A new study has shown that regularly eating later in the day can have negative health consequences.
A report on the findings - led by researchers from the University of Pennsylvania's Perelman School of Medicine in Philadelphia - were presented at this year's joint meeting of the American Academy of Sleep Medicine and the Sleep Research Society.
The study finds that compared with eating earlier in the day, eating later can have a negative impact on weight control, fat metabolism, and energy usage.
The researchers also found that a prolonged pattern of later eating results in higher levels of glucose and insulin (which is linked to higher risk for diabetes), and also of cholesterol and triglycerides (which are linked to cardiovascular problems).
Lead author Namni Goel, a research associate professor of psychology in psychiatry at the Perelman School of Medicine, explains that previous research has already shown that sleep loss can have a bad effect on weight and metabolism, and that this is partly attributed to eating later at night.
However, the new findings suggest that timing of meal times on its own, independent of sleep, can affect weight and metabolism.
Daytime and delayed meal time patterns compared
Prof. Goel says that the preliminary findings of their study - which is still ongoing - "give a more comprehensive picture of the benefits of eating earlier in the day."
For the randomized crossover trial, nine healthy-weight adults (five men and four women) aged between 23 and 29 underwent two different daily meal time patterns: a daytime pattern, and a delayed eating pattern - both of which lasted for 8 weeks.
The 8-week patterns were separated by a 2-week "washout" period to ensure that the first pattern did not carry over into the second.
The daytime pattern consisted of three meals and two snacks eaten between 8 a.m. and 7 p.m. The delayed pattern also consisted of three meals and two snacks, except that these were consumed between 12 p.m. and 11 p.m.
The sleep period was the same in both patterns, occurring between 11 p.m. and 7 a.m. This was confirmed by use of wearable activity monitors. Calories and exercise were also held constant between the two patterns.
The researchers measured the participants' metabolism, energy usage, blood markers, and weight at four points during the study: before the first 8-week meal time pattern, after the first 8-week meal time pattern, after the 2-week washout period, and then after the second 8-week meal time pattern.
Less healthy metabolic profile in delayed meal time pattern
A preliminary analysis of the results found that compared with daytime eating, a delayed meal time pattern led to weight gain.
It also found that "respiratory quotient" went up when meal times were later. Respiratory quotient is a ratio of the amount of carbon dioxide that the body produces compared with the amount of oxygen it consumes. It is used as an indicator of which nutrients the body is metabolizing. If the quotient goes up, then it means that the body is processing more carbohydrates and fewer lipids, or fats.
The results also showed evidence of a less healthy metabolic profile during the delayed meal time pattern. This was reflected in changes in fasting glucose, insulin, cholesterol, and triglycerides.
Hormonal differences were also marked. For example, in the 8 weeks of daytime eating, levels of ghrelin (a hormone that stimulates appetite) peaked earlier in the day, and levels of leptin (a hormone that produces the sensation of fulness) peaked later.
Such a combination could suggest that participants on the daytime eating pattern were more likely to receive eating cues earlier in the day, and by eating earlier, they also stayed satiated for longer.
The findings confirm those of similar, but much shorter, studies. However, the new study is the first long-term comparison of early and late meal time patterns that rules out possible influencing factors, such as the sleep-wake cycle, physical activity, and diet.
While acknowledging that "lifestyle change is never easy," senior author Kelly Allison, an associate professor of psychology in psychiatry and director of the Center for Weight and Eating Disorders at the Perelman School of Medicine, says that their findings suggest that "eating earlier in the day may be worth the effort to help prevent these detrimental chronic health effects." She concludes:
"We have an extensive knowledge of how overeating affects health and body weight, but now we have a better understanding of how our body processes foods at different times of day over a long period of time."