OMAD And 20:4 Intermittent Fasting
There are three similar intermittent fasting protocols that have quite short daily eating windows: 18:6, 20:4, and one-meal-a-day (OMAD). In general, these involve abstaining from calories for 18-23 hours a day, and then eating freely within the remaining time. The eating window can occur at any point of the day, but most people choose to eat between 2 p.m. and 8 p.m.
In a crossover study, 12 males completed an 18:6 diet and a 12:12 diet in random order. In both treatments, participants could only eat the three meals provided to them throughout the day. This study was designed purely to test the effects of meal timing, so there was no caloric deficit in the diets. This study found that the 18:6 group had significantly improved insulin levels, ꞵ-cell function, and insulin resistance during an oral glucose tolerance test, but there were no differences in glucose levels. Additionally, systolic and diastolic blood pressure were reduced, and morning total cholesterol and triglycerides were increased. The increased triglyceride levels are suggestive of increased fat oxidation due to a longer fasting window. Finally, inflammation was not changed between groups, but levels of 8-isoprostane (a marker of oxidative stress) was significantly lower in the 18:6 group than in the control group.
In a crossover trial, 21 men and women between the ages of 40 and 50 completed a standard diet and an isocaloric one-meal-a-day diet (with the meal being consumed between 4 and 8 p.m.) in random order. Each diet was eight weeks long and had no caloric deficit, and the diets were separated by an 11-week washout period. This study found that, when eating one meal a day, participants had higher systolic and diastolic blood pressure, blood urea nitrogen, albumin, liver enzymes, and total, HDL, and LDL cholesterol, and lower body weight, fat mass, cortisol, hemoglobin, red blood cell count, and hematocrit. Additionally, participants were significantly more hungry and had lower fullness when eating one meal a day. Unfortunately, this study collected their vitals and blood samples at different times of day between the groups. Because a number of these values (especially blood pressure and cortisol) are subject to variation during the day, this is a significant source of confounding.
The participants from this study also performed an oral glucose tolerance test, which was reported in a secondary analysis. This study found that one meal a day significantly worsened glycemic response to an oral glucose tolerance test and higher fasting glucose compared to the standard diet. The authors noted, however, that because the oral glucose tolerance tests were performed in the morning, the one-meal-a-day group had their glycemic control assessed much closer to the time they last ate, which may have reduced their performance.
In a randomized controlled trial, 28 healthy, untrained males underwent an eight-week resistance training program (with three nonconsecutive training sessions per week) plus either a normal diet or a 20:4 diet to be implemented on non-training days. The subjects were not given any instruction on what or how much to eat. This study found that, on the days they fasted, participants in the 20:4 group ate about 650 fewer calories, although their weight did not change. Additionally, the percentage of each macronutrient did not change. Finally, although strength improved in both groups, there were no differences between groups. Notably, the normal diet group did have increased lean mass compared to the 20:4 group, but this difference did not reach statistical significance.
In a crossover study, 30 otherwise healthy individuals followed a normal day of eating and a 24-hour water-only fast, in random order, with no washout period. This study found that a day of water-only fasting increased growth hormone, hemoglobin, red blood cell count, hematocrit, and total, LDL, and HDL cholesterol, and decreased fasting glucose, fasting insulin, insulin resistance, triglycerides, bicarbonate, and body weight compared to a normal day of eating. There were no changes in blood pressure.