Diets
Does regular (A1) milk upset your stomach more than A2?
The claim, precisely: A1 beta-casein worsens gastrointestinal symptoms
Probably not — clean independent trials pin the symptoms on lactose, not the protein type.
Evidence ladder
How far up the ladder this claim has climbed. A high consensus on a low rung means "consistent so far," not "proven in people."
Top evidence so far: Human trials (RCT / n-of-1)
How the studies fall
What the evidence shows
Whether A1 (vs A2) beta-casein worsens GI symptoms is CONTESTED and confounded: the positive trials are funded by the a2 Milk Company, while the best independent/objective-endpoint trials are null or find symptoms track LACTOSE, not casein type. A real distinct 'A1 intolerance' in lactose-tolerant people is not established.
The evidence (3)
| Source | Grade | Stance | Quality | Finding |
|---|---|---|---|---|
| Greenway F, et al. 2026 · (crossover) | RCT | tested-null | moderate | RCT w/ objective SmartPill transit: NO difference A1-free vs conventional |
| He M, et al. 2017 · Nutr J | RCT | supports | low | n=600 RCT: A1/A2 milk -> higher GI symptom scores vs A2-only (a2 Milk Company funded) |
| Mannila M, et al. 2025 · (crossover) | RCT | contradicts | moderate | Genotype-stratified RCT: symptoms tracked LACTOSE; A1/A2 difference vanished once lactose controlled |
Educational only, not medical advice. Grades and scores reflect published evidence weighted by study design and quality; see the methodology.