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It’s well-established that what you eat – and how much you eat – strongly influences body weight and health. But growing evidence suggests that the time of day that you eat at matters, too.
A number of studies have found that eating your largest meal earlier in the day benefits weight loss and metabolic health, while eating it late can hamper both. (Metabolic health is measured by waist circumference, blood pressure, blood glucose, cholesterol and blood triglycerides.)
Now, a new study from Johns Hopkins University has found that eating dinner close to bedtime worsens glucose tolerance and slows fat-burning. And, depending on your usual bedtime, you may be more susceptible to the metabolic consequences of late eating.
About the study
The research, published last month in the Journal of Clinical Endocrinology and Metabolism, looked at how 20 healthy volunteers, ages 18 to 30, metabolized an early and a late dinner.
Each participant was admitted to the research unit on two occasions; during one visit, they ate dinner at 6:00 p.m.; at the other visit, the identical dinner was eaten at 10:00 p.m.
All participants went to bed at 11 p.m. During the day and overnight, researchers measured participants’ rate of fat-burning and took blood samples every hour.
After eating the late dinner, blood glucose spiked 18-per-cent higher and the amount of fat metabolized from the meal decreased by 10 per cent compared with eating the early dinner. Levels of cortisol, a stress hormone thought to be involved in weight gain, also increased after eating the late meal.
When the researchers looked at participants’ responses according to their usual bedtimes, they found variations.
People used to going to bed at 11:00 p.m. had a 30-per-cent spike in blood glucose and a 20-per-cent reduction in fat-burning after eating the late meal compared with the early meal. Those whose typical bedtime was between 2 and 3 a.m. weren’t affected much.
It’s thought that eating a late dinner may disrupt the body’s circadian rhythm, the 24-hour cycle that governs calorie-burning, digestion and the metabolism of glucose and fat, as well as other bodily processes.
Our metabolism has adapted to eating during the day and sleeping at night. Over time, misalignment of the body’s sleep-wake cycle could lead to weight gain and contribute to Type 2 diabetes and cardiovascular disease.
This study looked at the effects of eating one late dinner; it’s not known if habitually eating dinner close to bedtime leads to weight gain or Type 2 diabetes.
The researchers also didn’t collect information about the participants’ usual eating time and meal frequency, which could have affected their responses to late eating.
As well, it’s unclear if the slower fat metabolism was caused by a disruption to circadian rhythm or how soon participants went to sleep after eating (e.g., our metabolic rate decreases during sleep).
Even so, this isn’t the first study to suggest it may be prudent to pay attention to meal timing.
A 2016 randomized controlled trial from the University of Nottingham found that among 80 overweight women assigned to a calorie-reduced diet, those who ate 50 per cent of their calories at lunch and 20 per cent at dinner (rather than the opposite) lost more weight and experienced a greater improvement in insulin sensitivity.
Another trial published in the journal Obesity in 2013 found that among 93 overweight women prescribed a 1,400-calorie diet, those who ate most of their calories at breakfast instead of dinner lost more weight and had greater improvements in fasting glucose and insulin levels.
Observational data also suggests that making breakfast, instead of dinner, the largest meal of the day helps prevent weight gain over time.
Takeaway tips: When should you eat for your metabolism?
- If you skimp on breakfast (defined as eating within two hours of waking up), consider moving more of your daily calories to the morning and eat fewer of them in the evening. To downsize dinner, use a smaller plate.
- If you eat dinner late, try to shift it earlier. When eating it late, have protein and vegetables; limit starchy foods. Avoid snacking after dinner.
- If you work night shifts, eat your main meal when you wake up in the afternoon, rather than later in the night. Eat a small meal and snacks during your shift.
Leslie Beck, a Toronto-based private practice dietitian, is Director of Food and Nutrition at Medcan.