By Elizabeth Weise, USA TODAY

Genetically engineered crops are safe for humans and animals to eat and have not caused increases in cancer, obesity, gastrointestinal illnesses, kidney disease, autism or allergies, an exhaustive report from the National Academies of Science released Tuesday found.

Work on the 388-page report began two years ago and was conducted by a committee of more than 50 scientists, researchers and agricultural and industry experts convened by the National Academies of Sciences, Engineering, and Medicine. It reviewed more than 900 studies and data covering the 20 years since genetically modified crops, often called GMOs for genetically modified organisms, were first introduced.

Overall, genetically engineered (GE) crops saved farmers in the United States money but didn't appear to increase crop yields. They have lowered pest populations in some areas, especially in the Midwest, but increased the number of herbicide-resistant weeds in others. There's also no evidence that GE crops have affected the population of monarch butterflies, the report said.

The review was thorough and systemic, assessing many of the issues that have been raised about genetically engineered crops over the years, said Gregory Jaffe, director of biotechnology at the nonprofit watchdog group the Center for Science in the Public Interest in Washington. The group was not involved in the report's creation.

The genetic material of GE plants is artificially manipulated to give them characteristics they would not otherwise have. The two most common are pest resistance and the ability to withstand certain herbicides. That allows farmers to spray fields with herbicide, killing weeds while not harming the crops. Drought tolerant traits are newer and also becoming popular.

The report, "Genetically Engineered Crops: Experiences and Prospects," was meant to be an objective overview of current research into the safety and environmental and social effects of these increasingly popular crops and the foods made from them.

To gauge whether foods made from genetically modified crops were safe for human consumption, the committee compared disease reports from the U.S. and Canada, where such crops have been consumed since the mid-1990s, and those in the United Kingdom and western Europe, where they are not widely eaten.

No long-term pattern of increase in specific health problems after the introduction of GE foods in the 1990s in the U.S. and Canada was found.

There was no correlation between obesity or type 2 diabetes and the consumption of GE foods.

Celiac disease, which makes humans intolerant of gluten, increased in both populations. Patterns in the increase in autism spectrum disorder in children were similar in both the United Kingdom and the U.S., the committee reported.

Overall the report concluded that there were no differences in terms of a higher risk to human health between foods made from GE crops and those made from conventionally bred crops.

Groups opposed to genetically engineered crops criticized the report for arriving at watered-down scientific conclusions due to agricultural industry influence.

Food & Water Watch, a government accountability group in Washington, said the committee's ties to the biotech industry and other corporations create conflicts of interest and raise questions about the independence of its work.

"Critics have long been marginalized," said Wenonah Hauter, the group's executive director.

Overall, the report found that GE crops save farmers money in terms of time spent tilling and losses to weeds and insects but can have both positive and negative effects on pests, farming practices and agricultural infrastructure.

Pest-resistant crops have resulted in lower pest populations overall in some areas of the Midwest, especially European corn borer, the report found.