“Having an extremely low body-fat percentage can affect the cardiovascular system’s ability to function normally,” says cardiologist Kevin Campbell , ., fellow of the American College of Cardiology. For example, in one study published in the International Journal of Sports Physiology and Performance , when bodybuilders prepared for competition by lowering their body fat, their heart rate dropped to 27 beats per minute. Too low heart rates, called bradycardia, can lead to dizziness, passing out, and cardiac arrest. Meanwhile, other electrolyte imbalances due to too-low of a body fat percentage, not to mention caloric intake, can lead to “cardiac arrhythmias and sudden cardiac death, he says.
To a modern physicist working with Einstein's general theory of relativity , the situation is even more complicated than is suggested above. Einstein's theory suggests that it actually is valid to consider that objects in inertial motion (such as falling in an elevator, or in a parabola in an airplane, or orbiting a planet) can indeed be considered to experience a local loss of the gravitational field in their rest frame. Thus, in the point of view (or frame) of the astronaut or orbiting ship, there actually is nearly-zero proper acceleration (the acceleration felt locally), just as would be the case far out in space, away from any mass. It is thus valid to consider that most of the gravitational field in such situations is actually absent from the point of view of the falling observer, just as the colloquial view suggests (see equivalence principle for a fuller explanation of this point). However, this loss of gravity for the falling or orbiting observer, in Einstein's theory, is due to the falling motion itself, and (again as in Newton's theory) not due to increased distance from the Earth. However, the gravity nevertheless is considered to be absent. In fact, Einstein's realization that a pure gravitational interaction cannot be felt, if all other forces are removed, was the key insight to leading him to the view that the gravitational "force" can in some ways be viewed as non-existent. Rather, objects tend to follow geodesic paths in curved space-time, and this is "explained" as a force, by "Newtonian" observers who assume that space-time is "flat," and thus do not have a reason for curved paths (., the "falling motion" of an object near a gravitational source).