Transcript for The 'Whole 30': Dieters Eliminate Certain Foods to Shed Pounds
Well there is a controversial new 30-day diet out there that some say can fundamentally change your relationship with food and lead to a healthier lifestyle. But, only if you are willing to say a lot of noes. Here is my "Nightline" co-anchor juju Chang. Julie says she never felt better in her life. It feels amazing. I feel like I am living in the body that I was meant to live in. Reporter: She has tons of energy and claims getting sick as a thing of the past. I don't get sick. I don't have seasonal allergies anymore. Reporter: All this in 30 days. Her secret -- the whole 30 diet. All righty. Reporter: She says transformed her relationship with food. And rebooted her lifestyle. It is more than diet and exercise. It is about the life that you are given on the other side of it. Reporter: It starts out with extreme restrictions. A lot of no-noes. No carbs, no beans, no dairy, no eggs. Like the Paleo diet you eliminate all food that can't be hunted or gathered. So just protein and fruits and vegetables. 30 days, no wiggle room, no gray area. Reporter: Those committed enough to tough it out swear by the results. Push through your heels. Reporter: Something Julie's trainer saw firsthand. She is much happier. When we talk to her. When she comes in. She will be happier with the kids. All kinds of things like that. Transform not only in the gym but you take it outside all abut attitude. Reporter: For Julie. Day five was the hardest. Day five was at my kids' soccer game feeling absolutely miserable, from, from probably sugar detox. Feeling low energy. But I kept plugging on knowing that -- from other people's results that on the other side was going to be some great -- great life changes. Reporter: She says once she got past the initial pain there was no going back. I have conquered a life team of bad habits in 30 days. Reporter: She lost 50 pound going from this to this. There is never going to be any great time to try this. There is not going to be a perfect day. Jump in. Give it a go. Reporter: The diet was profiled in the best-selling book "It starts with food" and fueled by #whole30 which is burning up the internet with more than quarter million snaps on instagram. It is a short term nutritional reset to help people learn what foods affect them and how. Reporter: The diet is the brain child of certified sports nutritionist, Dallas and Melissa Hartwig. We have developed so many habits. We use food as comfort, love, reward, punishment, we have cravings, we feel like we are not in control of the foods that we eat. We have built rules into the whole 30 to help people break their cravings but also re-establish a healthy connection with the food that they're eating. Reporter: In addition to cutting out grains like rice or wheat, there is of course, no processed sugar or alcohol. So a lot of the foods contain different types of protein structure that can be a problemen the human body. We are not adapted to eat a lot of the foods. Reporter: One surprising no-no. The bathroom scale. So many people are so obsessed with that number on the scale. It scan make or break your day. The scale will blind you to amazing progress you can make and improvement you can see, energy levels, sleep quality, athletic performance, recovery, improvements in your medical condition. That's yummy? Reporter: Like countless devotees, Ashley follows the diet on social media. Snapping photos of her meals to share with friend and collect encouragement. Like Julie, the beginning was a challenge. The first day it is exciting. Frying something new. This is fun. A couple days after it wears on you. You miss your diet coke. You want to run through the drive through and grab some things. Roim th Reporter: Then she said something miraculous happens. Once everything is out of your system. Probably for me it was about, day eight or nine. That's just when my energy just kicked up. I could totally tell a difference. And I just felt amazing. Reporter: The key to the whole 30 is slowly reintroducing food types like gluten or dairy or sugar. One by one. To see how your body reacts. For Julie Halama, results were startling. Sugar cravings came back with a vengeance. And it only took, you know, one bit of sugar to, to be, craving sugar kind of all day long. But, dairy, I feel like, I have some joint pain, some old injuries that kind of come back and just skacause me discomfort eating dairy. Reporter: The popular diet is not without its naysayers. Awe off it takes a lot of meal planning. You need to have a lot of time. Money. I don't think the whole 30 diet is a realistic diet plan. It will encourage rapid weight loss which is something that we all may love. But in fact, dieters may regain the weight that they have lost once they start reintroducing their favorite foods back into their diet. The larger concerns, say critics, is poor nutrition. When you start eliminating food groups like dairy, legumes, whole gra whole grains you miss out on calcium, vitamin D, B vitamins, fiber, anti-ox dantz important for overall health. Reporter: The diet's inven inventers defend it. There is nothing you are macing from food you are om omitting for 30 days that you can't get from the healthy food on the plate. You are not missing anything. Minerals, vitamins, calcium. Fiber. Now Julie is on maintenance running from herb kids to her company. Grabbing healthy lunches. You are quite healthy. Thank you. Reporter: And feeling good too. It feels great when I get up and I worked really hard. My husband thinks he has a new wife. Reporter: For "Nightline," juju Chang in New York.
This transcript has been automatically generated and may not be 100% accurate.