ProLon is a five-day meal program designed to trigger the biological effects of fasting while still allowing you to eat. It works by keeping your calorie intake low enough, and your macronutrient ratios specific enough, that your body shifts into a fasting-like metabolic state. Your cells begin breaking down damaged components for energy and your body starts burning stored fat, even though you’re consuming small meals each day. This approach is called a fasting-mimicking diet (FMD), developed through research at the University of Southern California.
The Basic Concept Behind Fasting Mimicking
When you stop eating entirely, your body eventually switches fuel sources. Instead of running on glucose from food, it begins converting stored fat into molecules called ketones and recycling old, damaged cellular parts for energy. This metabolic shift activates a cleanup process where cells break down and repurpose their own worn-out components, then rebuild fresh ones when you resume normal eating.
The problem with a traditional water-only fast is that most people find it extremely difficult to sustain for multiple days. ProLon’s approach is to provide carefully formulated food, mostly plant-based soups, bars, crackers, olives, and herbal teas, in precise amounts that keep your body in that fasting metabolic state. The foods are high in unsaturated fats and low in protein and sugars, a combination that avoids triggering the nutrient-sensing pathways that would tell your body “food is here, stop fasting mode.”
What Happens During the Five Days
The program follows a specific calorie arc. Day one provides roughly 1,100 calories to ease the transition. Days two through five drop to around 750 to 800 calories. The macronutrient balance shifts heavily toward fat (about 40 to 50 percent of calories) with relatively low protein and carbohydrates. This ratio is critical: protein and certain amino acids activate growth-signaling pathways in your cells that would counteract the fasting response, so keeping protein low is what allows the mimicking to work.
By day two or three, most people enter a state of mild ketosis, meaning the body is pulling energy from fat stores. Simultaneously, a cellular recycling process ramps up. Think of it as your cells running a deep cleaning cycle, dismantling damaged proteins and old organelles and repurposing the raw materials. After the five days end and you return to normal eating, your body shifts into a rebuilding phase where stem cell activity increases and fresh cellular components replace what was cleared out.
What the Research Shows
A meta-analysis published in Diabetology & Metabolic Syndrome pooled results from randomized controlled trials of fasting-mimicking diets and found significant reductions in blood pressure: systolic pressure dropped by about 4 mmHg and diastolic by about 2 mmHg on average. These are modest but meaningful shifts, roughly equivalent to what some people achieve with dietary salt reduction.
The same analysis found that effects on total cholesterol, LDL cholesterol, HDL cholesterol, and triglycerides were not statistically significant across the pooled trials. Markers of inflammation (C-reactive protein) also didn’t reach significance in the overall analysis, though some individual studies did show reductions. Imaging studies from USC researchers found decreases in abdominal fat and liver fat, both of which are closely tied to metabolic disease risk. The program also appears to reduce biological age markers, according to USC research published in 2024.
So the strongest evidence points to benefits in blood pressure, visceral fat, and liver fat rather than dramatic changes in cholesterol numbers. Weight loss does occur during each five-day cycle, though some of that is water weight that returns. The more lasting body composition changes seem to come from repeated cycles.
How Often You’re Supposed to Do It
ProLon recommends completing one five-day cycle per month for three consecutive months when you first start. After that initial phase, the suggested frequency depends on your goals. Some people continue monthly for up to six months, while others shift to once every three months as maintenance. Each cycle is followed by a gradual refeeding period where you ease back into normal eating over a day or two, starting with light soups and small portions before returning to your regular diet.
The refeeding phase matters more than people realize. Jumping straight into large, heavy meals after five days of caloric restriction can cause digestive discomfort and may blunt some of the metabolic benefits. The transition day gives your digestive system time to ramp back up.
Who Should Avoid It
The program is not appropriate for everyone. People who are pregnant or breastfeeding, anyone under 18 or over 70, and those who are malnourished or fighting an active infection should not use it. Anyone with a history of eating disorders should also avoid caloric restriction programs like this, as the structured deprivation can trigger harmful patterns.
If you have diabetes or take medications that affect blood sugar or blood pressure, the five-day calorie drop could interact with your treatment in ways that need medical oversight. The same applies if you plan to do strenuous physical activity or spend extended time in extreme temperatures during the five days, since your energy reserves will be significantly reduced.
What the Experience Actually Feels Like
Most people report that day two and day three are the hardest. Hunger peaks during this window, and some people experience headaches, irritability, or fatigue as their body transitions fuel sources. By day four, many find that hunger subsides noticeably as ketosis stabilizes. Energy levels often feel surprisingly normal by the final day, though intense exercise is not recommended throughout.
The food itself is prepackaged and portion-controlled, which removes decision-making but also means you’re eating the same limited options for five days. The soups and bars are designed for function over flavor. Some people find the structure helpful because there’s nothing to plan or count. Others find the small portions psychologically challenging, especially on the lower-calorie days when a single meal might be a small cup of soup and a handful of olives.
The cost is another practical consideration. Each five-day kit typically runs between $150 and $250, and since the recommended protocol involves multiple cycles, the total investment adds up. The kit includes all food for the five days, so you’re not buying groceries on top of it, but the per-calorie cost is high compared to assembling similar low-calorie meals yourself. The counterargument from ProLon is that the specific macronutrient ratios were validated in clinical trials, and DIY versions haven’t been tested the same way.