Creatine kinase is expressed in eyes. The eyes can take creatine up from the blood via two different transporters, the classic SCL6A8 (creatine transporter) and MCT12. It seems that expression of the receptors and accumulation of creatine occur in a relatively higher level in photoreceptors, which perceive color. Similarly to many other tissues, they appear to protect the cells during periods of low oxygen availability.
Testosterone is a steroid from the androstane class containing a keto and hydroxyl groups at the three and seventeen positions respectively. It is biosynthesized in several steps from cholesterol and is converted in the liver to inactive metabolites. It exerts its action through binding to and activation of the androgen receptor. In humans and most other vertebrates, testosterone is secreted primarily by the testicles of males and, to a lesser extent, the ovaries of females. On average, in adult males, levels of testosterone are about 7 to 8 times as great as in adult females. As the metabolism of testosterone in males is greater, the daily production is about 20 times greater in men. Females are also more sensitive to the hormone.
In people who are fairly active, the glutes are usually one of the strongest muscle groups in the entire body, due to the need to support the sacrum and femur, areas of the lower and mid body where the glute muscles attach to. Together the glute muscles help with exercises or activities such as: lifting and lowering when sitting, thrusting, climbing stairs, jumping, and balancing the lower body. For the overall most functional lower body strength, the glutes are exercised in proportion to other muscles of the legs, including the quadriceps and calves.
High extracellular creatine concentrations induce the expression of a factor that inhibits the creatine transporter (CrT). To date, neither the identity of nor mechanism for this putative CrT-suppressing factor has come to light. Future studies that are able to identify this creatine transport-suppressing factor and how it works may provide valuable insight into possible supplementation strategies that might be used to increase creatine uptake into muscle cells.
Creatine synthesis primarily occurs in the liver and kidneys. On average, it is produced endogenously at an estimated rate of about 8.3 mmol or 1 gram per day in young adults. Creatine is also obtained through the diet at a rate of about 1 gram per day from an omnivorous diet. Most of the human body's total creatine and phosphocreatine stores are found in skeletal muscle, while the remainder is distributed in the blood, brain, and other tissues.
More recent studies on the regulation of CrT creatine transport activity have identified the protein kinase (Janus-Activating Kinase 2) JAK2, which suppresses the rate of creatine uptake via CrT without affecting creatine binding. JAK2 is a regulatory protein involved in stabilizing the cellular membrane and controlling water concentrations in response to osmotic stress. Similar to c-Src (a positive creatine transport regulator), Jak2 can also be activated by growth hormone signaling. The growth hormone receptor seems to activate these two factors independently, as gh-mediated activation of c-Src does not require JAK2. Given that c-Src is a positive regulator of CrT, JAK2 is a negative regulator, and the fact that downstream signals from both are induced by growth hormone, it is tempting to speculate that JAK2 activation downstream of the gh receptor may function as a homeostatic response to limit c-src induced creatine uptake. This has not been studied, however, and the effects of gh-induced JAK2 signaling on CrT activity have not been examined.
One study investigating the effects of creatine supplementation on people with osteoarthritis undergoing knee arthroplasty (surgical procedure for osteoarthritis), who received creatine at 10g daily for 10 days prior to surgery and 5g daily for a month afterward, failed to find benefit with supplementation. This study failed to find any differences in muscular creatine stores or weight changes.
As scientific research progressed, it became apparent that the best types of protein came from milk and eggs. That led to the next great revolution in sports nutrition, namely the engineered food, pioneered by Scott Connelly, M.D., a critical care specialist from Northern California who teamed with a young entrepreneur named Bill Phillips from Golden, Colorado.
In regard to practical interventions, concurrent glycogen loading has been noted to increase creatine stores by 37-46% regardless of whether the tissue was exercised prior to loading phase. It is important to note, however, that creatine levels in response to the creatine loading protocol were compared in one glycogen-depleted leg to the contralateral control leg, which was not exercised. This does not rule out a possible systemic exercise-driven increase in creatine uptake, and the increase in creatine noted above was larger than typically seen with a loading protocol (usually in the 20-25% range). Consistent with an exercise-effect, others have reported that exercise itself increases creatine uptake into muscle, reporting 68% greater creatine uptake in an exercised limb, relative to 14% without exercise.
Having a strong butt will get you far—literally. Our glutes are responsible for powering us through everything from long runs to tough strength workouts to a simple jaunt up a flight of stairs. Strong glutes that can take on the brunt of the work can help us avoid overcompensating with smaller muscles during lower-body exercises. Plus, beyond just helping us move, the glutes play an important role in "stabilizing our entire lumbo-pelvic-hip complex," says Cori Lefkowith, NASM-certified personal trainer and owner of Redefining Strength in Costa Mesa, California. That translates to better form, more efficient movement, and a reduced risk of straining your lower back and hips.