Scientific studies examining the effects of Ginkgo biloba (EGb 761) on indicators of systemic stress
Two recent studies examined the ability of Ginkgo biloba (EGb761) to attenuate indicators of stress, namely elevated blood pressure and serum cortisol levels.
Cortisol is the nemesis of any athlete undergoing strenuous training. Cortisol is catabolic, meaning that it literally causes the body to breakdown its own tissues for conversion into immediate energy (ATP) during moments of stress. The so-called fight-or-flight response. Unfortunately, the body mignt interpret excessive exercise as a stressful situation, which will recruit the actions of cortisol and potentially undermine muscle anabolism (tissue building). This hormonal imbalance is the physiological basis of OverTraining Syndrome, or OTS. In essence, training too heavily, eating inadequately, and resting insufficiently, shifts the balance from total body anabolism to catabolism-defeating the purpose of training in the first place.
Cortisol is a steroid hormone secreted by the adrenal glands located on top of the kidneys. The adrenal glands are provoked to release cortisol by another hormone known as adrenocorticotropic hormone (ACTH) that is released from the pituitary gland at the base of the brain. Again, cortisol counteracts the ability of our anabolic hormones, namely growth hormone, testosterone and insulin, to produce muscle growth.
Visit our page concerning the anabolic effects of growth hormone.
Scientific study #1: EGb 761 suppresses cortisol release in response to stressors
The first study (Ref. 1) used a combination of two stressors on which to study the effect of EGb 761, static exercise (handgrip: 3 minutes at 30% of maximum voluntary contraction) and mental load (memory test: memorize the ordering of 25 words in three minutes). Cortisol, blood pressure and heart rate measurements were taken before the administration of EGb 761 to establish a baseline value. Subjects were then given a single dose of EGB 761 and 30 minutes later stressed in the manner described above. Heart rate and blood pressure were taken during the handgrip trial, while salivary cortisol levels were measured one hour later.
A college pharmacology course served as a backdrop for the study from which the participants for the study were selected. The study was placebo controlled and conducted in a double-blind manner, meaning that neither investigator, nor experimental subject, knew who was taking placebo or EGb 761. That is, it was unknown who was taking the ginkgo extract, or who was taking placebo, until after the results of the study were fully analyzed.
The administration of ginkgo biloba had litttle effect over the resting levels of cortisol observed throughout the day, agreeing well with the second study being discussed. Ginkgo did, however, reduce the release of cortisol following stress, but with special considerations (see below).
Diurnal Cortisol Rhythm
Serum cortisol levels obey a circadian rhythm; they are highest in the morning and recede throughout the remainder of the day until almost totally disappearing around midnight.
In males, the stress-evoked release of cortisol was abolished with the administration of a single dose of EGb 761, but only in the afternoon when the ability of stress to increase serum cortisol levels over baseline is most apparent. Due to the large drop in serum cortisol levels in the early morning hours, however, the ability of exercise to increase serum cortisol levels was largely hidden by the declining wave and hence, precluded any apparent effect of EGb 761. That is, baseline and stress measurements were made one hour apart, which was much too slow to discern a change in serum cortisol in response to exercise, or ginkgo, treatment.
Interestingly, women did not exhibit a stress-provoked rise in serum cortisol at any point in the day and accordingly, ginkgo biloba had no effect over serum cortisol levels in women.
Heart rate and blood pressure consistently increased in response to the combined stressors. EGb 761 reduced blood pressure during the handgrip trial in all cases, males and females, at all times of the day, but without altering heart rate.
Therefore, at least in males, Ginkgo biloba appears to reduce the release of cortisol and rise in blood pressure following a combination of physical and mental stresses. These results therefore suggest that ginkgo supplementation might represent a possible way to circumvent the release of cortisol following prolonged exercise and possibly help circumvent the development of Overtraining Syndrome.
Scientific study #2: EGb 761 does not decrease resting cortisol levels
Earlier animal studies have suggested that Ginkgo biloba, or more specifically, one of ginkgo’s components may inhibit the synthesis of the glucocorticoid hormones. This would be good news with reference to cortisol, but may have negative repercusions as far as the steroid sex hormones are concerned, especially testosterone and progesterone.
To paraphrase, the second study (Ref. 2) found that Ginkgo biloba produced a small, yet statistically insignificant, drop in serum cortisol levels after administration of EGb 716 for fourteen days. Other products of steroid hormone synthesis (17a-hydroxyprogesterone, bound testosterone, free testosterone, free androgen index, androstenedione, dihydrotestosterone, sex hormone-binding globulins, etc) were similarly unchanged by ginko administration. With respect to the little change in resting steroid hormone production following ginkgo administration, this study agreed well with the findings of the first study. It thus appears that ginkgo biloba does not alter the resting production of the steroid hormones humans.
Comparison of the two studies
Here is a list of the principal differences between the two studies examining the effects of EGb 761 over serum cortisol levels.
- Dose: 120 milligrams (Ref.1) versus 240 milligrams (Ref. 2) per day; 120 milligrams is the typical dose recommended in the medical literature.
- Duration: Single dose (Ref.1) versus 14 day trial (Ref 2.).
- Sample Size: Seventy subjects (Ref.1; 33 males and 37 females) versus eleven subjects (Ref.2; 6 males and 5 females) participated in the study. The larger sample size was a major advantage of the first study (Ref.1). Too many studies are restricted to small sample sizes, making it more difficult to interpret the statistical significance of the generated data.
- Assay: Salivary sample (Ref.1) versus blood test (Ref.2).
- Product: European manufacturered extract EGb 761 elixir (Ref.1) versus American produced tablets of EGb 761 (Ref.2).
- Study Design: Stress-induced (Ref.1) versus resting cortisol levels (Ref.2).
Both studies utilized relatively young subjects of between 20 and 40 years of age. I would like to see similar studies conducted on older group subjects given that this demographic is where Ginkgo biloba is currently showing the most promise.
Selected Scientific References
EGb 761 reduces the stress of exercise
(Ref. 1) Jezova, D. et al. (2002) Reduction of rise in blood pressure and cortisol release during stress by Ginkgo biloba extract (EGb 761) in healthy volunteers. American Journal of Physiology and Pharmacology, Volume 53 (3), pages 337-348.
(Ref. 2) Markowitz, J. S. et al. (2005) Effect of Ginkgo biloba extract on plasma steroid concentrations in healthy volunteers: A pilot study. Pharmacotherapy, Volume 25 (10), pages 1337-1340.
Read our recent newsletter detailing the positive effects of extracts of Ginkgo biloba on indicators of exercise-induced stress.