It depends on what he means by pure sugar. Table sugar is half fructose and has a lower GI than dextrose or maltodextrin. Pure glucose (dextrose) has a GI ranging from 100-114 (depends on the amount I think). Maltodextrin has a GI of 140 apparently, it's not listed at glycemicindex.com so I can't confirm. A potato has a GI ranging from 50-111 depending on what kind. Plus, dextrose and/or maltodextrin are way cheaper than potatos.
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