The fascinating science behind everything throughout the course of a three-day juice cleanse, from headaches and hunger to that second-day euphoria.

By By Paige Fowler and Andrea Stanley
Photo: Getty Images / FGorgun

With everyone from celebrities to your cube-mate joining the cleansing craze, you've likely done at least one juice detox-and experienced the roller coaster of highs and lows physically and mentally.

Luckily, hangriness aside, none of the side effects of a short cleanse (read: three-day cleanse, max!) are dangerous. "A three-day juice cleanse is not a harmful thing," says Robynne Chutkan, M.D., an integrative gastroenterologist and author of Gutbliss: Feel Light, Tight, and Bright-the Healthy Way. "It's not going to dramatically change your health." (Related: Superfood Juice Shots That Are Tailored to Your Health Needs)

But before you embark on a 72-hour, cold-pressed sipping spree, know this about the three-day juice cleanse. "Juice cleanses are a fad," says Kimberly Sasso, R.D. "There is no current, clear evidence that shows any health benefits to juice cleanses." And all the claims of detox benefits are nothing more than pulp fiction, she adds (that said, your armpits, hair, and beauty routine *could* use detoxing). "There's a perception that juicing can 'cleanse' your intestines and 'reboot' your system-neither of which are true. Your liver and kidneys do all of the necessary cleaning."

What's more, even the "best" three-day juice cleanse might do more harm than good. "Juicing reduces the fiber content and leaves only some of the vitamins, minerals, and some phytonutrients," Sasso says. Plus, an all-juice diet is painfully low in protein. The result: "The body starts breaking down its energy stores, including muscle," Sasso says. That happens whether you're doing a three-day juice cleanse or a 10-day cleanse-anything over a day.

That doesn't mean you need to stop gulping your greens altogether, though. Sasso suggests blending rather than juicing. "Blending maintains the fiber content of plants," she says. If it's a meal replacement you're looking for, be sure to include a protein source, like protein powder, hemp seeds, or Greek yogurt.

All of that aside, curious what *actually* happens to the body on a three-day juice cleanse? Check out this day-by-day snapshot of what you can expect to encounter, and decide for yourself. (And BTW, you can eat clean without obsessing over it.)

Day 1

"Right off the bat, you may notice you're thinking about what you could be eating, should be eating, and want to be eating, so cravings can occur right away," Dr. Chutkan says. Blame habit-meals are ingrained in your daily life-and the lack of chewing food may be throwing you for a loop. Luckily, these distracting thoughts and cravings usually subside as you complete the day and see you can make it without solids, she adds.

If you're a low-carb eater, you could experience hypoglycemia while juicing since your body isn't used to all the sugar. "Each juice floods your system with simple sugars, which requires your body to produce more insulin more quickly," says Jo Ann Carson, Ph.D., R.D., a professor of clinical nutrition at the University of Texas Southwestern Medical Center. "If your body sends out more insulin than is needed, your blood sugar could drop too low, and you may be faint or light-headed." (Related: 10 Low-Carb Snacks to Eat on the Go)

Low-carb or not, by the end of the day, your energy may be low. Each day on a cleanse provides approximately 900 to 1,000 calories compared to the 1,500 to 2,000 calories you're probably eating most days. "When you restrict calories, your brain isn't getting as much glucose, which is its main energy source, so you may feel sluggish and grouchy," Carson says. If you typically exercise later in the day, plan to do a lower-intensity activity such as yoga, walking, or swimming, and go to bed early. (And FYI, it takes 24 to 72 hours for food to travel through your GI, so expect to have a normal bowel movement on day one since it's the by-product of what you ate the day or so before.)

Day 2

Welcome to the honeymoon day. "If you've ever heard people describe cleansing as a spiritual experience, this is the day that gives them that feeling," Dr. Chutkan says. "You wake up feeling lighter since your digestive tract isn't as full as when you're eating solid foods." You could also have more energy. "Digestion is a very active process and takes lots of resources from the body, such as blood flow and energy to break down food," she explains. "Even though you're taking in fewer calories, your body isn't doing this work, so that energy is available to be used elsewhere."

Another component to the juicing "high" could simply be an undiagnosed food allergy. During a cleanse, you're eliminating nearly every major food group other than fruits and vegetables. "Allergies to soy, corn, eggs, nuts, wheat, dairy, and artificial sweeteners are so common in the U.S.," Dr. Chutkan says. Take these things out, and you may feel less bloated and puffy, and you may experience fewer headaches or achiness within 48 hours if you have an allergy and don't know it, she says. If you feel dramatically better, it's worth investigating further to identify what your trigger might be. (Keep in mind that if you're allergic or intolerant to gluten, it could take four to six weeks of being gluten-free before symptoms improve.)

On the other hand, if you're a caffeine drinker (one to two cups daily), withdrawal may rear its ugly head since you have to cut out coffee, tea, soda, and chocolate during a cleanse. Caffeine is a stimulant to the central nervous system, and regular consumption causes some degree of dependence on it. "Without caffeine, blood vessels in the brain can dilate, causing headaches," Dr. Chutkan says. You may also experience central nervous system depression, which is like the opposite of stimulation and comes with a hefty dose of drowsiness and fatigue. (Heavy caffeine drinkers-four or more 8-ounce cups per day-may notice signs as early as the first day.)

Be prepared for a few extra trips to the bathroom on day two. Without fiber or protein to slow absorption, the sugar in each juice draws fluid into your digestive tract, resulting in softer, more frequent bowel movements, Carson says. This literally gives your body a "cleansing" feeling. Your bottles of juice aren't enough to replace these lost fluids, so be sure to chug plenty of water throughout the day to prevent dehydration.

At night, you may find yourself counting sheep (or steak) while you sleep. "If you feel hungry, it can be really difficult to sleep because your body is trying to tell you that you should eat," Carson says. "Plus the combination of a low-calorie diet and the lack of solid foods may confuse your GI tract, so you could wake up at night needing to have a bowel movement."

Day 3

"On the third day you're coming down from the high of day two," Dr. Chutkan says. "Some people experience stronger cravings because the end is in sight, and they find themselves planning exactly what they're going to eat the next day."

Despite what you may have heard, your metabolism is unlikely to stall in just three days since you're still taking in calories, carbohydrates, and vitamins and minerals. "It's really the digestive system that slows, simply because there's less to digest," Dr. Chutkan says. However, it's a temporary change that will return to normal once you start eating normally again.

"As much as people like to think they're getting so many nutrients on a juice cleanse, you're missing out on macronutrients like protein and fat that your cells need to function," Dr. Chutkan says. Doing a cleanse for more than three days could make you feel the effects of omitting these nutrients and possibly lead to loss of muscle, decreased brain function (since the brain needs fat to operate), and disruptions in neurotransmitters and hormones, which rely on protein and control multiple crucial bodily functions from menstruation to your mood.

One surprising perk you may enjoy by the end: increased libido. "Patients are often surprised when I ask them in a consultation how things are in the bedroom," Dr. Chutkan says. "Many people don't realize that the rectum and sigmoid colon are located directly behind the pelvis." When you're bloated, you don't want to have sex because gas could slip through or the pressure during sex feels uncomfortable. By day three of juicing, however, bloating should be way down and your digestive system is pretty empty-especially if you experienced the Drano effect the day before. (Related: 8 Simple Ways to Get Rid of Belly Bloat) "You may have an increase in confidence and feel better about yourself and your body because you've accomplished this difficult feat, which works wonders for your libido," Dr. Chutkan says.

With pizza still off the menu for the next 12 hours, a roll in the hay sounds like the perfect way to celebrate the conclusion of your cleanse.

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