It is a strange but well replicated fact, that if you leave small animals in a centrifuge for a really long time, they lose a lot of fat. Many of these experiments were done in the 1960’s and 1970’s as part of the study of the physiological effects of spaceflight.
Centrifugation makes animals smaller, leaner, more muscular, and denser-boned
If you put female rats in a centrifuge for 60 days, at 2.76 and 4.15 G (where G is the strength of Earth’s gravitational field), they lose 10% and 19% of their body weight, respectively, with reductions in the fat fractions of most components and increases in the water fraction of liver and gut.
Female rats exposed to 3.5 or 4.7 G for one year showed “marked depletion of body-fat depots” and “significant decrease in kidney and liver lipids.”
Chickens exposed to 1.75, 2.5, or 3 G for 24 weeks had significantly reduced body fat. The drop in body fat is linearly increasing in G, and _also _increases with body mass.
Rabbits exposed to up to 2.5 G had a drop in body fat and increase in body water, even as their food consumption increased.
Female rats centrifuged for 30 days at 2.76 or 3.18 G reduced body fat and fat-free body mass within the first week of centrifugation, without any difference depending on whether they were fed commercial chow, a high-fat diet, a high-protein diet, or fasted.
The drop in body fat from centrifugation can be quite large; chickens went from 13% body fat to 3% body fat at 3G, and mice have a 55% drop in total body fat after 8 weeks of 2G exposure.
Centrifuged mice have a drop in weight during the first few days, but slowly regain it. Hamsters born in centrifuges have a final body weight of about 30% lower than control hamsters.
Female rats centrifuged for 810 days at 2.76 G grew more slowly than control rats, but had the same absolute muscle mass; they have thicker bones and larger muscles for their size than uncentrifuged rats. They also have denser bones. They have a higher proportion of slow-oxidating muscle fibers (the kind used in distance running and other endurance activities). Centrifuged dogs (subjected to 2G for 3 months) also have denser bones.
Centrifuged rats also had more uptake of glucose into tissues and a stronger response to insulin than uncentrifuged rats; this is the opposite of “insulin resistance.” Centrifuged chickens also have higher glucose uptake.
Centrifuged rats have a sharp decrease in body temperature at about 3 days, and a subsequent recovery of normal body temperature.
Centrifuged rats have a prolonged decrease in locomotor activity and distorted circadian rhythms.
Centrifugation alters the vestibular system
The vestibular system is involved in balance.
The microscopic structure of the lateral vestibular nucleus (where many vestibular nerve fibers enter the brain) is altered in chronically centrifuged rats. Centrifuged hamsters have impaired balance during swimming tests.
Knockout mice that lack vestibular linear acceleration organs are known as “head-tilt mice.” They move normally, except for a head tilt, but cannot swim because they cannot orient to the gravitational force vector. Head-tilt mice, when centrifuged at 2G, do _not _experience the changes that chronic centrifugation causes in wild-type mice: they do not have a drop in body temperature, body mass, or body fat percentage. While wild-type mice under 2G dropped from 16% to 8% body fat, head-tilt mice started out at 8% before centrifugation and did not change. This implies that vestibular effects somehow cause the physiological changes associated with higher gravity.
Artificially stimulating the vestibular organs causes fat loss
A pilot study at the University of California San Diego’s Center for Brain and Cognition, one of whose authors was famed neuroscientist Vilayanur Ramachandran, tested galvanic stimulation of the vestibular nerves, a non-invasive procedure that involves passing current over the inner ear, on six overweight and obese subjects, with three controls, for a total of 40 hours, for an hour a day. There was a significant 8.3% decrease in truncal fat and a nonsignificant decrease in total body fat. Appetite was reduced, leptin was reduced, and insulin was increased.
This is not a huge reduction in fat. (It would be something like two pounds on me, over the course of a month.) On the other hand, this is a significantly lower “dose” of vestibular stimulation than centrifuged animals would receive. The animals that had body composition changes were centrifuged continuously over a period of months. It may be possible to slowly increase the time spent receiving galvanic stimulation.
Vestibular stimulation may affect hormone levels
There are a few case studies from India of “controlled vestibular stimulation” (swinging on a swing) causing various changes in physiology. A college student for whom swinging resulted in significantly lower blood pressure, blood glucose, and cortisol, and an 83-year-old diabetic man for whom swinging resulted in significantly lower glucose and blood pressure .
The vestibular system modulates autonomic activity, and vestibular stimulation activates vagus nerves in the pancreas which stimulate insulin production. There seems to be a parasympathetic response to vestibular stimulation, which goes with increased insulin production and lower hunger, both of which would reduce fat. (It also matches the intuitive observation that rocking and swinging is soothing: think of infants and rocking chairs.)
Other Vestibular Stimulation Weirdness
Galvanic vestibular stimulation also seems to reverse face blindness .
Galvanic vestibular stimulation is safe, if sometimes uncomfortable (causes motion sickness), and might have significant effects on body fat and other metabolic factors. It is probably worth investigating more on humans.
It’s trivial to set up; people who are interested in virtual reality frequently build their own vestibular stimulation rigs to increase the verisimilitude of immersive games. This seems like something with a lot of potential for venturesome self-experimenters to try out as well as something to investigate seriously in clinical experiments.
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