In May, the World Health Organization issued an alarming report that declared widely used non-sugar sweeteners like aspartame are likely ineffective for weight loss, and long term consumption may increase the risk of diabetes, cardiovascular diseases and mortality in adults.

A few months later, WHO declared aspartame, a key ingredient in Diet Coke, to be a “possible carcinogen”, then quickly issued a third report that seemed to contradict its previous findings – people could continue consuming the product at levels determined to be safe decades ago, before new science cited by WHO raised health concerns.

That contradiction stems from beverage industry corruption of the review process by consultants tied to an alleged Coca-Cola front group, the public health advocacy group US Right to Know said in a recent report.

It uncovered eight WHO panelists involved with assessing safe levels of aspartame consumption who are beverage industry consultants who currently or previously worked with the alleged Coke front group, International Life Sciences Institute (Ilsi).

Their involvement in developing intake guidelines represents “an obvious conflict of interest”, said Gary Ruskin, US Right to Know’s executive director. “Because of this conflict of interest, [the daily intake] conclusions about aspartame are not credible, and the public should not rely on them,” he added.

Aspartame was first approved for use in the US in the early 1980s over the objection of some researchers who warned of potential health risks. In recent years, as evidence of health threats has mounted, industry has ramped up a PR campaign to downplay the issues.

In the World Health Organization’s 14 July aspartame hazard and risk assessments, its cancer research arm, the International Agency for Research on Cancer (Iarc) classified aspartame as “possibly carcinogenic”. That same day, WHO’s Joint Expert Committee on Food Additives (Jecfa), which makes consumption recommendations, reaffirmed the acceptable daily intake of 40 mg/kg of body weight.

Ilsi describes itself as a nonprofit that conducts “science for the public good”, but it was founded in 1978 by a Coca-Cola executive who simultaneously worked for the company through 2021, US Right to Know found. Other Coca-Cola executives have worked with the group, and US Right to Know detailed tax returns that show millions in donations from Coca-Cola and other beverage industry players. Coke ended its official membership with the group in 2021.

Over the years, Ilsi representatives have sought to shape food policy worldwide, and Ruskin, who has written multiple peer-reviewed papers on the group, characterized the aspartame controversy as a “masterpiece in how Ilsi worms its way into these regulatory processes”.

US Right To Know identified six out of 13 Jefca panel members with ties to the industry group. This Jecfa panel prepared its assessment with another group of 13 experts, among which two also have Ilsi ties, the WHO acknowledged last month in a statement to the news outlet Le Parisien.

In a statement to the Guardian, a WHO spokesperson defended the industry consultants’ inclusion in the review process.

“For the meeting on aspartame, Jefca selected the experts likely to make the best contributions to the debate,” said spokesperson Fadéla Chaib. She said WHO’s guidelines only require disclosure of conflicts of interest within the last four years.

“To our knowledge, the experts you listed by name have not participated in any Ilsi activities for at least 10 years,” she said.

But that partially contradicts a statement made by WHO just weeks before to Le Parisien in which it acknowledged two people currently working with Ilsi were involved in the process. The Guardian had also asked about those two people identified in the Parisien story but were not listed “by name” in its email.

The WHO told Le Parisien: “We regret that this interest was not declared by these two experts either in the written statement or orally at the opening of the meeting.”

WHO’s inclusion of Ilsi-tied consultants in its review process is especially alarming because WHO has in place “much higher standards” to ensure there are no conflicts of interest in its processes, Ruskin said. He noted WHO only relies on publicly available, peer-reviewed science, while excluding corporate interest studies.

Ruskin said the move also marks a change in direction for WHO, which in 2015 distanced itself from Ilsi when its executive board found the group to be a “private entity” and voted to discontinue its official relationship.

Ruskin said the damage has been done. In the “avalanche” of media coverage of WHO’s designation of aspartame as a possible carcinogen, many outlets noted WHO’s split decision, or reported that WHO found the product to be safe. Those reports did not note Ilsi’s fingerprints on the safety assessment, Ruskin said.

“So much of the tone of it has been ‘There was a split decision at WHO and we shouldn’t be concerned, so go ahead and drink all you want,’” he said. “That has so poorly served the public.”

• This article was amended on 18 August 2023 to clarify how Jecfa panel members’ ties to Coca-Cola were uncovered by US Right to Know and Le Parisien.