Passive heat and exercise: a small look at post-meal blood sugar
A small crossover study compared a hot bath with cycling before a meal. Peak blood sugar differed slightly; total blood sugar exposure over the day did not.
The short version
In this small crossover study, 14 men either sat in a 40°C bath or cycled at a matched intensity for 60 minutes each, on separate days, then ate a standard meal. A stress protein, HSP70, rose by a similar amount after both. Peak blood sugar after the meal was slightly lower after the bath than after cycling, but total blood sugar exposure over the next few hours did not differ. With 14 men studied once each, this is a small, exploratory comparison, not evidence that a hot bath can substitute for exercise.
Keep in mind
A very small, single-session comparison in 14 men. The peak blood sugar difference did not carry through to total blood sugar exposure over the following hours, and this is not evidence that a hot bath can replace exercise.
What they found
HSP70 rose by a similar, comparable amount after both the hot bath and the cycling session; there was no meaningful difference between them.
Peak blood sugar after the meal was slightly lower after the hot bath (about 6.3 mmol/L) than after cycling (about 6.8 mmol/L). But the total blood sugar exposure over the following hours was no different between the two conditions.
The inflammation marker IL-6 rose after both sessions, more after cycling (about four-fold) than after the hot bath (about two-fold).
What this means for your practice
This is not evidence that a hot bath or sauna session can replace exercise for blood sugar control. The clearest finding, a similar HSP70 rise, is a mechanism, not a demonstrated health benefit.
If you are managing blood sugar or a metabolic condition, this small study is not a basis for changing your approach; that’s a conversation for your doctor.
Methods and limitations
What the researchers did
Fourteen men, some lean, some with overweight, took part in two sessions on separate days: a 60-minute bath at 40°C, and 60 minutes of cycling set to produce a similar amount of body heat. After each, they ate the same standard meal.
Researchers measured a heat shock protein (HSP70), an inflammation marker (IL-6), and blood sugar over several hours after the meal, comparing the two conditions in the same people.
This tests passive heating against exercise directly, but only as a single session in a small group, not repeated use over time.
How much can this tell us?
This is a small, single-session study in 14 men, so it can raise an interesting question about passive heat and metabolism, but it cannot answer it. A difference in one peak reading, with no difference in overall blood sugar exposure, is a modest and mixed result, not a clear win for heat over exercise.
The comparable HSP70 response is the more consistent finding here: it suggests passive heat and exercise both trigger a similar cellular stress response, at least acutely.
With men studied once each, this cannot tell us what happens with repeated use, in women, or over the longer term.
What makes it useful
- Direct, head-to-head comparison with exercise in the same people
- Crossover design: each person is their own comparison
- Measured multiple markers (HSP70, IL-6, blood sugar), not just one
What limits it
- Very small: 14 men, one session of each condition
- Men only
- The peak blood sugar difference did not hold up across total blood sugar exposure over the following hours
- A single acute session, with no data on repeated use
Where this study fits
Where this study fits
We score claims, not single studies. This one is part of the evidence for:
Heat switches on protective proteins in your cells. Some support
Low certaintyMedium effectBody signals
Heat helps control blood sugar. Not supported
Low certaintyNo effect foundBody signals
Sauna can stand in for exercise. Not supported
Low certaintyNo effect foundIllness and death
Read the original
Faulkner SH, Jackson S, Fatania G, Leicht CA. The effect of passive heating on heat shock protein 70 and interleukin-6: a possible treatment tool for metabolic diseases? Temperature. 2017;4(3):292-304.
DOI 10.1080/23328940.2017.1288688 · PubMed 28944271