Lily James in Bathing Suit Feels "Cold Sweat"

Lily James, star of Pam & Tommy, is chilling this weekend in a swimsuit. "My day, listening to" GLU, she said, jamming to "Cold Sweat," the new song from GLU, aka Michael Shuman, aka Mikey Shoes. The actress looked incredible as always, so how does she do it? Read five ways the star got that body below and what experts revealed to CelebWell about her diet and fitness secrets. Read on to see 5 ways Lily James stays in shape and the photos that prove they work—and to get beach-ready yourself, don't miss these essential 30 Best-Ever Celebrity Bathing Suit Photos!

1. She Had This Diet Plan

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The 33-year-old told Cosmopolitan UK, "On top of the fitness and the health, we also had a diet plan we had to follow which involved eating between five and six meals a day." Antoine Hamelin, a kinesiologist, a trainer with 20+ years of experience and CEO of First Step Fitness Trainings explains, "Choosing to spread your food consumption over many small meals can help you lose weight faster. Many people find it hard to eat a normal portion size if they consume 3 meals per day only. Thus, by eating frequent meals, they can eat smaller portions, more often. Eating many small meals can help you lose weight also because every time you eat, the body has to expand calories for digestion. This is especially true if you have a lean protein source with each meal -- like chicken or fish. Protein is by far the most costly nutrient to digest -- it takes 6 times more calories than it takes for carbohydrates. It makes it a great asset to lose weight faster and to recuperate from your training sessions as well."

2. High Protein Diet

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James told Cosmo UK as part of her training, she would eat "lots of protein shakes, fish, chicken and no cheating.” Aaron Guyett, CSCS Director of Education with Living Fit explains, "High protein helps you lose weight and stay in shape in three ways. First, it curbs your appetite and cravings, especially for simple carbs like sugar and breads. Second, it provides the nutrients needed to keep lean body mass, which is crucial if you are looking to burn more calories during exercise and at rest. Third, it helps to level out your glucose and insulin. Often we eat too many carbs, which spikes our glucose, and then our insulin spikes, leading to fatigue and hunger for more carbs. Riding this roller coaster is a common thread for most people, but a high protein diet helps reduce the spike in glucose and insulin. We suggest 1 gram of protein per pound of lean body mass, if you know your lean body mass, or you can consume 1.6-1.8 grams of protein per kg of body weight.”

3. Boxing

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The star told Entertainment Weekly, "And I did boxing with a really cool guy in LA, which I love too, being physically strong like I could literally beat someone up." TJ Mentus, an ACE Certified Personal Trainer, Certified USA Weightlifting Level 1 Coach and review board at Garage Gym Reviews explains, "Boxing helps to keep weight off by burning calories at a high rate. The reason that boxing is so effective is that it is an activity that significantly increases the heart rate. While working at higher heart rate zones, the body is using energy more rapidly and, in turn, burning more calories per minute than a lower intensity activity. When done with proper technique, boxing is a full-body workout that trains muscles to be more explosive or generate more power. As the muscles are challenged consistently in boxing workouts, the demand placed on them will force them to grow stronger to generate force more efficiently. For the mind, it helps to calm it by requiring complete focus on the task at hand. It is a sport that requires precision and excellent technique. This forces the mind to be present at the moment and not think about other external things that may be stressing it as they are not necessary in that moment."

4. Cardio

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James did a lot of cardio for months to help get her fit for the role and Mentus states, "Being able to perform cardio or handle cardiovascular stress is a vital function of the body. Cardio trains the heart and lungs to be stronger and operate more efficiently, which is how the body gets in better shape and more fit. Over time, performing cardio consistently will create adaptations such as lowering the resting heart rate and the respiratory rate, which signify being in better shape and physical health. It is great for keeping weight off by increasing the amount of calories burned. The rate of calories burned during cardio versus at rest is significantly higher, so including cardio can help better manage the amount of calories expended versus calories consumed."

5. Changes Up Her Routine

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To prep for Pam & Tommy, James did a variety of intense workouts. "We trained five times a week; running, circuits, weights, working out in the gym, and on the track. It was an insane time. When I first had to lift weights, do squats and run for ten minutes I was knackered and wanted to be sick," she told Cosmo UK. Hamelin says, "Losing weight, gaining muscle and increasing cardiovascular capacity are all adaptations to training. The human body will adapt to any stimulus -- brought about by different training methods -- if the stimulus is intense enough. This is why it is important to increase progressively the amount of weight used for different weightlifting exercises and to increase the pace from time to time when we do cardio for example. While adapting to an increase in stimulus is desirable and is what makes us progress, it can be a double-edged sword. When the body has fully adapted to an exercise or a training routine, the progression stops. Thus, from time to time, you must change your program, your exercises and other variables (like the rest times). To optimize your training, I recommend changing your strength training program every 4-6 weeks. On the other hand, cardio can be changed almost daily, using different methods or activities. For example, you can choose to run one day, and to ride your spinning bike the next. The difference in muscles used in both exercises is enough to add variety and to prevent staleness. Another way to mix it up is to use a moderate intensity for a longer period of time one day -- 45 minutes at an intensity at which you can carry a conversation -- and to use a high intensity the next -- doing short intense intervals for 20 to 25 minutes."