In short, just as it is best to get the majority of your sunlight exposure early in the day (before 2 or 3 PM) to allow your melatonin levels to rise in time for bedtime, it may be best to eat the majority of your daily calories early in the day or before the typical American dinner. In fact, getting late bright light exposure can worsen your blood sugar control, and eating late can negatively impact your sleep patterns!

Dinner of Champions

Peterson says that dinner may actually be the most important meal of the day. Our bodies are most vulnerable to junk food or sugar spikes in the evening, making it even more important to eat a healthy, and early, dinner.

She has data to back this up. Peterson and colleagues found that prediabetic men following a form of intermittent fasting called early time-restricted feeding substantially improved their insulin sensitivity, blood pressure and oxidative stress levels as compared to men eating the same meals on a 12-hour eating schedule. Moreover, men on an eTRF schedule experienced these metabolic health improvements without losing weight.

Peterson set out to discover whether time-restricted feeding has intrinsic benefits. To do this, she had to strictly control what and when research participants ate, to make sure that the benefits of time-restricted feeding in these participants weren’t in fact simply the benefits of eating less food. She recruited research volunteers, 8 prediabetic men, for a rigorous controlled feeding trial.

“We made all of their food for them for five weeks and then we required that they eat the food under supervision, either in our clinic or on Skype,” Peterson said. “We also made sure that we fed them enough such that they didn’t lose weight, so that we could study the impacts of intermittent fasting independent of weight loss. These are intense studies that are difficult to pull off. This was actually the first controlled feeding trial of any type of intermittent fasting in humans.”

In the study, all men on an intermittent fasting schedule started eating around 8 AM finished dinner by 3 PM. The men in the control group ate the exact same meals, but over a 12-hour period.

Schematic of an example of meal composition and timing used in Peterson’s study of early time-restricted feeding. Image created by Paige Jarreau.

Men with prediabetes were randomized to early time restricted feeding (eTRF, 6-hr feeding period, with dinner before 3 p.m.) or a control schedule (12-hr feeding period) for 5 weeks and later crossed over to the other schedule. eTRF improved insulin sensitivity, b cell responsiveness, blood pressure, oxidative stress, and appetite. We demonstrate for the first time in humans that eTRF improves some aspects of cardiometabolic health and that IF’s effects are not solely due to weight loss. — Sutton et al., 2018

Peterson found that prediabetic men on the early time-restricted feeding schedule experienced improved insulin sensitivity, or ability to process sugar in the bloodstream. One of the first metabolic health changes in individuals with prediabetes and diabetes is insulin insensitivity, which can have many negative downstream consequences. The men on the early time-restricted feeding schedule also experienced dramatic lowering of blood pressure, around 10 to 11 points on average, and reduced oxidative stress, measured in terms of damaged fat molecules known to cause atherosclerosis and heart disease.

“That was really exciting to see, because that extent blood pressure lowering is about what you would see with a medication such as an ACE inhibitor — and we didn’t even have anyone lose weight,” Peterson said.

In Peterson’s study, fasting men’s triglyceride levels went up, although this could just be because Peterson’s group only measured triglyceride levels in the morning before breakfast, after men who were fasting had already gone a much longer time without calories (18 hours or so) than men on a control meal schedule. Triglyceride levels can rise over time while fasting, due to fat burning.

Overall, Peterson is excited by the results of her first pilot study on early time-restricted feeding. She plans to investigate this form of intermittent fasting in future studies with larger numbers of research participants, both men and women. In these studies, she plans to measure biomarkers of metabolic health such as blood sugar control and lipid levels at multiple timepoints through fasting and feeding periods, to better understand daily fluctuations in both participants who practice early time-restricted feeding and participants who don’t. She also is planning a study to investigate if intermittent fasting can actually help people lose more fat and less muscle than standard caloric restriction.

Our trial tested eTRF [early time-restricted feeding] in men with prediabetes — a population at great risk of developing diabetes — and indicates that eTRF is an efficacious strategy for treating both prediabetes and likely also prehypertension. We speculate that eTRF — by virtue of combining daily intermittent fasting and eating in alignment with circadian rhythms in metabolism — will prove to be a particularly efficacious form of IF [intermittent fasting]. — Sutton et al. 2018

Time-restricted feeding studies are more complicated to conduct with female research participants, it turns out. Women’s blood sugar control and fat burning varies throughout the month based on their menstrual cycle, meaning that researchers studying fasting in women need to match metabolic health and biomarker measurements for different research participants with the participants’ menstrual cycles. In future studies, however, Peterson will study early time-restricted feeding in both men and women and account for these fluctuations.

“As a woman, I’m super sympathetic to the need for more studies of time-restricted feeding in female research participants,” Peterson said.