In a crossover study,[12] 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,[6] 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.[13] 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.
