Low-fat A2 milk was tolerated during treadmill running

May 18, 2026

Milk-based drinks are not the obvious first choice for most runners during exercise. This study asked a practical question: does low-fat, lactose-free A2 milk feel harder on the gut during running than cycling?

Reference: Garzón-Mosquera et al. Gastrointestinal tolerance to a standardized milk-based hydration strategy is similar across exercise modalities. Frontiers in Nutrition (2026) DOI: https://doi.org/10.3389/fnut.2026.1800364.

Study snapshot

A quick, practical summary for runners and coaches.

Quick answer

The study tested whether low-fat, lactose-free A2 cow’s milk caused different gut symptoms during treadmill running and stationary cycling. The overall gut symptom burden was similar during both exercise modes.

The main caution is that the study was short, small, and lab-based. It does not show whether milk is a good race drink for marathons, trail races, or ultras.

Key takeaways

Low-fat, lactose-free A2 milk was similarly tolerated during treadmill running and cycling.
The study tested physically active adults, not marathon runners or ultrarunners in real races.
Runners should treat the finding as a reason to test carefully in training, not as race-day proof.

How confident should we be?

Evidence confidence: Moderate

The crossoverCrossover means that all subjects completed all interventions (control and treatment) usually with a wash-out period in between. design was useful because each participant completed both exercise conditions. But the study was small, short, mostly male, and tested one milk drink in controlled lab conditions.

Bottom line

Low-fat, lactose-free A2 milk was not harder to tolerate during 60 minutes of treadmill running than cycling in this study. That is interesting, but runners should not overhaul their race nutrition based on it. The study supports cautious training-based testing, not a sudden dairy-based revolution in the aid station.

Read the deep dive below for a practical interpretation, actionable decisions, my thoughts, my Rating of Perceived scientific Enjoyment, the study details, full results, strengths, and limitations.

Running science research reviews for endurance runners

The deep dive

The details behind the headline result, including the practical meaning, full findings, limitations, and my interpretation.

Practical meaning

What does this research mean for runners and coaches?

The study suggests that running impact alone may not make a milk-based drink much harder to tolerate than cycling when the exercise intensity, drink volume, and drink composition are controlled.

That matters because running often gets blamed for gut symptoms. Sometimes fairly. The gut does get jostled about during running, and many runners know the horror of a mid-run stomach mutiny. But the study suggests the drink, the dose, the exercise intensity, the duration, and the individual runner may matter at least as much as the exercise mode itself.

This is practical, but only within a narrow lane. The study tested 60 minutes of treadmill running in young, physically active adults who did not have lactose intolerance or known adverse reactionsA harmful or unwanted effect caused by a treatment or intervention. It is a negative outcome. to cow’s milk. It did not test marathon pace, trail descents, heat stress, ultra-distance events, race nerves, high carbohydrate fuelling, or repeated aid-station intake over several hours.

For runners

This finding may interest runners who tolerate dairy and are curious about milk-based hydration during shorter training sessions. It is most relevant to controlled runs of about an hour, not long races.

For road runners, the study suggests that low-fat, lactose-free A2 milk is not automatically unsuitable during running just because running involves impact. For trail runners and ultrarunners, the relevance is weaker because terrain, duration, heat, fatigue, and cumulative fuelling load become much bigger issues.

For coaches

The study is useful for thinking about gut tolerance testing. It supports the idea that a drink tested during cycling may sometimes translate to running better than expected, provided the exercise intensity and intake are similar.

But coaches should still individualise. The study does not prove that milk is a good race-fuelling strategy. It shows that this specific milk drink produced similar overall gut symptom scores during two controlled exercise modes.

Practical decision

Should runners change anything?

Maybe. Runners who already tolerate milk could consider testing low-fat, lactose-free A2 milk during short, low-risk training sessions. That is a practical interpretation, not a direct race recommendation from the study.

Consider this if

You tolerate cow’s milk well.
You want to test a milk-based drink during shorter training runs.
You are doing a session where gut discomfort would be annoying, not disastrous.
You are curious about a drink that provides fluid, carbohydrate, sodium, potassium, and protein.

Do not overreact if

You are lactose intolerant or dairy-sensitive.
You are preparing for a marathon, trail race, or ultra.
You already have a race drink that works.
You need a high-carbohydrate fuelling plan for long events.

A sensible next step

Test a small amount in training before trying anything close to the study dose. Keep the first test easy, local, and preferably near a toilet. Heroism is optional; planning is better.

TIP: Never make any major changes to your training or lifestyle habits based on the findings of one study, especially if the study is small or provides low-quality evidence. Check whether other trials confirm the findings. If there is a meta-analysisA meta-analysis quantifies the overall effect size of a treatment by compiling effect sizes from all known studies of that treatment. on the topic, look at the effect sizeA standardised measure of the magnitude of an effect of an intervention. Unlike p-values, effect sizes show the size of the effect and how meaningful it might be. Common effect size measures include standardised mean difference (SMD), Cohen’s d, Hedges’ g, eta-squared, and correlation coefficients., the variability between studies, and the quality of evidence.

Expert interpretation

My thoughts

Running science from Thomas Solomon at Veohtu

This is a tidy and useful study because it challenges a common assumption: that running must be much worse than cycling for gut tolerance because of the impact.

Under the study conditions, that assumption did not really hold. The overall gut symptom burden was similar during treadmill running and stationary cycling, even with a fairly chunky milk dose. That’s interesting, and it gives coaches and runners a useful reminder: gut tolerance is not just about the sport. It is also about the drink, the amount, the intensity, the duration, the heat, and the person drinking the stuff.

Still, I would keep the practical interpretation modest. The study does not show that milk improves performance, works as a marathon drink, or suits runners with dairy sensitivity. It shows that young, physically active adults who tolerate milk could complete 60 minutes of controlled treadmill running while drinking low-fat, lactose-free A2 milk, with similar overall symptoms to cycling.

So, should runners start racing on milk? No. Should curious, dairy-tolerant runners test it carefully in training if they want to? Sure. But let’s not pretend this turns the marathon drinks table into a fridge aisle.

My Rating of Perceived scientific Enjoyment

RPsE: 5/10

I experienced low scientific enjoyment because, although the crossover design was tidy and the reporting was transparent, the study was small, short, lab-based, mostly male, and only partly relevant to real-world endurance racing. Plus, not comparing the low-fat, lactose-free A2 milk to a control or placebo kinda makes me wonder why this study was done.

Research question

What did the researchers ask?

The researchers asked whether exercise mode changes gut tolerance to a milk-based hydration drink.

More simply: does running make low-fat, lactose-free A2 milk harder on the stomach than cycling?

The question matters because running is often assumed to provoke more gut discomfort than cycling. The researchers wanted to isolate the effect of exercise mode by controlling the drink, the dose, the duration, and the exercise intensity.

Study design

What type of study was this?

The study was a randomized crossover trialA study in which a group of people is randomised to receive BOTH the treatment and the no-treatment control, and the outcome of interest is measured before and after both. The “crossover” means that all participants complete all interventions (the control and the treatment), usually with a washout period in between..

Each participant completed both exercise conditions: treadmill running and stationary cycling. The order was randomizedRandomization means assigning people to different parts of a study (e.g., groups in a randomised controlled trial) by chance, not by choice. This helps make the groups similar at the start and reduces bias, so any differences you see are more likely due to the treatment, not background differences. In a crossover study, randomization usually decides the order in which each person gets the treatments (for example, Treatment A first then B, or B first then A). This way, order effects—like learning, fatigue, or simple time passing—are less likely to skew the results. by coin flip, and the sessions were separated by at least 48 hours.

This design is useful because each participant acts as their own control. That reduces the influence of individual differences in gut sensitivity, fitness, and drink preference.

But the design can only answer the question under the study conditions. It cannot tell us whether milk works well during longer races, hotter environments, higher intensities, or repeated use over weeks and months.

Participants

Who took part?

The study included 28 physically active adults.

The participants were:

22 males and 6 females.
25.5 ± 5.0 years old.
67.3 ± 11.0 kg.
Familiar with both treadmill running and cycling.
Doing endurance exercise at least 3 times per week, for at least 30 minutes per session, for at least 6 months.
Free from diagnosed metabolic, gastrointestinal, or cardiovascular disease.
Not pregnant or lactating.
Not lactose intolerant and with no known adverse reaction to cow’s milk.

All 28 participants completed both trials. The authors reported no dropouts or adverse events.

Methods

What did the researchers do?

The participants completed two lab sessions:

60 minutes of treadmill running.
60 minutes of stationary cycling.

The exercise intensity was set at 70–80% of age-predicted maximum heart rate. The participants were also verbally instructed to keep their perceived effort between 12 and 15 on the Borg scale, which means roughly “somewhat hard” to “hard”.

The drink was a commercially available low-fat, lactose-free A2 cow’s milk from Dos Pinos. The milk contained only the A2 β-casein protein variant.

Per 250 mL, the milk provided:

8 g protein.
5 g fat.
12 g carbohydrate.
125 mg sodium.
375 mg potassium.

The milk was stored at 4°C and served at 12°C. No other food or drinks were allowed during the sessions.

The participants drank:

4 mL/kg body mass 10 minutes before exercise.
2 mL/kg every 10 minutes during exercise.
16 mL/kg total across the full protocol.

For a 70 kg runner, that total dose would be about 1.12 L of milk. That is not a casual splash. That is a serious meeting with dairy.

The researchers measured gut symptoms at baselineThe measurements taken before the intervention begins, used as the “before” or “pre-study” point for comparison with “after” or “post-study” measurements., before exercise, at 30 minutes, and immediately after exercise. They also measured palatability, heart rate, body mass change, urine output, urine specific gravity, and sweat rate.

Main findings

What did the study find?

The overall gut symptom burden was similar during treadmill running and stationary cycling.

Overall gut symptoms Similar between running and cycling

Palatability Acceptable in both conditions

Race performance Not tested

The researchers found no significant effect of exercise mode on:

Upper gut symptoms: ORThe odds ratio is a measure of association between an exposure and an outcome, representing the odds of the outcome occurring with the exposure compared to the odds without the exposure. An OR of 1 indicates no association; greater than 1 indicates increased odds; less than 1 indicates decreased odds. = 1.01, 95% CIA measure of uncertainty used in Frequentist statistics. The 95% confidence interval is a plausible range of values within which the true value (e.g., the true treatment effect) would be found 95% of the time if the data were repeatedly collected in different samples of people. If this range of values (the confidence interval) crosses zero, there is little confidence that the average value is the true effect. If the confidence interval does not cross zero, we can be confident that the average value is the true effect. 0.62 to 1.65; p = 0.972A p-value is a statistical measure that indicates the probability that the result is at least as extreme as that observed if the null-hypothesis was true. If P is small, the observed difference is big enough to disprove (reject) the null hypothesis. In very basic terms, P equals the probability that the effect could be explained by random chance, and a P-value of less than 0.05 means the results look so promising that there’s only a 1-in-20 (or 5%) chance that they would have occurred if the treatment had no effect at all. Common thresholds for statistical significance are 0.05, 0.01, and 0.001..
Lower gut symptoms: OR = 1.30, 95% CI 0.76 to 2.22; p = 0.339.
Systemic symptoms: OR = 1.22, 95% CI 0.71 to 2.12; p = 0.475.

In plain English, the running condition did not produce a clearly higher overall symptom burden than the cycling condition.

At the end of exercise, the medianThe middle value in a set of ordered numbers; if there is an even number of values, it is the average of the two middle numbers. symptom scores were:

Upper GI: 7.50 during running vs 8.50 during cycling.
Lower GI: 2.50 during running vs 4.00 during cycling.
Systemic: 3.00 during running vs 2.50 during cycling.

The possible maximum scores were much higher than these values, so symptoms were present but generally modest in severity.

The equivalence testing supported equivalence between running and cycling for total upper GI and systemic symptom scores. Lower GI equivalence was inconclusive. Symptom counts did not reach equivalence in any region, so the details are a bit more nuanced than “exactly the same”.

Some individual symptoms were more frequent during cycling:

Heartburn/acidity: OR = 7.40, 95% CI 2.16 to 25.39; p = 0.001.
Flatulence: OR = 2.45, 95% CI 1.22 to 4.92; p = 0.012.
Defecation urgency: OR = 231.40, 95% CI 4.58 to 11698.59; p = 0.007.
Headache presence: OR = 2.38, 95% CI 1.02 to 5.55; p = 0.044.

The defecation urgency result looks dramatic, but the numbers were small: 1 of 28 participants during running versus 3 of 28 during cycling at the end of exercise. So yes, notable. No, not a bowel-based apocalypse.

The milk remained acceptable in palatability. The rating declined from about 8.1 out of 9 before exercise to about 6.6–6.7 out of 9 by the end. Exercise mode did not affect palatability.

MeanThe average of a set of numbers, calculated by summing all the values and dividing by the total number of values. heart rate was slightly higher during running than cycling:

142 ± 6 bpm during running.
138 ± 6 bpm during cycling.

Final heart rate was also higher during running:

146 ± 7 bpm during running.
141 ± 5 bpm during cycling.

Both remained within the target intensity range. Sweat rate, urine output, body mass change, and total fluid intake were similar between conditions.

What helps my confidence in the findings?

The strengths

The crossover design reduced the influence of individual differences in gut sensitivity.
The researchers controlled the drink type, drink volume, exercise duration, and relative intensity.
The study directly compared treadmill running and cycling under standardised conditions.
The researchers measured symptoms repeatedly during the exercise protocol.
The study reported the methods, statistics, funding, conflicts, and data availabilityWhen authors make their data available, this increases transparency and data integrity, and allows other to check the statistical analyses. clearly.

What limits my confidence in the findings?

The limitations

The study was small, with 28 participants.
The sample was mostly male, with only 6 female participants.
The study lasted only 60 minutes, so it does not reflect marathons, ultras, or long trail races.
The study took place in the lab, not during real-world training or racing.
The participants were physically active adults, not specifically trained marathon runners, trail runners, or ultrarunners.
Exercise intensity used age-predicted maximum heart rate, not directly measured VO₂peakVO₂peak is the highest oxygen uptake measured during a test, even if VO₂max wasn’t fully reached. I.e., it is the best effort recorded, but not always your max. or threshold-based zones.
The researchers did not formally record RPERating of perceived exertion (RPE) is a simple way to score how hard exercise feels to you, not to a machine. You pick a number on a scale (often 1–10), where low numbers mean “this feels easy” and high numbers mean “I’m really pushing it.” It blends how heavy your breathing is, how tired your muscles feel, and how much effort you think you’re putting in. values, VO₂, treadmill speed, or cycling power output.
The gut symptom scale was not formally psychometrically validated.
The study did not measure biomarkers of intestinal integrity, such as I-FABP.
The study did not track gut symptoms beyond the immediate post-exercise period.

Funding and conflicts

Who funded the study?

The study was funded by the Dos Pinos Milk Producers Cooperative of Costa Rica under a cooperation agreement with the University of Costa Rica. The cooperative and the University of Costa Rica also supported the open-access publication fees.

The authors declared no commercial or financial relationships that could be construed as a potential conflict of interestA conflict of interest happens when a person or group has a personal, financial, or professional interest that could influence their judgment. It does not always mean they did something wrong. But it can create bias or make others question whether the decision or result is fully fair and trustworthy..

Because the study tested a milk product and received support from a milk producers cooperative, the funding deserves a cautious reading. That does not make the findings wrong. It simply means the methods, outcomes, and interpretation should be checked carefully. Science is at its best when it keeps its lab coat buttoned and its eyebrows slightly raised.

FAQ

Can runners drink lactose-free A2 milk during exercise?

Some runners may tolerate it during shorter training sessions, especially if they already tolerate cow’s milk. This study tested 60 minutes of treadmill running, not long races.

Is milk a good hydration drink for runners?

Milk contains fluid, carbohydrate, electrolytes, and protein. But this study mainly tested gut tolerance and palatability, not performance or race-day hydration effectiveness.

Did A2 milk cause stomach problems during running?

Gut symptoms increased during exercise, but the overall symptom burden was similar during running and cycling. The symptoms were generally modest, and no participants withdrew.

Should I use milk during a marathon?

Not based on this study alone. The study did not test marathons, ultras, hot races, high carbohydrate fuelling, or several hours of intake.

Is lactose-free milk better than regular milk for runners?

This study did not compare lactose-free milk with regular milk. It only tested one low-fat, lactose-free A2 milk drink.