Creatine is known to be present in the retina due to the expression of creatine kinase (CK) and the GAMT enzyme of creatine synthesis, which is also present in the mammalian retina. Creatine in the blood can be transported into the retina via the creatine transporter (confirmed in humans), and inhibiting transporter activity (by depleting the medium of chloride and sodium) reduces uptake by 80%. The fact that not all uptake was inhibited suggests that another transporter, such as the monocarboxylate transporter MCT12 (or SLC16A12), plays a role, perhaps moreso in the lens, where its levels were comparable to that of the major creatine transporter SLC6A8.
Cribb et al (2007)  observed greater improvements on 1RM, lean body mass, fiber cross sectional area and contractile protein in trained young males when resistance training was combined with a multi-nutrient supplement containing 0.1 g/kg/d of creatine, 1.5 g/kg/d of protein and carbohydrate compared with protein alone or a protein carbohydrate supplement without the creatine. These findings were novel because at the time no other research had noted such improvements in body composition at the cellular and sub cellular level in resistance trained participants supplementing with creatine. The amount of creatine consumed in the study by Cribb et al was greater than the amount typically reported in previous studies (a loading dose of around 20 g/d followed by a maintenance dose of 3-5 g/d is generally equivalent to approximately 0.3 g/kg/d and 0.03 g/kg/d respectively) and the length of the supplementation period or absence of resistance exercise may explain the observed transcriptional level changes that were absent in previous studies [30,31].
In the United States, the manufacturers of dietary supplements do not need to provide the Food and Drug Administration with evidence of product safety prior to marketing. As a result, the incidence of products adulterated with illegal ingredients has continued to rise. In 2013, one-third of the supplements tested were adulterated with unlisted steroids. More recently, the prevalence of designer steroids with unknown safety and pharmacological effects has increased.
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