How was Theodore Roosevelt able to protect consumers from big business?
He struggled against big business by breaking the trusts in order to make competition fairer.
Big business with its trusts are a threat to consumers because their share of the markets enables them to control prices and thus fleece out consumers. Market economy indeed requires faire competition and thus atomization of the market(many buyers, many sellers).
The Sherman Law in 1890 was passed under Harrison presidency to solve to problem. TR continued this policy and brought a series of legislation that weakened the trusts. This is the main reason why this period is called the Progressive Era.
According to Marixists, thses laws were aimed at relieving popular contestation that was embodied by the Populists.
Libertarians claim that these laws were a travesty that actually strengthened their power while pretending to weaken it, indeed regulations were passed which consolidated their hegemony.