Elizabeth Olsen is already an action movie pro with two Avengers movies and two Captain America films under her belt, but she wanted to get even stronger for Avengers: Infinity War.

The actress, who plays the Scarlet Witch in the film, out on May 4, worked with nutritionist Philip Goglia to lose body fat and add muscle.

“She was focused on body composition,” Dr. Goglia, celebrity nutritionist and founder of G-Plans, tells PEOPLE. “Fat is light and fluffy and takes up a lot of room, and muscle is dense and doesn’t take up a lot of room at all. She wanted to be dense, heavy and athletic, and she just really nailed it.”

Olsen, 28, also had a specific look in mind for her body shape.

“This newest Avengers movie she was more about muscularity,” Dr. Goglia says. “More stomach and shoulder shape, a better V-shape in her back and a much tighter waist and shoulder differential.”

And Olsen was doing plenty of workouts and training for the action-packed film, but Dr. Goglia emphasizes that nutrition and sleep is the key to a better body composition, not exercise.

“The training is great, but the important thing to remember about training is that it makes you weaker, it doesn’t make you strong,” he says. “Training is inflammatory: it breaks you down. The other pieces are what make you stronger, and that’s the kitchen and the bedroom, so you’ve got to make sure your kitchen is in order and you’re getting enough sleep.”

For Olsen, that meant seven to eight meals a day, with no gluten, dairy or yeast. She would start out the day with a teaspoon each of almond butter and jam before training, and then follow that with a protein shake or eggs and vegetables for breakfast, fish or chicken for lunch and vegetables and another fish for dinner, with snacks of fresh fruit, vegetables and hard-boiled eggs throughout the day.

Dr. Goglia says fatty fishes like salmon, sea bass, black cod or artic char are best, especially at dinner, to “promote deep sleep and hormone release.” And for a nighttime snack, he had Olsen eating a spoonful of honey or blackstrap molasses for iron.

“It shuttles oxygen to the red blood cell count to create more energy for the morning and better endurance capacity,” he ss.

Dr. Goglia also worked with Don Cheadle for the movie, and he says Cheadle, Olsen and the rest of the cast always does a fantastic job with their nutrition.

“We get any of these Marvel folks in — from Chris Hemsworth to Chris Evans — and they approach food just like any of my pro athletes would,” he says. “They’re great — they don’t stray.”