"I have to confess, my husband loves beer. So I had to figure out the truth," Ms Hari wrote on her website.
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"There is a long list of ingredients allowed in beer – like high fructose corn syrup, artificial flavours, stabilisers that are linked to intestinal inflammation ... even fish swim bladders.
"But, why are the ingredients not listed on the label? What's really in beer? Which brands can we trust?"
The rapid response by AB InBev and SABMiller – which capitulated to Ms Hari's demands within 36 hours – underscores the growing power of social media over corporate policy.
AB InBev, one of the sponsors of the World Cup, immediately invited the 34-year old Ms Hari and her husband "to our flagship St Louis brewery to show how our beers are made and the ingredients we use".
The Belgian-Brazilian company, the world's largest brewer, listed its ingredients for its Budweiser and Bud Light brands on its website after the food activist gathered just 44,000 signatories demanding to know what was in the beers.
SABMiller did the same for its Miller Lite and Coors beers. US labelling requirements for beer in the US differ from those in the European Union, where ingredients are listed.
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Ms Hari said she was "thrilled with Anheuser-Busch's quick response", telling her supporters that: "It's pretty amazing that making your voice heard can change the policies of a multibillion-dollar company overnight."
In the three years since she launched Foodbabe, Ms Hari, a former management consultant has claimed considerable success in obliging food and drinks companies – with a market value between them of $364bn – to remove or disclose ingredients.
Subway, the sandwich chain, removed a chemical used in yoga mats from its bread earlier this year while Kraft eliminated a dye from its Mac and Cheese ready meal. General Mills, Chick-fil-A and Chipotle have also removed ingredients following campaigns launched by Ms Hari.
Professor Paul Berryman, food scientist and head of Berryman Food Science consultancy, said: "It's increasingly important for companies operating in food and drink to take account of social lobby groups because they are so powerful and the information in them spreads like wildfire."