The Belgian moved to the San Siro from Manchester United in a £59m deal but turned up for pre-season as much as 10 pounds over his ideal match weight.
But after his electric start to life in the blue and black stripes, scoring two in two games, Lukaku has explained why he was below peak condition.
“Normally I have a fine digestive system,” Lukaku told ESPN. “I digest everything very quickly.
“That’s how it had been my entire life. But what the nutritionist said to me was, it had stopped working.”
Lukaku was then put on a special diet that included fish, sweet potatoes, shiitake pasta, and cooked and raw vegetables.
The change made an immediate difference, losing 10 pounds in just 12 days, with Lukaku satisfied after returning one of the lowest body fat percentages in the Inter squad after tests a few weeks ago.
“I’m good now,” Lukaku adds. “My body is in the zone.”
Antonio Conte will hope Lukaku can extend his promising start to life in Serie A with Udinese next on Saturday.
Champions Juventus are also perfect after two games on six points, with Inter hoping Lukaku can fire them to the scudetto to break the Old Lady’s eight-year reign.
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