They concluded that the Forceful Breath method can indeed raise core body temperatures into the mild fever range:
We were able, for the first time, to document reliable [core body temperature] increases during the FB type of g-tummo practice, all within the slight to moderate fever zone, validating the legends of the extraordinary capacity of g-tummo meditators to elevate their body temperature beyond normal. However, the results also suggest that temperature increases during g-tummo meditation are neither solely a by-product of meditation nor its goal, but instead may be a means to facilitate the achievement of “deep meditative states”.
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