FoundMyFitness · 2026-05-04 · Rhonda Patrick (host), Kelly Starrett
The Scientific Formula for a "Healthy Day" | Dr. Kelly Starrett
52 research-tied claims examined: 5 contradicted 2 overstated 5 context 29 supported 1 corroborated online 10 unverified
2 Overstated
The brainstem's ventilatory sensitivity and tolerance to elevated carbon dioxide levels can be trained and adapted through intentional breath-holding exercises.
"if we can get the brainstem to handle high circulating CO2s by practicing exposing that brainstem to these climbing CO2 levels, then we're able to tolerate higher workloads, strip more oxygen off, and not quickly scrub-breathe off that. And now we can actually get more work done because it's the CO2 that's triggering the breathing response, right? And we don't sort of realize that that's very trainable" (said at 0:55:27)
The claim that the brainstem's ventilatory sensitivity to carbon dioxide (the hypercapnic ventilatory response, or HCVR) is easily trained through breath-holding exercises is overstated. Randomized and prospective interventional studies show that while repeated breath-hold practice significantly improves breath-holding time, it does not alter central chemoreflex sensitivity or HCVR slopes over short-to-medium training periods (such as 14 days of daily apneas). Although some cross-sectional studies in long-term breath-hold divers (with years of exposure) have observed blunted ventilatory responses to hypercapnia, multiple controlled trials and physiological assessments of elite apneists demonstrate that resting central chemoreflex sensitivity to CO2 does not significantly differ from non-divers, indicating that increased breath-hold tolerance is primarily driven by psychological tolerance, lung volumes, and peripheral adaptations rather than readily trained brainstem chemosensitivity.
- partial: Ventilatory responses to hypercapnia in divers and non-divers: effects of posture and imme… (European journal of applied physiology 2001) · cited 36x in the literature
"The slopes of the linear regression, which characterised the sensitivity to CO2 and were determined with the rebreathing tests, revealed differences between the two populations (ventilation: P < 0.0001; mean inspiratory flow: P < 0.05)... These results confirmed a lower sensitivity to CO2 for trained divers." (abstract, results, passage verified)
pubmedfull study (doi) - contradicts: Central chemoreflex sensitivity and sympathetic neural outflow in elite breath-hold divers… (Journal of applied physiology (Bethesda, Md. : 1985) 2008) · cited 46x in the literature
"The slope of the hypercapnic ventilatory and MSNA response was similar in both groups. We conclude that repeated bouts of hypoxemia in elite, healthy breath-holding divers do not lead to sustained sympathetic activation or arterial hypertension." (abstract, results and conclusions, passage verified)
pubmedfull study (doi) - contradicts: Cardio-ventilatory responses to poikilocapnic hypoxia and hypercapnia in trained breath-ho… (Respiratory physiology & neurobiology 2014) · cited 13x in the literature
"BHDs presented carbon dioxide sensitivity similar to that of NDs (2.85±1.41 vs. 1.85±0.93Lmin(-1)mmHg(-1), p>0.05, respectively)... These results suggest that the exposure to repeated bouts of hypoxia/hypercapnia frequently experienced by trained breath-hold divers only enhances their chemo-responsiveness to poikilocapnic hypoxia, without altering HCVR." (abstract, results and conclusions)
pubmedfull study (doi) - contradicts: Repeated apneas and hypercapnic ventilatory response before and after apnea training. (Aerospace medicine and human performance 2015) · cited 7x in the literature
"Although breath hold time (BHT) increased after apnea training in the NBH group by ~46%, CO2 sensitivity slopes were not different among experimental conditions and groups (2.8 0.3, 2.9 0.4 L min(1) mmHg(1) in the CBH and 2.7 0.5, 2.7 0.3 L min(1) mmHg(1) in the NBH during the second and third visit, respectively). HCVR after five RBHE or 14 d of apnea training was not decreased despite the achieved BHT enhancement." (abstract, results and conclusions, passage verified)
pubmedfull study (doi)
Head-to-head comparison studies show that aerobic walking in a natural environment produces a profound antidepressant and mood benefit not observed when walking in an urban environment.
"there's even been studies comparing people that exercise in an urban versus natural environment and when I say studies that are comparing I mean head-to-head comparison this the study looked at it so they're doing the same you know aerobic walking exercise in either a natural setting or an urban environment. And it's that only in the natural setting that people are having this really profound anti-depressant effect. It's affecting their their their brain, their mood, their mental health" (said at 2:18:37)
Head-to-head randomized crossover and parallel-group trials have compared the mental health and neural effects of walking in natural versus urban environments. For example, a 90-minute walk in nature led to significant reductions in self-reported rumination and subgenual prefrontal cortex (sgPFC) neural activity compared to an urban walk (Bratman et al., 2015). Similarly, single 50-to-60-minute nature walks in individuals with major depressive disorder produced greater improvements in mood and larger reductions in negative affect compared to urban walks (Berman et al., 2012; Nejad et al., 2022). However, characterizing these findings as a 'profound antidepressant effect' overstates the evidence: the published literature consists primarily of small, single-session experimental studies measuring transient changes in acute affect, rumination, or cognition rather than sustained clinical remission of depression.
- partial: Interacting with nature improves cognition and affect for individuals with depression. (Journal of affective disorders 2012) · cited 809x in the literature
"Participants exhibited significant increases in memory span after the nature walk relative to the urban walk, p<.001, η(p)(2)=.53 (a large effect-size). Participants also showed increases in mood, but the mood effects did not correlate with the memory effects, suggesting separable mechanisms and replicating previous work." (abstract, results, passage verified)
pubmedfull study (doi) - supports: Nature experience reduces rumination and subgenual prefrontal cortex activation. (Proceedings of the National Academy of Sciences of the United States of America 2015) · cited 919x in the literature
"We show in healthy participants that a brief nature experience, a 90-min walk in a natural setting, decreases both self-reported rumination and neural activity in the subgenual prefrontal cortex (sgPFC), whereas a 90-min walk in an urban setting has no such effects on self-reported rumination or neural activity." (abstract, results, passage verified)
pubmedfull study (doi) - partial: The effects of walking in nature on negative and positive affect in adult psychiatric outp… (Journal of affective disorders 2022) · cited 38x in the literature
"Controlling for baseline levels of affect before the walk, individuals who walked in nature experienced overall lower levels of negative affect, F(1, 35.039) = 4.239, p = .047, compared to those who walked in urban settings. Positive affect did not differ across walk conditions." (abstract, results, passage verified)
pubmedfull study (doi)
Unverified means no publication matching the claim was located; it does not prove the claim false. Spotted an error? See the corrections policy - disputes from the people quoted are prioritized.