Does the length of the eating window matter?
Following her pilot study of an 8-hour eating window, Dr. Varady conducted further research with 4- or 6-hour eating windows to see if even shorter periods would precipitate greater weight loss, ideally a clinically significant loss of 5% of body weight.
She ran a 2-month randomized, controlled study in people with obesity, published in 2020, which was the first to examine both a 4-hour (3 p.m. to 7 p.m.; n = 19) or 6-hour (1 p.m to 7 p.m.; n = 20) eating window versus a diet without any food restrictions as a control (n = 19) (Cell Metab. 2020;32:366-78.e3).
Dr. Varady explained that they decided to shift the eating window to later in the day for this trial (in contrast to the earlier 8-hour study) to allow people to eat dinner at a sociable time, and thereby hopefully reduce dropouts from the study.
“Unlike with alternate-day fasting, most people find time-restricted feeding easy to incorporate into their lifestyles,” she remarked.
Both the 4- and 6-hour eating window groups experienced a mean 3.2% body weight loss, compared with controls, and this correlated with a 550-calorie reduction in their daily consumption, compared with their baseline calorie intake.
In terms of other outcomes – and in contrast to the 8-hour window study which showed very little changed other than a minor decrease in blood pressure – researchers saw some changes in metabolic risk factors with the 4- and 6-hour eating windows, Dr. Varady reported.
Compared with the control group, fasting insulin decreased in both time-restricted feeding groups by a mean of 15% (P < .05). Insulin resistance also decreased by 25% in the 4-hour group and by 15% in the 6-hour group, compared with the control group. Fasting glucose did not change in either group, however.
The researchers did not observe any effect on blood pressure or plasma lipids in the 4- or 6-hour eating window groups, compared with controls. However, measures of oxidative stress and inflammation decreased in both groups versus controls by approximately 35% (P < .05).
“These findings suggest that this form of severe time-restricted feeding is achievable and can help adults with obesity lose weight, without having to count calories,” Dr. Varady and colleagues conclude.
Is intermittent fasting better for weight loss than calorie restriction?
Ultimately, if weight loss is the primary goal, many want to know how time-restricted feeding compares with conventional daily calorie restriction.
Back in 2017, Dr. Varady published a year-long randomized, controlled study that compared alternate-day fasting with a calorie-restriction diet and a conventional/usual diet among 100 participants with obesity who were otherwise healthy.
Participants on the alternate-day fasting plan (n = 34) consumed 500 calories on fasting days for the first 6 months for weight loss (approximately 25% of energy needs) followed by 125% of energy needs on alternating “feast days”. For an additional 6 months, they ate 1,000 calories on fasting days – aimed at weight maintenance.
Those following the calorie-restriction diet (n = 35) reduced energy intake by 25% (approximately 500 kcal) for the first 6 months for weight loss, followed by enough calories sufficient for weight maintenance (so no further loss nor gain).
However, the study showed alternate-day fasting did not produce better weight loss than conventional calorie counting.
“Over the first 6 months [during the weight-loss period] both groups lost an average of 6% body weight. After 12 months it crept back to 5% weight loss,” reported Dr. Varady.
“Realistically, if the study continued for 2 or 3 years, they probably would have regained much of their weight,” she admitted.
Dr. Varady suspects it might be better for the alternate-day fasting participants to continue eating only 500 calories on their fast day during the weight-loss maintenance period rather than increasing calorie intake during this phase.
Heart rate and blood pressure did not change in either group, while triglycerides decreased in the alternate-day fasting group, and LDL cholesterol decreased in the calorie-restriction group.
Glucose level decreased in the calorie-restriction group but not the alternate-day fasting group, and insulin and HOMA-IR were unaffected in both groups, reported Dr. Varady, noting that these findings were in healthy people with obesity.
In people with obesity and insulin resistance – evaluated as a subgroup in a separate study by Dr. Varady of alternate-day fasting versus daily calorie restriction published in 2019 – she noted that when insulin levels and HOMA-IR were measured, there was a greater reduction in both variables in the fasting group, compared with the calorie-restriction group.
“For people at risk of diabetes, maybe fasting produces more potent effects on glycemic control?” she ventured.