Sporting a white coat and tagged with impressive credentials, Dr. Ronald Kleinman carries an aura of authority on camera as he says, "There are no cancer risks associated with agriculture produced through biotech. None whatsoever."

The online advertisement featuring the physician-in-chief at Massachusetts General Hospital for Children and professor of pediatrics at Harvard Medical School was rolled out on Tuesday by a campaign opposing California’s Proposition 37, which would require the labeling of genetically modified foods -- so-called GMOs.

But on Wednesday, Californians -- along with the rest of the world -- heard a very different message: A two-year study, led by Gilles-Eric Seralini of the University of Caen in France and published in a prestigious peer-reviewed journal, found that a widely grown GMO corn variety raised the rate of cancer and increased the risk of kidney and liver problems in rats.

The new finding fit perfectly with the "Yes on 37" platform. The campaign has warned that the industry proclaiming GMOs won't harm human health is the same one that once said DDT and Agent Orange were perfectly safe. But criticisms of the new study quickly emerged, suggesting insufficient sample sizes, a breed of rats prone to tumor growth, and a lead researcher who was seemingly already convinced of the dangers of GMOs. The study authors are now attempting to refute these and other points.

"This study has some significant flaws and needs to be examined by an independent panel before we decide if there's validity in their findings," Kleinman told The Huffington Post, adding that he hadn't changed his mind on the absence of a cancer link. "This is particularly true since there's a very large body of studies on this topic over a long period of time and this new one is totally inconsistent with what they show and with what's been observed over the past 10-plus years of use of the product."

Dr. Kleinman consults for General Mills and presents webinars on children's health for Coca-Cola, both major funders of the fight against Proposition 37 -- along with Monsanto, DuPont and others.

"They're flooding the state with a 32 million dollar advertisement campaign of deception," said Stacy Malkan, media director of Yes on 37, which has collected contributions totaling a fraction of that amount.

So, what does this leave the public to believe? Where does the science stand on the safety of GMOs?

It's not easy to say. And that's exactly the problem, according to experts. There have been a handful of studies that have hinted at human health concerns, including allergies. And there have been studies, even a recent review of studies evaluating five different genetically modified crops, suggesting there are no concerns. But overall, few studies have looked at the range of potential effects that the introduction of foreign genes may have on a food's safety.

“Industry says these are the most carefully tested foods ever," said Charles Benbrook of the Center for Sustaining Agriculture and Natural Resources at Washington State University. "But there have been no studies on prenatal effects, on birth defects, on the long-term impact of exposure during fetal development."

Many GMO critics point to what they see as undue industry influence over what gets studied and published, as well as a regulatory system that they say bends to industry's wishes. In general, a 90-day animal study is all they need to win approval of a new genetically modified crop. But in Seralini's study, rats only started developing tumors after four months.

"If we learned anything from this new study," Malkan said, "it's that there are legitimate questions that need to be answered with legitimate research."

Margarida Silva, a biotechnology expert at the Portugese Catholic University in Porto, published a review in 2011 reporting a link between favorable outcomes in GMO studies and author affiliation with industry.

"Conflict of interest is very real, and it does change the study outcome," said Silva, who suggests that Seralini's affiliation with a non-governmental organization that opposes GMOs does not present the same hazards. "The NGO didn't stand to lose dozens of millions of Euros," she said.

As university research funding has been drying up, many academics are now turning to industry for help. This, according to Silva, is the problem. "It's not likely that people who develop such convenient, comfortable connections to industry are going to be interested in doing research that might effect that funding source," she said.

What's more, because companies patent their genetic alterations and researchers generally must get approval before doing any studies, independent research tends to be difficult -- even if they have the millions of dollars needed for a study like Seralini's.

Seralini did not immediately respond to a HuffPost request regarding his source of the corn, or if he sought any approval.

Tom Helscher, director of corporate affairs for Monsanto, told The Huffington Post that the company's toxicologists are currently reviewing Seralini's study, but that the results were unlikely to affect their products. "Based on our initial review, we do not believe the study presents information that would justify any change in the safety evaluation of GM corn products or glyphosate herbicides, or alter their approval status," he said.

Washington State's Benbrook also found several issues with the study. "On the other hand," he said, "the study is actually more carefully designed and has the same sample size as the original study conducted on behalf of Monsanto and submitted to regulatory agencies in support of the approval of the tech in the first place. If industry thought that study had a good design, it's a little disingenuous of them to be criticizing this study.

"I do think that this new study again raises an alarm bell, and ought to be followed up by much more careful studies with larger sample sizes, more than one species of lab animal and conducted by people that have no dogma on the side, haven't done research in the area and don't have an opinion in area," he continued.

Benbrook has also warned about the increased use of pesticides that has come with the explosive growth of GMO crops designed to be resistant to the chemicals.

As HuffPost previously reported, growing resistance among insect pests and weeds to the chemical concoctions leads farmers to apply larger quantities of chemicals to achieve the same level of pest control. Of note, Seralini's study found that animals exposed to Round-Up, the herbicide that the corn was engineered to tolerate, suffered serious health effects independent of whether or not they ate the GMO corn.

Benbrook is also concerned about a new sweet corn Monsanto has engineered to produce a toxin poisonous to insect pests. A study published last year found these Bt toxins in the umbilical cord blood of pregnant women, which contradicted the biotech industry's claim that the toxin couldn't survive the acidic conditions of the human stomach.

Set to hit supermarkets this year, Monsanto's sweet corn will be the first GMO food humans eat in an essentially raw form, he explained.

On Monday, before the "No on 37" ad roll-out and before the publication of the controversial study, protesters took to the streets around the U.S. to protest Monsanto and call for a "Yes" vote on Proposition 37.

"Old Man Santo had a farm, and he raised GMOs," protesters sang as part of an Occupy Monsanto rally in San Diego. "Now he sells them in the stores. No one ever knows. Not a label here or a label there, makes you want to holler and pull out your hair."

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  • A picture taken on October 9, 2008 shows an ultralight helicopter hovering above a field where Greenpeace activists and Austrian organic farming association BIO AUSTRIA wrote the message 'NO GMO' (Genetically Modified Organism) by planting light green coloured organic buckwheat in a field of organic peas in Breitenfurt, some 60 kms south east from Vienna. (DIETER NAGL/AFP/Getty Images)

  • Thirty-five tons of corn put by Greenpace activists at Mexico City's Zocalo Square as a protest against the sowing of transgenic corn, form a map of Mexico on February 26, 2009. (Ronaldo Schemidt/AFP/Getty Images)

  • People walk on a plateform past an advert against genetically modified (GMO) food on February 15, 2011 at a subway station in Paris. (MIGUEL MEDINA/AFP/Getty Images)

  • Greenpeace activists demonstrate against genetically modified organisms (GMOs) on November 24, 2008 in front of EU headquarters in Brussels. Greenpeace called on the European Union to suspend the authorization of GMOs until the EU is capable of evaluating the risks they pose. (DOMINIQUE FAGET/AFP/Getty Images)

  • Greenpeace activists stand a protest in front of Los Pinos presidential residence in Mexico City against the farming of transgenic corn in Mexico, on June 26, 2009. (ALFREDO ESTRELLA/AFP/Getty Images)

  • A Greenpeace activist impersonating Brazil's Chief of Staff Dilma Russeff takes part in a protest against the authorization to grow transgenic rice during a meeting of the National Biosecurity Technical Commission (CYNBIO) at the Science and Technology Ministry in Brasilia October 15, 2009. (JOEDSON ALVES/AFP/Getty Images)

  • Greenpeace activists distribute samples of transgenic rice as part of a protest against the authorization to grow transgenic rice during a meeting of the National Biosecurity Technical Commission (CYNBIO) at the Science and Technology Ministry in Brasilia October 15, 2009. (JOEDSON ALVES/AFP/Getty Images)

  • The logo of French 'Les faucheurs volontaires' (Volunteer trimmers of GMO) is seen as demonstrators stand in front of the booth of French union 'la confederation paysanne' (farmers union) during an action against GMO at the International Agricultural Fair on March 6, 2010 in Paris. The European Commission authorised, on March 2, the cultivation of a genetically modified potato, developed by BASF, the first such green light for 12 years. The issue of so-called 'frankenfoods' has long been a matter of fierce debate in Europe and the commission stressed that the Amflora potato in question would be able to be grown only for 'industrial use' including animal feed, rather than for human consumption. (BERTRAND LANGLOIS/AFP/Getty Images)

  • A couple waves after a parody of union between German chemical giant BASF (L) and the European Food Safety Authority (R) - Autorite europeenne de securite des aliments- (EFSA) during the International Agricultural Fair on March 6, 2010 in Paris. (BERTRAND LANGLOIS/AFP/Getty Images)

  • A giant banner depicting a farm, is seen as Greenpeace activists hold banners to protest against the genetically modified (GMO) food production in front of the parliament building of Budapest on February 10, 2010. (ATTILA KISBENEDEK/AFP/Getty Images)

  • A grey-cow is pictured near Greenpeace activists in traditional Hungarian costume standing in front of a giant banner depicting a farm as others hold a banner reading 'GMO-free Europe' to protest against the genetically modified (GMO) food production in front of the parliament building of Budapest on February 10, 2011 during a demonstration. (ATTILA KISBENEDEK/AFP/Getty Images)

  • Greenpeace activists hold a banner to protest against the genetically modified (GMO) food production in front of the parliament building of Budapest on February 10, 2010. (ATTILA KISBENEDEK/AFP/Getty Images)

  • A man dressed up as a bee holds a placard during a demonstration organized by French Professional Beekeepers Federation (FFAP) to protest against the use of pesticide on September 14, 2011 along the Saint-Bernard quay in Paris. (JACQUES DEMARTHON/AFP/Getty Images)

  • Anti-Genetically Modified Organisms (GMO) demonstrators protest in front of Colmar courthouse on September 28, 2011, eastern France, during the trial of 60 militants accused of destroying MGO plants. (FREDERICK FLORIN/AFP/Getty Images)

  • An anti-GMO activist holds a banner reading 'Science without conscience is but the ruin of soul' during an action to call for the ban of the 'MON 810', a variety of genetically modified maize (corn) developed by Monsanto Company on January 23, 2012 at a Monsanto storehouse in Trebes near Carcassonne, southern France. (ERIC CABANIS/AFP/Getty Images)

  • Is Branding Food With "GMO" the Kiss of Death?

    Prop 37 in California proposes that genetically modified food be labeled "GMO". If you knew your food was genetically modified, would you still eat it?