Just in time for summer. Kate Upton's trainer is dishing on how the bikini model keeps her body so fit to Elle magazine. David Kirsch shared some of Upton's secrets and said the 21-year-old follows a strict diet and has a rigorous workout schedule.

 "She is amazing about the diet," Kirsch told Elle. "She does a few egg whites with spinach for breakfast, and when she wants to, she does a 48-hour cleanse. She loves my Thermo Bubbles for snacks and my Super Charged Greens. She has a very good sense of what she needs to eat for her body."

Kirsch - who also trained Liv Tyler, Karolina Kurkova and Hiedi Klum - said part of Upton's regime is a no-sugar, no-carb diet.

"It's sugar, alcohol, and processed carbs that we avoid. I don't have her on a specifically gluten-free diet, but by the nature of what she eats, it is gluten free," he said, adding that his young daughters are on a gluten-free diet and "you can see the difference."

"They are only four years old and it definitely changes their mood," Kirsch said. "You can see that for sure."

The trainer said Upton isn't completely forbidden from drinking alcohol and she can have a few (just a few) glasses of wine.

"It's not going to kill [your fitness], but I need to at least put the fear of God or fear of something into my clients," he told the magazine. "There should be the guilt factor. A night of binge drinking can really undermine what you are doing. If you are on a shoot, you want to look your best, and if you are drinking, you aren't going to be able to do that. I don't care who you are."

As for her workout, the Sports Illustrated model does circuit training, cardio and boxing.

"We do extreme, intense circuit training. We do three or four circuits and change them up a lot," Kirsch told the magazine. "There's lots of core, butt, and thigh shaping and toning. We work with resistance bands, stability balls, do lateral lunges for the inner thighs, and for arms we do boxing. She has her own pair of boxing gloves, and she will beat me up. She loves it! It's a very efficient way to work out and get your cardio in."