Last reviewed: February 15, 2025

Last updated: February 15, 2025

Written by: Jay Hastings, CEO of PlexusDx

Jay Hastings is the CEO of PlexusDx, a precision health company focused on genetic testing, blood biomarker insights, and personalized wellness recommendations. He has more than 20 years of experience across healthcare innovation, genomics, laboratory operations, healthcare investing, and strategic finance.

Medically reviewed by: Jayden Lee, PharmD, EMBA

Jayden Lee, PharmD, EMBA, is the PlexusDx Medical Science Liaison with a PharmD and MBA specializing in pharmacogenomics and clinical product development, with a proven ability to bridge the gap between genomic research and practical patient outcomes. Dr. Lee has more than 10 years of professional experience in clinical pharmacy, academia, and research.

Largely, yes. The ability to digest milk sugar into adulthood is an inherited trait, and it is the minority pattern worldwide. According to MedlinePlus Genetics, approximately 65 percent of the human population has a reduced ability to digest lactose after infancy. That reduction is the default human condition; the ability to keep producing lactase throughout life is the inherited variation on it. There are also non-genetic causes of the same symptoms, which is why "is it genetic" has a more interesting answer than yes or no.

The Enzyme and the Sugar

Lactose is the principal sugar in milk. It is broken down by lactase, an enzyme produced by cells lining the small intestine. When lactase activity is insufficient, undigested lactose passes into the colon, where bacteria ferment it.

That fermentation is what produces the symptoms. MedlinePlus describes abdominal pain, bloating, flatulence, nausea and diarrhea beginning 30 minutes to 2 hours after consuming lactose-containing dairy products.

The National Institute of Diabetes and Digestive and Kidney Diseases distinguishes lactose intolerance — the symptoms — from lactose malabsorption, the underlying failure to digest. Not everyone with malabsorption develops symptoms.

What Is Inherited, Precisely

The LCT gene provides instructions for making the lactase enzyme. In most humans, LCT expression declines gradually after infancy, a pattern called lactase nonpersistence. That decline is the ancestral, majority condition.

Expression of LCT is controlled by a regulatory element located within a nearby gene called MCM6. Some people have inherited changes in that element which lead to sustained lactase production in the small intestine and the ability to digest lactose throughout life.

So the inherited difference is not in the enzyme itself but in the switch that keeps the enzyme gene switched on. That is an unusual and elegant piece of biology, and it explains why the trait is inherited as a stable family pattern.

The Population Pattern Tracks Dairying History

MedlinePlus reports that lactase nonpersistence is most prevalent in people of East Asian descent, affecting 70 to 100 percent of people in those communities, and is also very common in people of West African, Arab, Jewish, Greek and Italian descent.

Prevalence is lowest in populations with a long history of dependence on unfermented milk products as an important food source. Only about 5 percent of people of Northern European descent are lactase nonpersistent.

This is one of the clearest examples in human biology of a dietary practice and an inherited trait appearing together across populations. It is a population-level pattern, not a rule about any individual — ancestry does not determine what any one person can digest.

The Non-Genetic Causes Matter Too

Congenital lactase deficiency is a rare disorder caused by variants in the LCT gene itself, inherited in an autosomal recessive pattern. Affected infants cannot break down lactose in breast milk or formula and develop severe diarrhea. MedlinePlus notes it is most common in Finland, affecting an estimated 1 in 60,000 newborns.

Secondary lactase deficiency is acquired rather than inherited. Damage to the small intestinal lining — from infection, celiac disease, Crohn's disease or other causes — reduces lactase activity, and the capacity often returns once the underlying condition is treated.

That distinction has real consequences. New-onset dairy intolerance in an adult who previously tolerated it is not usually a genetic change; it is a reason to ask what has changed in the gut, and that is a question for a clinician.

Tolerance Is a Spectrum, Not a Switch

MedlinePlus notes that most people with lactase nonpersistence retain some lactase activity and can include varying amounts of lactose in their diets without experiencing symptoms. The trait is quantitative rather than binary.

Food form matters as much as quantity. Many affected individuals have difficulty with fresh milk but tolerate cheese or yogurt, because fermentation breaks down much of the lactose before the food is eaten. Hard aged cheeses contain very little lactose.

Colonic bacteria adapt as well, which is part of why gradual reintroduction is sometimes tolerated better than an abrupt reintroduction. What amount suits you is something worked out with a clinician or registered dietitian, not predicted in advance.

What Testing Can and Cannot Tell You

Clinical testing for lactose malabsorption includes hydrogen breath testing and lactose tolerance testing, both of which measure what your gut actually does with a lactose load. Those are functional tests, and they are ordered and interpreted by a clinician.

Genetic testing addresses a different question: whether you carry the inherited variation associated with sustained lactase production. It describes a biological tendency rather than a measurement of your current digestion, and it cannot account for secondary causes at all.

Neither replaces a clinical evaluation. Symptoms that look like lactose intolerance can also arise from celiac disease, inflammatory bowel disease, irritable bowel syndrome or infection — which is why self-diagnosis by elimination diet risks missing something treatable.

How Your Genetics Relate to GLP-1 Pathways

Not everyone responds to GLP-1 medications the same way. Genetic variants — including GIPR rs1800437, FTO rs9939609, and MC4R rs17782313 — relate to the biological pathways these medications act on. These are pathway-level associations only and do not predict how much weight you will lose or how you will respond to any specific medication. PlexusDx maps 14 pathways, 49 peptides, and 150+ genetic insights so you and your provider can see how your genes relate to these pathways. It does not recommend, prescribe, or determine which medication, dose, or peptide is right for you. The PlexusDx Precision Peptide Genetic Test ($298) gives you and your provider pathway-level genetic context to support a more personalized conversation. Genetics is a guide, not a guarantee.

Access Personalized GLP-1 Care Through PlexusDx

PlexusDx offers seven prescription GLP-1 protocols to all 50 states — no membership, no insurance required, async intake or live consult. The Tirzepatide Oral is $349/mo month-to-month, or from $279/mo on the 6-month plan. Medications are dispensed from licensed 503A compounding pharmacies following strict quality and safety standards. Add a Precision Peptide Genetic Test for $298 to personalize your protocol from day one.

Frequently Asked Questions

Is lactose intolerance genetic?

The common adult form largely is. LCT gene expression declines after infancy in most people, a pattern called lactase nonpersistence, and it is controlled by a regulatory element within the nearby MCM6 gene. Inherited changes in that element sustain lactase production through life. MedlinePlus reports roughly 65 percent of the human population has reduced lactose digestion after infancy.

Why do some populations tolerate dairy far better?

Prevalence tracks historical dependence on unfermented milk. MedlinePlus reports lactase nonpersistence affects 70 to 100 percent of people of East Asian descent and is also very common in people of West African, Arab, Jewish, Greek and Italian descent, while only about 5 percent of people of Northern European descent are affected. That is a population pattern, not an individual rule.

Can lactose intolerance develop later in life?

Yes, in two ways. Lactase nonpersistence itself emerges gradually as LCT expression declines after infancy, so symptoms can appear in adolescence or adulthood. Secondary lactase deficiency is acquired from damage to the small intestinal lining — infection, celiac disease, Crohn's disease — and lactase capacity often recovers once the underlying condition is treated.

Why can I eat cheese but not drink milk?

Because fermentation and ageing break down much of the lactose before the food reaches you. MedlinePlus notes that affected individuals often have difficulty digesting fresh milk but can eat certain dairy products such as cheese or yogurt without discomfort. Most people with lactase nonpersistence also retain some lactase activity and tolerate varying amounts of lactose.

Should I get tested, and what kind of test?

Speak with a clinician first, because symptoms resembling lactose intolerance also occur in celiac disease, inflammatory bowel disease, irritable bowel syndrome and infection. Clinical options include hydrogen breath testing and lactose tolerance testing, which measure what your gut does with a lactose load. Genetic testing describes an inherited tendency rather than current digestion.

Medical and Editorial Standards

Medical review process: This article was reviewed for medical accuracy, scientific clarity, evidence alignment, and appropriate discussion of genetics, medications, supplements, biomarkers, and health-related claims.

Sources and evidence: PlexusDx educational content is developed using peer-reviewed research, clinical literature, reputable medical references, and, where applicable, public health or regulatory guidance.

Commercial transparency: PlexusDx offers genetic testing, blood biomarker testing, personalized supplement recommendations, and related precision wellness services. Product mentions are intended to help readers understand available options and should not be interpreted as medical advice.

Important disclaimer: PlexusDx educational content is for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about medications, supplements, genetic testing, lab testing, or health-related care.