Although these studies offer valuable preliminary data ( 6 , 7 , 10 ), they are limited in that they enrolled only metabolically healthy individuals. A key question that remains unknown is whether ADF produces superior changes in glucoregulatory factors compared with CR in populations at risk for developing diabetes (i.e., insulin‐resistant individuals with overweight and obesity). Accordingly, the goal of this study was to compare the effects of ADF with those of daily CR on body weight and glucoregulatory factors in participants with insulin resistance and overweight or obesity. We hypothesized that ADF would produce greater reductions in insulin resistance compared with CR in this sample despite similar weight loss.

Whether these diets produce comparable changes in glucoregulatory factors, however, is less clear. In a study by Harvie et al. ( 7 ), women with overweight participated in an intermittent fasting (two ~ 650‐kcal fast days per week) or daily CR regimen (~ 1,500 kcal daily) for 6 months. By the end of the study, both groups lost similar amounts of weight, but participants in the intermittent fasting regimen observed greater decreases in fasting insulin levels and insulin resistance compared with those in the CR regimen. In line with these findings, Hutchison and Heilbronn ( 10 ) demonstrated superior reductions in fasting glucose levels and insulin resistance in ADF (three 0‐kcal fast days per week) versus CR (~ 1,500 kcal daily) after 2 months of treatment in women with overweight. In contrast, Catenacci et al. ( 6 ) observed no change in fasting insulin levels or insulin sensitivity in ADF (three 0‐kcal fast days per week) or CR (~ 1,600 kcal daily) after 2 months in men and women with overweight and obesity.

Current guidelines for the treatment of obesity recommend moderate calorie restriction (CR) (20%‐30% reduction in energy needs daily) ( 1 , 2 ). Another form of dietary restriction that has gained attention in recent years is alternate‐day fasting (ADF) ( 3 , 4 ). ADF regimens generally consist of an ad libitum “feast day” alternated with a 25% energy intake “fast day.” Accumulating evidence has suggested that ADF and CR produce similar weight loss (6%‐8%) over 2 to 12 months in healthy adults with overweight and obesity ( 5 - 8 ). Research has also indicated that both diets are safe and well tolerated ( 5 , 6 , 9 ).

Results are presented as means ± SEMs. Normality was assessed by using the Kolmogorov‐Smirnov test, and no variables were found to be skewed. One‐way ANOVA with a Tukey post hoc test was used to test the differences among groups at baseline and to test percentage change differences from baseline to month 6 or month 12. ANCOVA, with baseline as a covariate, was used to test the differences between groups at months 6 and 12. Bonferroni post hoc tests were used for these comparisons. A two‐tailed P < 0.05 was used for statistical significance. Data were analyzed by using SPSS Statistics software for Mac (version 25; IBM Corp., Armonk, New York).

Blood samples were obtained after a 12‐hour fast at baseline and at months 6 and 12 (after a feast day in the ADF group). All participants were asked to avoid exercise, alcohol, and coffee for 12 hours before each visit. Samples were centrifuged for 10 minutes at 1,000 g at 4°C to separate plasma from red blood cells and were stored at −80°C until analyzed. Plasma total cholesterol, direct low‐density lipoprotein cholesterol (LDL‐C), high‐density lipoprotein cholesterol (HDL‐C), and triglyceride concentrations were measured in duplicate using enzymatic kits (Roche Diagnostics, Indianapolis, Indiana). Glucose was quantified using the glucose oxidase procedure (Beckman Autoanalyser II; Beckman Coulter Inc., Fullerton, California). Insulin was quantified using an electrochemiluminescence assay (Pacific Biomarkers, Seattle, Washington). Insulin resistance was calculated by applying the homeostatic model assessment of insulin resistance (HOMA‐IR) formula: (insulin [microinternational units/milliliter] × glucose [milligrams per deciliter])/405. Participants with HOMA‐IR values > 2.73 were considered insulin resistant ( 15 , 16 ). Blood pressure and heart rate were assessed at baseline and at months 6 and 12 after the weigh‐in following a 10‐minute rest. C‐reactive protein (CRP) was measured by IMMUNITE 1000 high‐sensitivity CRP kits (Diagnostic Products Corp., Los Angeles, California). Plasma tumor necrosis factor α (TNF‐α) and interleukin 6 (IL‐6) were measured by using enzyme‐linked immunosorbent assay (R&D Systems, Minneapolis, Minnesota).

Body weight was measured monthly after an overnight fast (after a feast day), with the participant in a hospital gown, by using a digital scale (HBF‐500; Omron, Bannockburn, Illinois). Fat mass and lean mass were assessed in the fasted state at baseline and at months 6 and 12 (after a feast day) by using dual‐energy x‐ray absorptiometry (QDR 4500W; Hologic, Arlington, Massachusetts). Visceral fat mass was measured at baseline and at months 6 and 12 by using a 1.5‐T magnet (Siemens, Erlangen, Germany), as described previously ( 14 ). Participants were instructed to lie in the magnet in a supine position with arms extended above the head. Contiguous sections were obtained every 1 cm from the ninth thoracic vertebra to the first sacral vertebra. This resulted in a range of 21 to 35 magnetic resonance imaging axial images per person, depending on the participant’s height. Images were analyzed using validated software ( 14 ).

Dietary intake and adherence were measured using a 7‐day food record and were analyzed with Nutritionist Pro software (version 5.2; Axxya Systems, Stafford, Texas). Adherence to the ADF and CR interventions was assessed by comparing actual energy intakes at months 6 and 12 with prescribed energy intakes at months 6 and 12. All participants were instructed to maintain their level of physical activity during the 12‐month study. Activity was assessed over a 7‐day period at baseline and at months 6 and 12 by a validated ( 13 ) pattern recognition monitor (Sense Wear Mini; BodyMedia, Pittsburgh, Pennsylvania).

Controls were asked to maintain their body weight during the 12‐month trial by not changing their usual eating and activity habits. Participants in the control group did not receive any food or dietary counseling but visited the research center at the same frequency as those in the intervention groups to control for any investigator‐interaction bias. During their visits, the study coordinator measured their body weight and blood pressure and also asked controls whether they had made any changes to their eating or activity habits.

The 12‐month trial included a 6‐month weight loss phase followed by a 6‐month weight maintenance phase ( 5 ). Baseline total energy expenditure was measured using doubly labeled water ( 11 ). Doubly labeled water was used to estimate energy needs for the energy intake prescriptions. During the 6‐month weight loss phase, ADF and CR participants reduced their net energy intake by approximately 25% per day. To achieve this, ADF participants consumed 25% of baseline energy needs as a lunch (between 12:00 pm and 2:00 pm) on fast days and 125% of baseline energy needs over three meals on alternating feast days. CR participants consumed 75% of baseline energy needs over three meals every day. From months 0 to 3, ADF and CR participants were provided with all meals. The meals were picked up from the research center on a weekly basis and transported via a rolling insulated cooler. All of the provided meals were consumed outside of the research center. The provided meals had the following macronutrient distribution: 30% of energy as fat, 55% of energy as carbohydrates, and 15% of energy as protein. From months 4 to 6, when food was no longer provided, intervention participants met one‐on‐one each week with a nutritionist to learn how to continue with their diets on their own. During the 6‐month weight maintenance phase, participants were instructed to maintain their body weight. ADF participants consumed 50% of energy needs as a lunch on fast days and 150% of energy needs over three meals on alternating feast days. CR participants consumed 100% of energy needs over three meals every day. During the maintenance phase, ADF and CR participants met one‐on‐one each month with a nutritionist to learn cognitive behavioral strategies to prevent weight regain ( 12 ).

This is a secondary analysis of a study that compared the effects of ADF versus CR on body weight ( 5 ). Independently living individuals were recruited from the University of Illinois at Chicago campus by flyers. Participants were included if they were 18 to 65 years old, had a BMI of 25.0 to 39.9 kg/m 2 , and were previously inactive (< 60 min/wk of light activity for the 3 months prior to the study). Individuals were excluded if they had a history of type 1 or type 2 diabetes or cardiovascular disease, were taking weight loss medications, were not weight stable for 3 months prior to the beginning of the study (> 4‐kg weight loss or gain), and were perimenopausal, pregnant, or smokers. Participants ( n = 100) were randomly assigned by a stratified random sample (based on age, sex, and BMI) to one of three groups for 12 months: (1) ADF ( n = 34), (2) CR ( n = 35), or (3) control ( n = 31) (Figure 1 ). All participants provided informed consent to participate in this study. The protocol was approved by the Office for the Protection of Research Subjects at the University of Illinois at Chicago.

Changes in body weight are displayed in Figure 2 and Table 3 . In insulin‐resistant participants, ADF and CR produced comparable body weight reductions ( P < 0.0001) by month 6 (ADF: −10% ± 1%; CR: −8% ± 1%) and by month 12 (ADF: −8% ± 2%; CR: −6% ± 1%), relative to controls. Fat mass and BMI also decreased ( P < 0.05) similarly in the ADF and CR groups at months 6 and 12 compared with controls (Table 3 ). Changes in lean mass and visceral fat mass in the ADF and CR groups were not significantly different from those in controls at month 6 or month 12 (Table 3 ).

Energy intake, macronutrient intake, and physical activity are shown in Table 2 . ADF participants consumed less energy than prescribed on the feast day at months 6 and 12 and more energy than prescribed on the fast day at months 6 and 12. CR participants consumed more energy than prescribed at month 6 and less energy than prescribed at month 12. Percentage energy intake from protein, carbohydrates, and fat did not change over the course of the trial in any group. Cholesterol and fiber intake also remained unchanged. Physical activity (measured as steps per day) did not change significantly by month 6 or month 12 in the intervention or control groups.

For the present study, only participants with insulin resistance who completed the entire 12‐month study were included in the analysis (insulin resistance was defined as HOMA‐IR > 2.73) ( 15 , 16 ). The total number of completers with insulin resistance in each intervention group was as follows: n = 11 in the ADF group, n = 17 in the CR group, and n = 15 in the control group (Figure 1 ). Baseline characteristics of the insulin‐resistant participants were comparable between the ADF, CR, and control groups (Table 1 ).

Discussion

This study is the first to show that ADF produces superior reductions in HOMA‐IR (a marker of insulin resistance) compared with daily CR in a cohort of participants at risk for developing diabetes. Interestingly, these improvements were noted despite similar weight and fat mass loss in the ADF and CR groups. Other metabolic disease risk factors, such as plasma lipids, blood pressure, and inflammatory mediators, were not preferentially altered by one diet versus the other.

The impact of intermittent fasting versus daily CR on glucoregulatory factors has been tested in a handful of clinical trials (5-7, 10). Studies performed in metabolically healthy individuals generally demonstrate either superior improvements by fasting compared with daily restriction (7, 10) or no effect (5, 6). For instance, Harvie et al. (7) and Hutchison and Heilbronn (10) noted greater decreases in insulin resistance (~ 25%‐35% reductions) by intermittent fasting versus daily restriction (~ 15% reductions) after 2 to 6 months of intervention in healthy participants with overweight and obesity. In both of these studies (7, 10), the fasting and daily restriction groups achieved the same degree of weight loss (~ 5%‐6%). In contrast, in a study by Trepanowski et al. (5), HOMA‐IR, fasting glucose, and insulin remained unchanged after 12 months of ADF and CR, with ~ 5% weight loss. Likewise, Catenacci et al. (6) observed no changes in glucoregulatory parameters after 2 months of ADF or CR in metabolically healthy individuals with obesity despite ~ 6% to 8% reductions in body weight. To our knowledge, only one study (17) has examined the effects of fasting on glucoregulatory factors in participants with obesity and marked insulin resistance. In this pilot study by Hoddy et al. (17), individuals participating in a 2‐month ADF regimen were divided into tertiles according to degree of insulin resistance based on HOMA‐IR. Results from this study revealed that ADF decreased fasting insulin (~ 25% reduction) and HOMA‐IR (~ 30% reduction) only in participants in the highest tertile of HOMA‐IR (i.e., > 3.7) (17). Participants who were not insulin resistant at baseline experienced no such improvements (17). The degree of weight loss was similar for each tertile (~ 4%). This study is limited, however, in that it did not involve a CR comparison group or a no‐intervention control group. Taken together, these findings suggest that fasting generally produces superior reductions in insulin resistance compared with daily CR despite similar weight loss. Moreover, it is likely that these effects may be more pronounced in participants who display higher levels of insulin resistance at baseline.

Our findings also show that neither ADF nor CR had any effect on other metabolic disease risk factors, such as plasma lipids, blood pressure, and heart rate, in this subanalysis of insulin‐resistant participants. It should be noted, however, that these participants were not hypercholesterolemic or hypertensive at baseline. Participants in all of the groups had LDL‐C, HDL‐C, and triglyceride levels and blood pressure within the clinically healthy range. Thus, it is not surprising that plasma lipid levels and blood pressure were not further improved. Our results differ somewhat from those of other trials that have compared intermittent fasting with daily CR (7, 10). In the 6‐month study by Harvie et al. (7), women with obesity experienced reductions in LDL‐C and triglyceride levels and in systolic and diastolic blood pressure by fasting, although these participants were not hypercholesterolemic or hypertensive at baseline. Similarly, Hutchison and Heilbronn (10) reported greater reductions in LDL‐C levels from ADF compared with daily CR after 2 months of diet in normocholesterolemic women with overweight. The reason why our findings differ from those of Harvie et al. (7) and Hutchison and Heilbronn (10) is uncertain. It is possible, however, that our participants were not as responsive to the fasting intervention because their level of adherence was suboptimal. More specifically, the ADF participants consumed ~ 400 to 600 kcal/d more than prescribed on the fast day and ~ 600 kcal/d less than prescribed on the feast day. In comparison, in the studies by Harvie et al. (7) and Hutchison and Heilbronn (10), adherence to the fasting interventions was higher because participants were much more compliant with their energy prescriptions. Thus, the suboptimal adherence to the ADF intervention in the present study could partly explain why our findings differ from those of previous fasting trials.

Circulating concentrations of CRP, TNF‐α, and IL‐6 are generally elevated in participants with insulin resistance and obesity (18-20). Weight loss has been shown to decrease concentrations of these proinflammatory factors and improve insulin sensitivity (21-23). Very few studies have examined the impact of intermittent fasting versus daily CR on inflammatory mediators (5, 7, 8). Harvie et al. (7) observed potent reductions in plasma CRP levels from intermittent fasting and CR in women with obesity (and elevated baseline CRP levels) after 6 months of intervention. In contrast, Trepanowski et al. (5) and Hutchison and Heilbronn (10) saw no change in circulating CRP levels from ADF or CR in metabolically healthy individuals with overweight and obesity. In the present study, we observed no effect of ADF or CR on circulating levels of CRP, TNF‐α, and IL‐6. However, plasma levels of these inflammatory mediators were not elevated at baseline, which could explain why no improvement was observed. Future trials in this field should examine the effect of fasting on other mediators of inflammation, such as interleukin 1β (IL‐1β), and chemokines such as monocyte chemoattractant protein‐1 (MCP‐1). IL‐1β is a proinflammatory mediator produced by monocytes and macrophages that contributes to the development of type 2 diabetes by destroying β‐cell mass and suppressing β‐cell function (24, 25). MCP‐1 is a chemokine that has been shown to impair insulin signaling in skeletal muscle (26, 27). Expression of MCP‐1 in adipose tissue is increased with obesity (28, 29). Whether intermittent fasting can improve circulating levels of IL‐1β and MCP‐1 in a way that protects against the development of diabetes is of great interest.

Key strengths of this study are the inclusion of a no‐intervention control group and the 12‐month trial duration. However, this study does have some limitations. First, we used HOMA‐IR to assess insulin resistance. Although HOMA‐IR is an adequate surrogate marker of insulin resistance (30), our findings need to be validated by more robust techniques such as the hyperinsulinemic‐euglycemic clamp. Second, we were not able to assess the number of prediabetic individuals in our sample because we did not measure hemoglobin A 1c or perform oral glucose tolerance testing at baseline in our original study (5). Third, there is a high degree of variability in the threshold of HOMA‐IR (i.e., 1.6‐3.8) (15) used to define insulin resistance. We chose a cutoff of 2.73 based on a US National Health and Nutrition Examination Survey population study (15). However, this higher cutoff value may introduce bias in our study because part of the effect may be due to higher baseline HOMA‐IR values. Fourth, we used 7‐day food records to measure dietary intake over the course of the trial. It is well known that individuals with obesity underreport food intake by 20% to 40% in diet records (31, 32); thus, our estimates of energy intake are most likely inaccurate. Fifth, we did not analyze when (i.e., time of day) the extra fast‐day calories were consumed and how this affected the total fasting duration. Sixth, the residual acute effects of fasting need to be considered when interpreting these data. In a recent study by Antoni et al. (33), intermittent fasting and CR produced similar reductions in postprandial insulin concentrations after matched weight loss (5%). In the present study, measurements were taken within 48 hours of the fast day. Therefore, the residual acute effects of fasting may have contributed to the improvements observed here.

The suboptimal adherence to the ADF protocol is also a major limitation to the study. Participants consistently struggled to stick to their fast‐day calorie goals, which puts into question the sustainability and tolerability of ADF long term. On the other hand, it is interesting that ADF participants consumed almost twice as many calories on fast days but still observed greater metabolic effects compared with CR participants. This suggests that simply reducing energy intake by ~ 1,000 kcal/d a few days per week may have significant metabolic benefits.