Cyclo (His-Pro) is also known as histidyl-proline diketopiperazine, and is a derivative of thyrotropin releasing hormone (TRH). It is a naturally occurring substance in humans and can be found in the blood, cerebral spinal fluid, semen, brain, spinal cord, and digestive tract. The concentration of cyclo (his-pro) is very high in the prostate.
Cyclo (his-pro) appears to be metabolically active with high levels of zinc. Zinc is very high in the prostate of humans and other animals, hence the belief why cyclo (his-pro) is also high in prostate.
Cyclo (his-pro) has been shown to regulate the absorption of zinc in the digestive tract. It can increase the overall absorption rates of zinc. Zinc is involved in the metabolism of insulin. The connections between cyclo (his-pro), zinc, and insulin, led to research on the efficacy of cyclo (his-pro) as a possible treatment for Type II diabetes.
Type II diabetes results in hyperglycemia caused by decreased tissue sensitivity to insulin. The tissues that are mostly affected are muscle and fat cells. Cyclo (his-pro) and zinc appear to improve the tissue sensitivity to insulin. 
Cyclo (his-pro) has also been shown to decrease prolactin secretion, but this effect has yet to be studied for its possible therapeutic benefit.
Cyclo (his-pro) can be found in various foods at concentrations much higher that that which normally occurs in human blood. Most supplements contain much higher levels as well. Supplemental cyclo (his-pro) is usually found in the form of ground, dried animal prostate, with cow prostate being utilized the most. Cow prostate contains approximately 1% of cyclo (his-pro). The following table lists the amounts of cyclo (his-pro) in common food sources. 
|Food Source (100 g)||Amount (pmol/g)||Food Source (100 g)||Amount (pmol/g)|
|Non-dairy creamer||121||Hot dog||73|
|Whole Milk||6||Chocolate Milk||15|
|Fish Sauce||5209||Dried Shrimp||6576|
Table 1: Food Sources Containing Cyclo (His-Pro)
Cyclo (His-Pro) is a relatively new supplement and little is known about its therapeutic benefits. The only use currently documented for cyclo (his-pro) is in individuals with Type II diabetes.
Individuals who were given powdered prostate containing cyclo (his-pro) had an overall decrease in their fasting blood sugar. They also had better glycemic control measured via decreased hemoglobin A1C levels. Cyclo (his-pro) also decreased insulin in the blood and lowered cholesterol. 
The current recommended dosage is 200 - 300 milligrams (mg) per day.
There are no known toxicities, side effects, or drug interactions with cyclo (his-pro). This is believed to be due to the fact that it is a naturally occurring compound. 
1. http://www.pdrhealth.com/drug_info/nmdrugprofiles/herbaldrugs/101840.shtml Cyclo (His-Pro) February 2005.
2. Hilton CW, Prasad C, Vo P, Mouton C. Food contains the bioactive peptide, cyclo (his-pro). J Clin Endocrinol Metab. 1992 Aug; 75(2): 375-378.
3. Song MK et al. Effects of bovine prostate powder on zinc, glucose, and insulin metabolism in older patients with non-insulin dependant diabetes mellitus. Metabolism. 1998; 47(1): 39-43.