Summary

Your MCM6/LCT result is one of the most definitive findings in consumer genetics — it tells you whether you maintain the ability to digest lactose (milk sugar) into adulthood, directly shaping how dairy fits into your diet with zero ambiguity.

Genotype spectrum

AA (T/T — lactase persistent, homozygous)

Dairy is a convenient, nutrient-dense food source you can use freely. Milk, yoghurt, cheese, and whey protein are all available to you without GI consequences.

AG (T/C — lactase persistent, heterozygous)

Same as AA — full dairy access. You are genetically indistinguishable from AA in terms of lactose digestion.

GG (C/C — lactase non-persistent)

You have a definitive answer. No more guessing whether dairy "agrees" with you.

Practical takeaway

For GG Carriers (Lactase Non-Persistent)

Immediate dietary adjustment:
• You do not need to eliminate all dairy. Most lactose-intolerant individuals tolerate 12g of lactose (one cup of milk) per sitting, especially when consumed with other foods. Split dairy intake across the day rather than consuming large amounts at once.
• Fermented dairy is your best friend. Yoghurt (especially Greek — strained, lower lactose), aged hard cheeses (parmesan, cheddar, Swiss, Gouda — virtually lactose-free), kefir (bacterial cultures pre-digest lactose).
• Avoid or limit: liquid milk, ice cream, soft fresh cheeses (ricotta, cottage cheese), cream-based sauces in large portions.
• Whey protein isolate (not concentrate) has most lactose removed and is usually well-tolerated. Check labels for "lactose-free" whey isolate if sensitive.
• Lactase enzyme supplements (e.g., Lactaid) taken immediately before dairy are effective. Carry them for social eating situations.

Calcium strategy (critical):
• Target: 1000mg/day (1200mg if >50 years old or postmenopausal).
• Non-dairy calcium sources: kale (250mg/cup cooked), bok choy (160mg/cup), broccoli (60mg/cup), sardines with bones (325mg/can), canned salmon with bones (180mg/half can), calcium-set tofu (350mg/half cup), fortified plant milk (300mg/cup — check labels, shake well), fortified orange juice, almonds (75mg/quarter cup).
• Vitamin D supports calcium absorption — ensure adequate status (see pillar guidance).
• Track calcium intake for 1-2 weeks to establish whether you're meeting targets without dairy. Most people who avoid dairy unknowingly under-consume calcium.

What "working" looks like:
• GI symptoms resolve when dairy is modulated (not necessarily eliminated).
• Calcium intake meets daily targets from a mix of food sources.
• No nutritional gaps from dairy avoidance.

Expected response: Immediate — symptoms resolve within days of appropriate dairy modulation. This is not a condition that needs weeks to respond.
For AG or AA Carriers (Lactase Persistent)
• Dairy is fully available to you. Use it as a practical protein and calcium source.
• Whey protein is highly bioavailable and well-tolerated for you. Excellent post-training

Evidence detail

What This Gene Does

LCT produces lactase, the enzyme that breaks down lactose (the sugar in milk) in your small intestine. Every human produces abundant lactase as an infant — it's essential for digesting breast milk. In most of the world's population (~65-70%), lactase production declines after weaning and effectively switches off by adulthood. This is the ancestral, default state: lactase non-persistence.

A regulatory mutation in the nearby MCM6 gene (not in LCT itself) keeps the LCT gene switched on throughout life. This mutation arose ~7,500-10,000 years ago in Northern European pastoral populations where dairy farming created strong selective pressure. If you carry the A allele (T on the coding strand), your lactase stays on. If you're GG, it switches off. This is a binary, dominant trait — one copy of A is enough.

Mechanism

The mechanism here is unusually clean and well-understood:

1. LCT gene on chromosome 2 encodes lactase-phlorizin hydrolase, the brush border enzyme in the small intestine that cleaves lactose (a disaccharide) into glucose and galactose for absorption.
2. MCM6 regulatory region: The rs4988235 variant sits ~14kb upstream of LCT in intron 13 of the MCM6 gene. It functions as a cis-acting enhancer of LCT transcription.
3. A allele (persistence): Maintains enhancer activity throughout life → lactase continues to be produced at childhood levels → lactose is digested normally.
4. G allele (non-persistence): Enhancer activity declines after weaning → lactase production drops to 5-10% of infant levels by adulthood → lactose passes undigested to the colon.

What happens when lactose isn't digested:

Undigested lactose reaches the colon, where gut bacteria ferment it, producing hydrogen gas, carbon dioxide, methane, and short-chain fatty acids. This causes the classic symptoms: bloating, gas, cramps, diarrhoea. Severity depends on dose (more lactose = more symptoms), gut microbiome composition (some bacteria produce more gas than others), and intestinal transit time.

Why fermented dairy works for GG genotypes:

During yoghurt and cheese production, bacterial cultures digest most of the lactose. Hard aged cheeses (parmesan, cheddar, Swiss) contain <1g lactose per serving — well below the ~12g threshold that most lactose malabsorbers can handle. Yoghurt contains live bacteria that continue digesting lactose in the gut (bacterial beta-galactosidase).

Dominant inheritance:

One A allele is sufficient because the enhancer operates on the cis chromosome — you only need one active copy of the LCT gene to produce enough lactase for full digestion. This is why AG and AA are phenotypically identical.

MCM6 Lct Lactase

Summary

Genotype spectrum

Practical takeaway

Evidence detail

Sources (7)