Do adaptations to stress exposure show up as changes in blood chemistry or heart function? The answer appears to be “yes”. In his Newsweek article “Lessons in Survival”, Ben Sherwood reported on a very interesting study of elite Army Airborne and Special Forces soldiers that probed the differences between those who could and could not endure an extremely stressful 19-day mock-prisoner-of-war camp. The Resistance Training Laboratory, located at a secret location near Fort Bragg, North Carolina, subjected participants to sleep deprivation, blaring music, semi-starvation and — worst of all — intense interrogation techniques used by enemy forces during WWII, Korea and Vietnam. “The goal is to simulate hell on earth like the Hanoi Hilton in Vietnam or Al Qaeda’s torture chambers,” according to Sherwood. In another test of mettle, at the Navy Diving and Salvage Training Center in Florida, trainee divers were put through stressful routines such as being thrown into a pool with their hands and feet bound, and underwater ocean swimming from 3 miles offshore to a target on shore.
Biological markers. After these ordeals, Dr. Andy Morgan of Yale Medical School found that the top performers had very different physiological responses from those who couldn’t hack it. He did extensive physiological monitoring and found that those who passed these tough tests had much higher levels of NPY (a neuropeptide) and DHEA (a hormone that buffers the effects of cortisol, a stress hormone). In addition, those who did best also had “metronomic heartbeats” — very little heart rate variability (HRV), compared to most normal people, who show a lot of variability in the intervals between heartbeats.
These biological markers of stress-resistant individuals show that they are somehow different than most of us. Perhaps this is worth looking for what it can teach the rest of us. Certainly, there are other health known health benefits that have been reported for NPY and DHEA.
Autonomic nervous system. Regarding HRV, however, Sherwood raises a caveat, noting that numerous studies have associated metronomic heartbeats (low HRV) with cardiovascular disease, diabetes, and even sudden death. However, other research into the HRV paints a more complex picture. A paper on the Intellewave Method, by Dr. Alexander Riftine, indicates that low HRV may have different implications for the state of the autonomic nervous system, depending on the frequency of the heartbeat variability, derived from heart rhythmograms. When spectral analysis (a mathematical technique based upon Fourier Transforms) is applied to the heart rhythmograms, the heartbeat frequency variations are resolved into high frequency (HF) and low frequency (LF) patterns. The LF variations are associated with the sympathetic nervous system (SNS), where as high frequency HF variation correlate with the parasympathetic nerous system (PSNS). The balance between SNS and PSNS states predicts much about an individual’s fitness. According to Dr. Riftine, these states cluster into nine typical combinations. The ninth state–an elevated PSNS with a reduced SNS state–is “rather unusual because normally an increase in PSNS is accompanied by an increase in SNS. This rare condition is found in water polo athletes, long-distance runners, navy seals and persons with special heart training for deep sea diving.”
The physiological analysis of individuals who have successfully adapted to tolerate stress is a promising area ripe with lessons for the rest of us, as it could be used to assess, predict, and track our progress in getting stronger and more stress-hardy.