You won’t find one if you are talking about a randomized, blind, controlled clinical study with human subjects to establish the safety of GM crops. This model is not used to establish food safety. There are several studies of nutritionally altered GM crops with expected biological effect. These were paid for by Monsanto, and I will talk below about why these were conducted. In general, a question on human studies was answered on this site previously, but please allow me to explain the circumstances around why human clinical studies are normally conducted.
When people ask about double-blind, placebo-controlled studies, they are often thinking about testing for pharmaceuticals or even nutritional supplements. These studies are done to determine if the anticipated clinical benefit of the product is detectable, and they include specific endpoints or variables that are selected for relevance to the expected effect. For instance, if a drug or supplement affected heart disease by lowering LDL levels, you would measure LDL and other lipoproteins in the blood. In order to do these studies, it is necessary to write a detailed protocol that needs to be reviewed by an Ethics Review Board. Those boards will not let researchers conduct studies that are not justified, do not have an expectation of safety and are fishing expeditions (i.e., measure numerous endpoints without an appropriate hypothesis). This site lets you see clinical trials being conducted, and you can even find out whether there is a particular study you could participate in as test subject.
Clinical studies are done for pharmaceuticals, but only after there have been significant safety, mechanism of action and pharmacokinetics data collected in rodent studies. It is here that fundamental distinctions arise between food and drug testing. Drugs are expected to produce a significant physiological effect and must be proven to do so. Further, the intent is to provide drugs at a dose sufficient to reliably produce a pharmacological effect, with an expectation that side effects may occur. Thus, human clinical trials are conducted primarily to determine the efficacy of the product being tested. Such studies are not formal safety studies as performed in animals, where we push to a maximally tolerated dose for the duration of the study and subject the animals to autopsy at the conclusion of the study. Rather, we dose to desired pharmacological effect and collect data that can be ethically collected in living subjects. The primary purpose of the study is efficacy, however, and safety data are collected secondarily.
Food is a lifelong, essential “exposure” in humans, and there is no rational basis for performing lifelong or other long-term studies on humans. This would require significant ongoing dietary alterations that cannot practically be sustained in voluntary subjects. Rather, foods (and even food additives) are placed into the market based on animal data and scientific knowledge sufficient to establish safety, usually with a wide margin of safety.
In the case of GM foods, animal testing and an extensive body of knowledge inform us that no adverse effect is anticipated with the ingestion of DNA, RNA or digestible dietary proteins. There is, in general, no hypothesis of nutritional benefit of GM foods versus that of a conventional variety (unless nutritional characteristics have been altered).
In the case of a particular GM food that was envisioned to have a nutritional benefit, the following three human clinical studies were done in collaboration with Monsanto:
Lemke, S.L., K.C. Maki, G. Hughes, M.L. Taylor, E.S. Krul, D.A. Goldstein, H. Su, T.M. Rains and R. Mukherjea. 2013. Consumption of stearidonic acid-rich oil in foods increases red blood cell eicosapentaenoic acid. J Acad Nutr Diet 113:1044-1056.
Krul, E.S., S.L. Lemke, R. Mukherjea, M.L. Taylor, D.A. Goldstein, H. Su, P. Liu, A. Lawless, W.S. Harris and K.C. Maki. 2012. Effects of duration of treatment and dosage of eicosapentaenoic acid and stearidonic acid on red blood cell eicosapentaenoic acid content. Prostaglandins Leukot Essent Fatty Acids 86:51-59.
Lemke, S.L., J.L. Vicini, H. Su, D.A. Goldstein, M.A. Nemeth, E.S. Krul and W.S. Harris. 2010. Dietary intake of stearidonic acid-enriched soybean oil increases the omega-3 index: randomized, double-blind clinical study of efficacy and safety. Am J Clin Nutr 92:766-775.
These were conducted by an independent clinical research organization and/or academic collaborators and were double-blinded and had a placebo control. The purpose of these studies was to demonstrate that oil from a GMO soybean, modified to contain an omega-3 fatty acid (SDA, or stearidonic acid) could produce changes in a patient lipid profile similar to those found with ingestion of the long-chain omega-3 fatty acids found in fish: EPA and DHA. SDA is converted by humans to EPA, and beneficial changes in red-cell lipid profile were expected and were demonstrated in these studies. Studies were preceded by establishment of safety based on familiarity with other fatty acids, as well as animal studies; doses were designed to achieve an expected effect; and appropriate bio-ethical reviews and consent were undertaken in these studies to assure proper management of issues related to studies using human subjects. Such studies can establish benefit of a nutritionally enhanced product.
Such studies are not necessary or appropriate for ordinary DNA, RNA or digestible dietary proteins in foods, as our vast history of human experience shows us these components are safe to eat.