Key Takeaways
- Twin studies attribute 47% to 90% of the variation in BMI within a population to genetic differences.
- Data show the shared family environment affects BMI most in mid-childhood.
- Trial participants still reported more hunger one year after losing about 30 pounds on a diet.
- Tirzepatide, sold as Zepbound for weight management, activates both GIP and GLP-1, receptors involved in appetite.
- People with a personal or family history of medullary thyroid carcinoma, or with MEN 2, must not use tirzepatide.
Obesity is partly inherited. Twin studies attribute most of the variation in BMI between people to genetic differences.
That figure describes a population and cannot say how much of one person's weight comes from genes. High genetic risk does not make obesity certain.
What We Inherit: Property, Traits, and Risk
Families pass on property by law or custom, traits through genes, and health risks through both genes and household habits.
A comparative legal analysis of Nigerian succession law describes how statute, customary law, and Islamic law decide who receives property. Some customs pass titles and status as well as land and goods.
Heritability is the share of variation in a trait across a population that comes from genetic differences. A heritability of 0.7 does not mean that genes cause 70% of a trait in one person.
Spoken language runs in families but has no genetic contribution, so it is not heritable. Heritability also says nothing about how easy a trait is to change.
How Much of Body Weight Is Genetic?
A review of 88 twin estimates found BMI heritability of 47% to 90%, with a middle value of 75%.
| Study type | Who was studied | Genetic share of BMI variation |
|---|---|---|
| Twin review | 140,525 twins | 47% to 90%, middle 75% |
| Family studies | 42,968 family members | 24% to 81%, middle 46% |
| Adoption studies | Adoptive and biological relatives | 20% to 60% |
| Child twins | 87,782 twin pairs under age 20 | 41% to 42% at age 4, 75% at age 19 |
| Adult twins | 140,379 adult twin pairs | 75% to 77% in the 20s, 57% to 59% in old age |
| DNA-based studies | 25,465 and 44,126 unrelated people | 27% to 30% |
An adoption study of 540 adults in Denmark found that adoptees' weight tracked their biological parents' BMI, not their adoptive parents' BMI. Researchers still debate why DNA-based estimates are lower than twin estimates.
Family Habits vs Genes
In twin data, the shared family environment affects BMI mainly in mid-childhood and is no longer detected after about age 15. Adult twins show no shared-environment effect.
Parental obesity more than doubled the risk of adult obesity for children under 10 in a US cohort study. The study cannot separate genes from the household.
In a Norwegian population study, BMI rose in both high and low genetic risk groups from the 1960s to the 2000s. The gap between high and low genetic risk widened. The study attributed the overall rise mainly to the environment.
About one in six people with the highest genetic risk scores still had a healthy BMI in a study of UK adults.
Why "Just Eat Less" Fails for Some People
Eating less can cause weight loss, but the body can respond with more hunger and lower energy use.
One trial put 50 nondiabetic adults with excess weight on a 10-week very-low-calorie diet. They lost an average of about 30 pounds.
One year later, several appetite hormones had not returned to pre-diet levels. Leptin was still lower and ghrelin still higher. Participants also reported more hunger than before the diet.
A six-year follow-up of 14 people after extreme weight loss found resting metabolism about 500 calories a day lower than expected for their size and age.
One model, called the set point, holds that the body defends a particular weight. Researchers debate this model, which does not explain the rise in obesity.
A consensus scientific statement describes obesity as a disorder of the body's energy-regulation system, not only passive weight gain. Taking a weight-loss drug is one medical treatment for obesity.
Treatments That Act on Biology: Tirzepatide
The prescription drug tirzepatide activates the receptors for GIP and GLP-1, two hormones released by the intestine.
Both receptors are found in brain areas involved in appetite regulation. Tirzepatide can reduce calorie intake, likely through its effect on appetite. It is a once-weekly injection.
| Product | What it is | FDA-approved uses |
|---|---|---|
| Mounjaro | Tirzepatide brand | Type 2 diabetes in adults and children 10 and older, and cardiovascular risk reduction in high-risk adults with type 2 diabetes |
| Zepbound | Tirzepatide brand | Chronic weight management, and moderate to severe obstructive sleep apnea in adults with obesity |
| Compounded tirzepatide | Tirzepatide mixed by a compounding pharmacy | None, and not reviewed by the FDA for safety, effectiveness, or quality |
Using Mounjaro for weight loss is off-label, meaning the FDA has not approved it for that use.
The FDA approved Zepbound for adults with a BMI of 30 or higher, or 27 or higher with a weight-related condition such as high blood pressure.
In a 72-week trial of 2,539 such adults without diabetes, average weight loss was 15.0%, 19.5%, and 20.9% on the three maintenance doses, versus 3.1% on placebo.
In a withdrawal trial in the Zepbound label, everyone took the drug for 36 weeks. Those switched to placebo then gained 14.0% in body weight over 52 weeks. Those who stayed on the drug lost a further 5.5%.
A post hoc analysis of the 72-week trial looked at 32 carriers of a faulty MC4R gene, the most common single-gene cause of obesity. Carriers on tirzepatide lost 18.3% of their weight, versus 19.9% for noncarriers. The Zepbound label reports no results by family history of obesity.
Common side effects can include nausea, diarrhea, vomiting, and constipation.
The FDA declared the tirzepatide shortage resolved in December 2024. The temporary allowance for compounding pharmacies ended in March 2025. The FDA has received more than 730 adverse event reports tied to compounded tirzepatide as of May 31, 2026.
Talking to a Doctor About Family History
A family health history helps a clinician decide which screening tests a person needs and when to start them. Details to bring include:
- Parents, siblings, children, grandparents, aunts, and uncles, with major conditions and age at diagnosis
- Whether one or both parents had obesity, since that raises the chance of obesity
- For a child, whether extreme obesity began before age five with extreme hunger or a family history of extreme obesity, since consensus guidance suggests genetic testing only then
- Any personal or family history of medullary thyroid carcinoma, or a diagnosis of multiple endocrine neoplasia syndrome type 2 (MEN 2), since both tirzepatide labels list these as contraindications
- Current BMI and weight-related conditions, since Zepbound eligibility rests on these, not on family history of obesity
Both tirzepatide labels carry a boxed warning. In rats, tirzepatide causes thyroid C-cell tumors. It is unknown whether it causes them in humans.
Sources
- Nigerian succession law: Otu and Nabiebu, Eur J Soc Sci, 2021
- Heritability definition: MedlinePlus Genetics, What Is Heritability?, 2021
- Twin and family heritability: Elks et al., Front Endocrinol, 2012; Silventoinen et al., Am J Clin Nutr (CODATwins), 2016; Silventoinen et al., Am J Clin Nutr (CODATwins), 2017
- Adoption: Maes et al., Behav Genet, 1997; Stunkard et al., N Engl J Med, 1986
- DNA-based heritability: Yang et al., Nat Genet, 2015; Wainschtein et al., Nat Genet, 2022
- Family environment and parental risk: Silventoinen et al., Int J Obes, 2010; Whitaker et al., N Engl J Med, 1997
- Gene-environment interaction: Brandkvist et al., BMJ (HUNT), 2019
- Polygenic scores: Khera et al., Cell, 2019
- Hunger and metabolism after weight loss: Sumithran et al., N Engl J Med, 2011; Fothergill et al., Obesity, 2016
- Weight regulation: Speakman et al., Dis Model Mech, 2011; Schwartz et al., Endocr Rev (Endocrine Society statement), 2017
- Tirzepatide labels: Zepbound label, 2026; Mounjaro label, 2026
- FDA approval: FDA Approves New Medication for Chronic Weight Management, 2023
- Tirzepatide trial: Jastreboff et al., N Engl J Med (SURMOUNT-1), 2022
- MC4R: Loos and Yeo, Nat Rev Genet, 2022; Bhatnagar et al., Nat Med, 2025
- Compounded tirzepatide: FDA Clarifies Policies for Compounders as National GLP-1 Supply Begins to Stabilize, 2024 to 2026; FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss, 2026
- Family history: CDC About Family Health History, 2024; NIDDK Factors Affecting Weight and Health, 2023
- Pediatric genetic testing: Styne et al., J Clin Endocrinol Metab (Endocrine Society guideline), 2017
This article is for general information and does not provide medical advice.