What you control
Eating faster increased headache risk §
In the ICE-H trial, 145 middle-school students (mean age 12.7) each got the same portion: 100 ml of ice cream. The only variable tested was consumption time: one group ate it over more than 30 seconds, the other in under 5.
| Group | Participants | With headache | Incidence |
|---|---|---|---|
| cautious (>30 s) | 72 | 9 | 12.5% |
| fast (<5 s) | 73 | 20 | 27.4% |
The relative risk computed from these numbers is 2.19, published as 2.2 (95% confidence interval: 1.03–4.94). The number needed to harm is 6.71: for roughly every seven people who switch from cautious to fast eating, one extra headache occurs. Of the 29 total headaches reported, 17 (58.6%) lasted under 10 seconds.
What happens inside the artery
What the Doppler study measured §
In a second study, 77 healthy adults drank 200 ml of water at 22 ± 2°C and, five minutes later, 200 ml of ice water (0°C), as fast as possible. Transcranial Doppler ultrasound measured blood flow velocity in both middle cerebral arteries. In 39 of the 77 participants (50.6%), the cold stimulus provoked a typical headache, on both sides of the head, with a stabbing sensation.
The pain didn't appear right away: median latency was 20 seconds, and median duration was 12 seconds. The cold stimulus was identical for every participant. What differed was how hard the blood vessel reacted, measured by its flow velocity.
Rise in blood flow velocity, by group
Bars are proportional to the published percentages (p=0.003). Hensel et al., Frontiers in Neurology, 2019.
Those who also had tearing (19 of 39, or 48.7%) showed an even larger increase in flow: 12.8% versus 5.9% for those without tearing (p=0.006). In a quarter of those with a headache (10 of 39, 25.6%), a second headache followed shortly after the first one resolved, with a latency of 12 seconds (range 3 to 26 seconds).
Why you feel it in your forehead
How forehead pain might arise §
The phenomenon has a familiar precedent. In a heart attack, pain can show up in the left arm, because the nerves from the heart and the nerves from the arm send signals to the same neurons in the spinal cord: the brain gets one combined signal and misplaces it. Doctors call this pattern referred pain.
For brain freeze, the Doppler study authors suggest involvement of the trigeminal nerve and nerve circuits that may reduce resistance in cerebral vessels. Cold receptors in the mouth and throat can activate the trigeminal and other nerves. Neither study directly measured the convergence of nerve signals that might explain why the pain is felt in the forehead.
The authors describe a possible "trigeminal-parasympathetic vasodilator reflex." Ultrasound measured the speed of blood in the arteries; it did not measure the volume passing through them. The findings suggest neural and vascular mechanisms may be involved, while their exact roles remain unclear.
Limits
What these two studies don't cover §
- The samples are small by epidemiological standards: 145 schoolchildren in one trial, 77 adults in the other. The confidence interval on the relative risk (1.03–4.94) is wide.
- The ICE-H trial used schoolchildren and ice cream. The Doppler trial used adults and ice water. The populations and stimuli of the two studies differ, though the described mechanism is the same.
- Neither study directly measured trigeminal convergence. It is the accepted model in the neurology literature, not an observation made in either experiment.
- Headache was reported by participants in both studies, not confirmed by a separate objective marker.
Sources
Studies cited §
- Kaczorowski M, Kaczorowski J. "Ice cream evoked headaches (ICE-H) study: randomised trial of accelerated versus cautious ice cream eating regimen." BMJ, 2002. pmc.ncbi.nlm.nih.gov/articles/PMC139031
- Hensel O, Burow P, Mages S, Wienke A, Kraya T, Zierz S. "Increased Blood Flow Velocity in Middle Cerebral Artery and Headache Upon Ingestion of Ice Water." Frontiers in Neurology, 2019. pmc.ncbi.nlm.nih.gov/articles/PMC6611440