The ultimate guide to obesity

The ultimate guide to obesity
42% of US adults are obese— which raises the risk of diabetes and heart disease. We dig into its causes to help you gain back control of your health.
Written by Orchid Team
Medically reviewed by Dr. Jean Yun
  • Dr. Jean Yun, MD, is board certified in Internal Medicine, Endocrinology and Obesity Medicine. She has 5+ years of clinical experiences treating adult patients with different endocrine conditions and endocrine-related comorbidities. She has been involved in different medical and technical consultant roles improving patients' access and success in their health pursuits.

What is obesity?

Obesity is a condition in which an individual has an “excessive amount of bodyfat”. There are multiple ways to be diagnosed  with obesity, but the most common is having a Body-Mass Index (BMI) of 30 or more, which takes both height and weight into account. Obesity is also further classified into classes, from Class I (BMI between 30 and 34.9) to Class III (BMI > 40) obesity.  

What are the risk factors for obesity?

  • Genetics
  • Food environment
  • Behavioral factors

How do my genetics affect my risk of obesity?

Within a given environment, a given BMI is highly heritable, with an estimated heritability of 47% to 90%. This means that differences in genetics within a given environment are highly correlated with differences in body weight. The genetics of obesity are still being worked out, but the basics are the following: most differences in obesity in the general population are due to many variants of small effects, though with some exceptions; and the genetic variants that affect obesity risk are concentrated in the brain.

How does the food environment influence obesity?

Obesity rates have increased rapidly over the last 100 years, likely because of changes in the food environment, such as more processed food, higher food energy density, and higher palatability.

How do behavioral factors influence obesity?

Apart from the changing food environment driving more calorie intake, other behavioral changes like reduced activity levels may play a role. 

What are the consequences of obesity?

In general, the medical complications from obesity become more common and severe as someone’s weight becomes more extreme. The recommended treatments for obesity also scale with the degree of obesity, as well as other medical conditions individuals may have. Obesity matters because it raises the risk of a number of other diseases: type 2 diabetes, heart disease, obstructive sleep apnea, osteoarthritis, and many others. Supporting that, weight loss of 5-10% of body weight can bring disease risk closer to normal.

How common is obesity?

Obesity is very common: from 2017-2020, 42% of US adults were obese

Why do some people develop obesity?

Obesity is not perfectly understood, but individual differences in appetite and weight regulation are likely central to why some individuals develop obesity. In today’s food environment, filled with cheap, convenient, and hyper-palatable food, some people find it more difficult to maintain a normal body weight and there may be some biological reasons why. The exact details are still being worked out, but the leptin–melanocortin pathway is especially important, with both rare and common obesity-related variants showing up in genes in that pathway.

What’s my BMI?

Here is a calculator developed by the National Heart Lung and Blood Institute to calculate your own BMI.

Figure 1 NHLBI BMI calculator

Is there anything I can do to reduce my child’s risk of developing heart disease?

As explained in the severe obesity whitepaper, genetics plays an important role in determining the risk of obesity. Using Orchid’s embryo scoring, you can prioritize the embryo with the lowest genetic risk for obesity and reduce their risk of obesity.  

Is there anything I can do to reduce my risk of obesity?

Broadly speaking, there are lifestyle changes, medication, and surgery, which can all have varying effects on your weight. 


The classic advice of eating less and doing more exercise are, in theory, the cure-all for obesity.

Unfortunately, while strictly limiting calories reliably produces weight loss, only a fraction of people who try to lose weight succeed in sticking to an effective diet in the long-run. If you can stick with a diet, you’ve got a great shot at losing weight.

If you’re like most people, long-term weight loss will be difficult, since your body will tend to “defend” your previous weight: appetite will tend to increase as you lose weight. A more in-depth explanation of these changes is found here. Differences in how people respond to attempted weight loss, such as increases in appetite or a reduction in energy expenditure, likely account for why many people find it very difficult to lose weight.

For these reasons, while reducing calories and exercising more are excellent advice, most individuals with obesity will not normalize their weight through lifestyle changes alone.


Until recently, there had been few effective and safe long-term medications for weight loss. Some apparent successes had later been withdrawn because of safety concerns, such as rimonabant and fenfluramine. However, a recent class of drugs that more directly target appetite, the GLP-1 agonists, are both safe and effective. Liraglutide and semaglutide, although expensive, are approved for use in obese adults and obese adults with at least 1 weight-related condition, respectively.

Treatment guidelines for adults with obesity generally recommend lifestyle changes as the first step, with medication recommended for adults with a BMI > 30, or a BMI >27 with an obesity-related condition.


There are surgical treatments for obesity (bariatric surgery) that are more effective than drug therapy, even the new GLP-1 agonists, though the surgery comes with short-term (acute surgical complications) and long-term (vitamin deficiencies) complications. 

In general, surgical treatment of obesity is offered for patients with a BMI > 35 and at least one obesity-related medical comorbidity (especially Type 2 Diabetes). Because it is a major surgery, patient selection is important, and doctors are cautious about performing the surgery in sick patients. However, for those who qualify, it produces very impressive weight loss and health benefits. In some cases, perhaps because of the dramatic weight loss, it can reverse Type 2 diabetes, and is associated with a longer lifespan.

How is obesity diagnosed? 

Here is a calculator developed by the National Heart Lung and Blood Institute to calculate your own BMI.


  1. Obesity is a chronic condition with large effects on your health, such as increased risk of type 2 diabetes and heart disease.
  2. There are medical and surgical interventions that can effectively treat obesity.
  3. Diet and exercise, if strictly adhered to, can cause substantial weight loss, but long-term weight loss success without medication or surgery is uncommon.

Where can I learn more?

  1. The Hungry Brain by Stephan Guyenet PHD
  2. Dr. Peter Attia podcast on the VIRTA diet
  3. Podcast with research scientist Kevin Hall on different diets and weight loss
  4. The Art and Science of Low Carbohydrate Living by Jeff S. Volek, PhD RD and Stephen D. Phinney, MD PhD
  5. Healthy eating resources for patients
                 1. Just Enough for You: About Food Portions
                  2. USDA MyPlate
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