Kim and Kourtney Kardashian in Bathing Suits "Grow and Evolve"

Happy birthday, Kourtney Kardashian! Her younger sister, Kim, celebrated the special day yesterday by sharing a photo of the duo rocking matching blue swimsuits while taking a dip in the ocean. "Each year around the sun with you I learn and grow and evolve more because of you! Thank you for teaching me to always go for it and follow my heart even if no one else understands. They just aren't on our growth path lol Happy Birthday @kourtneykardash! Happiness looks so good on you. I love you so much!!!" she captioned the series of snaps. How do the sisters maintain their famous figures? Read on to see 8 of Kourtney Kardashian and Kim Kardashian's top stay-fit secrets 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!

Kim achieved her best body ever with the help of bodybuilder Melissa Alcantara. "I've been working out really hardcore with a bodybuilder for one year. September is our one-year anniversary," Kim revealed to E! News in 2018, via People. Kim lifts "heavy weights," per her trainer who told Women's Health that 85% of their training is done with weights and the other 15% is cardio. 

Kourtney tries to move every single day. "I think I take time for myself to work out every day, which I never did—especially having three young kids," she told E! News. "I just never made it a priority, and now it just makes me feel so much better—mentally and physically." She does a variety of workouts including strength training, jump roping, running, lifting weights, stairmaster cardio, Pilates, and floor exercises. (For her website's advice on how to burn fat in 15 minutes, go here.)

Kim has a no-excuses policy when it comes to exerise. "Just like anybody else, there are days where Kim's not really feeling it, but she will do it," her trainer told People in 2020. "Then there are a lot of days when she's killing it! She never questions the stuff that I make her do, she's just like, 'Okay,' and I love that."

Kourtney relies on workout buddies to keep her accountable, including friends like Sarah Howard and her sister, Khloe. "I know for my body, I need four to five days a week of working out. Khloe and I really push each other to do that," she told E!

Kim doesn't starve herself thin. In fact, she eats to fuel her workouts. "My trainer Mel always says that before and after you train, you should eat simple carbs, like sweet potatoes, and small amounts of fat and protein, like chicken. You should also have veggies with your meals, since you need them to help effectively break down and absorb your protein, fat, and carbs," she wrote on her app in 2018, according to Harper's Bazaar.

Kourtney is a big fan of supplements. "Every morning, when I wake up, I take collagen supplements on an empty stomach," Kourtney dished on Poosh. She starts her day with water mixed with one tablespoon of organic apple cider vinegar. "Both of these habits have a ton of health benefits that affect everything from your skin to the way your body processes nutrients," she said. She also drinks a vegan probiotic shot, and then avocado pudding with the fruit and E3Live blue-green algae, bone broth protein powder, and MCT oil. "It only takes a few minutes to make and it provides healthy fats to give me energy before a workout," she says about her routine. 

Kim famously followed the Atkins diet, but has been gravitating toward plant-based living for the past few years. "My 40s are about being Team Me. I'm going to eat well," she told Vogue. "Plant based and two workouts a day for 30 days to get our mind and body right this year!" she wrote in a January 2021 Instagram post.

Kourtney, who recently gained weight during IVF treatments, as the medication she was given put her into menopause, has major self-love whether she is at her thinnest or curviest. "I'm a woman with a BODY," she wrote in response to a user asking if she was expecting when she had gained a little weight. "This is me when I have a few extra pounds on, and I actually love it," shesaid to a follower during another body shaming incident. "I have given birth three amazing times and this is the shape of my body."