Parents are currently advised to switch toddlers to reduced-fat milk at age 2. David M. Goehring/Via Flickr hide caption

toggle caption David M. Goehring/Via Flickr

Thousands of years ago, a mutation in the human genome allowed many adults to digest lactose and drink milk. iStockphoto.com hide caption

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Dairy farmer Bob Andrews feeds heifers in the same barn his grandfather used. He says today "the harder you work, the further you get behind." David Sommerstein/NCPR hide caption

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Milk Producers Peer Over The Dairy Cliff

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European dairy farmers spray police officers with milk during a demonstration outside the European Parliament in Brussels on Monday. Geert Vanden Wijngaert/AP hide caption

toggle caption Geert Vanden Wijngaert/AP

Laura Chase, a member of the MOOMilk dairy cooperative, sweeps her barn in a film still from documentary, Betting The Farm. Pull-Start Pictures hide caption

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A customer studies a display of milk and dairy products at Bryan's Grocery in San Francisco in March. Justin Sullivan/Getty Images hide caption

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In one piece from the edgy campaign, a henpecked husband cowers with cartons of milk in his arms. California Milk Processor Board hide caption

toggle caption California Milk Processor Board