesity increase late-life dementia risk A population-based twin study. Neurology, 76(18), 1568-1574.

30. Drager, L. F., Togeiro, S. M., Polotsky, V. Y., & Lorenzi-Filho, G. (2013). Obstructive sleep apnea: a cardiometabolic risk in obesity and the metabolic syndrome. Journal of the American College of Cardiology, 62(7), 569-576.

31. Mayo Clinic. Childhood Obesity.