As a registered dietitian who fields questions on the daily about nutrition and weight loss, I’ve noticed that fasting has overtaken the keto diet as the buzziest weight-loss topic. Of the numerous ways to fast, my clients have tried several, and so have I. But one trending method was compelling enough for me to pay for, and to recruit my husband to try along with me.
The five-day meal program called ProLon is actually a fasting-mimicking diet. In other words, the plan shifts your body into fasting mode and allows your cells to reap the benefits of fasting, while still allowing you to eat a limited amount of food. Here’s a primer on what the diet is, its benefits, cost, potential downsides, and more.
What intrigued me most about ProLon is that it’s based on years of research from one of the most well-known longevity experts in the country, and the studies on its outcomes are impressive. It’s also in line with my own eating philosophies: ProLon is 100% plant-based, gluten-free, and non-GMO, with no artificial additives.
ProLon was created by Valter Longo, PhD, director of the Longevity Institute at the University of Southern California in Los Angeles. The company’s research partners include well-respected organizations, such as the Mayo Clinic and the Texas-based MD Anderson Cancer Center.
After over a decade of studies in both animals and humans, Dr. Longo developed a patented eating plan that activates the body’s fasting mode, but provides enough food to simultaneously nourish cells and support metabolic balance. This “fasting with food” approach also reduces some of the risks tied to extreme fasting, which may include very low blood sugar and/or blood pressure, reduced immune function, fatigue, and low productivity.
The specific combination of limited plant protein, calorie restriction, good fats, and low-glycemic index carbohydrates in ProLon regulates the body’s nutrient-sensing pathways, meaning cells believe they’re not being fed. At the same time, the diet triggers anti-aging pathways responsible for cellular repair and regeneration.
ProLon is considered to be a type of periodic fasting, as opposed to time-restricted feeding (for example, narrowing your eating window to 4-8 hours a day) and intermittent fasting (such as eating just 500-600 calories two nonconsecutive days a week).
The studies on ProLon’s outcomes are pretty remarkable, particularly given that the plan is just five days in length. One published study compared a control group with no diet intervention to a group that followed ProLon five days a month for three months. The ProLon group lost body weight and belly fat while maintaining lean muscle mass. They also experienced reductions in blood pressure, cholesterol, and CRP, an inflammatory marker, in addition to an increase in stem cell production, as well as reduced levels of insulin-like growth factor (IGF-1), a hormone linked to cancer risk when elevated.
According to scientists, these health-protective results appear to be limited to the precise makeup of the ProLon plan, and are not seen in other periodic fasts, such as a juice cleanse, or even a longer term keto diet. The latter has actually been shown to trigger unhealthy outcomes, including constipation, vitamin and mineral deficiencies, and increased cholesterol.
In addition, the majority of the ProLon results were sustained, even after participants resumed their normal diets. For example, 115 days after the protocol, 60% of the weight lost and reductions in waist measurements were maintained.
Other research-based ProLon outcomes noted include reductions in glucose, insulin, triglycerides, and blood pressure, as well as improved stress resistance and cognitive performance.
Based on all of this incredible research, I was excited to try ProLon myself, especially before advising clients about the plan. My husband, Jack, agreed to try it with me, and after ordering ProLon online, our kits arrived by mail.
ProLon’s packaging is stellar. The sleek, modern, glossy white kit reminded me of unboxing a new MacBook or iPhone. Inside sits five smaller boxes, marked Day 1, Day 2, etc. Each contains all of the food allowed for that specific day, which is made up of 73 carefully chosen plant ingredients. The protocol does not allow you to change the order of the days or swap foods from one day’s allotment to another. You can, however, decide how you’d like to arrange your daily meals, although a suggested menu card is provided.
Foods provided include dried soup blends (which require adding hot water), proprietary nut and cocoa-based bars, pouches of olives and kale flax crackers, and a few supplements, including algal oil for plant-based omega-3 fatty acids, and a multivitamin/mineral. The kit also contains herbal teas and a large water bottle used to sip a flavored glycerol-based drink, which is used days two through five to help maintain fullness.
A very straightforward instruction sheet is provided, which lays out how to follow the plan. An FAQ on the brand’s website answers additional questions about things like coffee (one cup of black per day is allowed), adding lemon to water (one wedge per day is OK), sprinkling herbs into the soup (one teaspoon per day is permitted), and drinking sparkling water (fine, as long as it’s all natural with no artificial sweeteners).
Overall, I’ll admit that getting through the five days wasn’t easy, especially for Jack, who is taller with a muscular build and has a more physically demanding job than mine.
Day one started out just fine. In fact, it was a bit of a breeze. That day provides the most food, and all of it was tasty and enjoyable. It also wasn’t a significant switch, because we’re already plant-based. And having all of our food for the day pre-planned made it decision-free and easy to follow.
By nighttime I thought, “OK, I can do this, it’s only five days.” But by day two I was already missing fresh food. I typically eat five cups of non-starchy veggies each day, plus two cups of fruit, in addition to fresh avocado and plant proteins like lentils and beans. By the second day, I was already craving a salad or my usual fresh fruit and nut butter snack. I grew weary of eating only packaged foods more quickly than anticipated, but I was committed to seeing it through. Jack, who normally eats much larger portions and significantly more calories, was already feeling the hunger. However, he was encouraged by a reduction in bloating and was already feeling lighter.
Day three was by far the most difficult for me. At times it felt like everything was in slow motion, and I was 100% hangry. My hubs, on the other hand, experienced a euphoric-like high and mental clarity that can accompany fasting, which unfortunately I never quite achieved.
Day four was slightly easier, because we were over the hump. But the dry packaged food had become pretty unappealing, and we were doing anything to make it more palatable. I soaked the kale flax crackers into my soup to mix up the texture, we savored every morsel of the bars, and Jack drank the juice from the olive pouches. We were also not sleeping well and were pretty distracted by thoughts of foods we couldn’t wait to eat again.
By day five we were ready for ProLon to be over. We were both experiencing fatigue and to be honest, it’s a bit of a blur. But what I do remember is that around midnight that night we broke down and drank smoothies before going to bed. The ProLon protocol encourages slowly transitioning back to solid food on day six, but on night five we threw in the towel. I whipped up a few frozen cherry smoothies with plant protein powder and almond butter (which by the way tasted absolutely incredible after five days of packaged food), and we finally slept soundly.
I did not weigh or measure myself, since weight loss isn’t one of my goals, and after five days my clothes fit about the same. (Note: this makes sense, as the research shows that those with the highest BMIs experienced the greatest weight loss results.)
Jack lost 12 pounds and three inches from his waist. Now, some of this was undoubtedly due to water weight and less bloating. After resuming his normal diet, he maintained about a one-inch waistline difference and six pounds of weight loss. However, his calorie needs are much greater than mine, at about 2,200 per day. ProLon provides 1,100 calories on day one, and 750 calories days two through five, which was too extreme of a shift for Jack.
That said, I have had male clients try ProLon with excellent results, but they weren’t as muscular as Jack, so I think everyone’s experience is a bit different. In the three-month study mentioned earlier, the average weight loss was 5.1 pounds, primarily from abdominal fat. And in case you’re wondering, no, you should not extend the program past five days or complete more than one cycle per month. After three months, the company encourages repeating the diet “from time to time” when looking for rejuvenation.
ProLon isn’t cheap. One kit costs $249, so about $50 per day, although this does include a consultation with a registered dietitian or nurse practitioner. Strenuous exercise should not be preformed while following the plan, and activities like driving long distances, prolonged sun exposure, or being in a hot tub or sauna should be avoided. Researchers say that exercise may actually negatively impact the outcome of the fast, so if anything, only slow walking, stretching, and light yoga are advised. The plan also encourages paying attention to how you feel, as side effects like lightheadedness can occur.
You should not try ProLon if you’re pregnant or nursing, underweight, or have a history of an eating disorder. And anyone with a chronic medical condition, including type 2 diabetes, or heart or liver disease, should get a doctor’s approval prior to trying ProLon.
As for allergies, all of the ingredients in ProLon food are visible online, so if you have any food allergies or intolerances you should check to see if you can follow the plan. ProLon states that the diet is not appropriate for those with allergies to nuts or sesame, for example.
In my opinion, the pros of ProLon are based on the research about its unique benefits beyond weight loss. I also practice time-restricted feeding, but some of the longevity outcomes in Dr. Longo’s research are unique to the ProLon protocol.
Bottom line: I will do ProLon again, but Jack is unlikely to join me, and I agree that it’s not ideal for him. While it was challenging, I think it will be easier for me to complete again—now that I know what I’m in for!
For clients who ask me about ProLon, I offer a few caveats apart from those above. First, if any kind of strict eating plan tends to trigger you emotionally, skip ProLon. In other words, if a previous cleanse led to a pizza binge by day two, ProLon is not the plan for you. If you’re a premenopausal woman, don’t schedule ProLon just before your period, when cravings can be more intense. And if your work is physically demanding, it’s safest to wait to follow ProLon when you have five consecutive days off.
Finally, don’t go it alone. If you don’t have a significant other, friend, or family member to do it with you, follow ProLon under the supervision of a registered dietitian who can coach you through the five days. I’ve done this with clients, and the support and personalized guidance can invaluable. ProLon can be an investment in your health. Just be sure to examine if it’s the right one for you before you hit the ‘add to cart’ button.
Cynthia Sass, MPH, RD, is Health's contributing nutrition editor, a New York Times best-selling author, and a private practice performance nutritionist who has consulted for five professional sports teams.