Weight management: does meal timing really matter?

In TRF, all daily calories are consumed within a fairly short time window in any given 24-hour period (in this case between 8am and 2pm).

Evidence for TRF

It was back in 2016 when US scientists investigating TRF and its metabolic effects concluded that ‘TRF supports robust metabolic cycles and protects against nutritional challenges that predispose to obesity and dysmetabolism’⁽⁷⁾. In plain English, there was evidence that TRF and the thrifty gene therapy were supported by their findings. Other scientists looking into this approach found that restricting food intake to a 4-12 hour slot in the day, without reducing calorie intake, was been associated with improved body weight maintenance, and they went on to propose TRF as weight reduction strategy^(8-10).

However, in later randomized clinical trials, scientists found that while TRF had very beneficial effects on cardiovascular risk factors, including abdominal fat loss, blood glucose regulation, insulin resistance, blood pressure, and blood lipids, it actually resulted in similar weight loss compared with eating the same foods and calories throughout the day^(11-14). Confusingly however, these later studies were limited by small sample sizes, relatively short durations, and participants with specific health conditions such as pre-diabetes or overweight/obesity.

New research

Despite the positive findings in studies on TRF, it’s clear that questions remain about the potential benefits of time‐restricted eating patterns in the real world, over extended periods of time, especially given that TRF still imposes significant restrictions when combined with social and athletic demands⁽¹⁵⁾. It’s one thing when positive findings emerge from a short-term intervention study supervised and supported by researchers. But what’s really needed is longer-term data in the general population, where the effects of TRF on weight and body composition can be measured and compared to other patterns of food and calorie intake.

In a brand new study by US researchers at the John Hopkins University in Baltimore, researchers have done just this⁽¹⁶⁾. Published in the Journal of the American Heart Association, this study looked at data gathered over a period of six months from 547 participants from the general population, using a mobile app to record meal timings and sleep patterns. In particular, the scientists sought to evaluate the relationship between the time period between waking up and breakfast consumption, the time between the last meal of the day to sleep onset, the number of meals consumed during the day and the spacing of those meals, and how the participants’ weight trajectories changed over time. For each eating occasion, participants first indicated the time of the meal using a 24‐hour wheel on the app, and then selected the type and estimated size of meal from a drop‐down menu (ie, small meal (less than 500 calories), medium (500–1000 calories) or large meal (more than 1000 calories) – see figure 2. The weight and height information for all participants was collected (from medical records) from up to 10 years before enrolment and by measurements for a further six years after enrolment (ie during the study and for six years afterwards).

Figure 2: Screenshot of the meal intake timing app

What they found

When all the data was collected and analyzed, the key findings were as follows (see figure 3):

·        The average time from the first to the last meal was around 11.5 hours. The average time from waking up to breakfast was around an hour and a half, while the time between the last meal and sleep onset was around four hours.

·        Meal timings were not associated with weight change, either during the study period, or during the 4-month follow up.

·        The time period in which meals were consumed (time window) had no bearing on weight loss or gain.

·        The time period between waking and breakfast and between the last meal and sleep onset did not have any association with weight gain or loss.

·        The total daily number of large meals (estimated at more than 1,000 calories) and medium meals (estimated at 500-1,000 calories) were each associated with weight gains.

·        A greater frequency of small meals (estimated at less than 500 calories) was associated with weight loss over the study period.

·        The findings above were the same regardless of the participant’s initial starting weight.

The senior study author Wendy Bennett, who is an associate professor of medicine at Johns Hopkins University School of Medicine concluded the even though prior studies have suggested TRF and/or intermittent fasting can improve the body’s metabolic rhythms, the data from this study, which looked at a large group with a wide range of body weights did not support this assertion.

Figure 3: Factors affecting weight gain or loss

Factors associated with weight gain lie to the right of the zero line. Factors associated with weight loss (ie more smaller meals) lie to the left of the zero line. Purple dots indicate the association is significant. The total number of meals, particularly medium and large meals, is significantly associated with weight gain. Consuming more, smaller meals is associated with weight loss. 

Implications for athletes

Although TRF has definite health benefits, and has become a popular among those seeking to control or lose weight, the notion that it is an effective method of achieving these goals is thrown into doubt by these findings. From the large body of data gathered in this study, it appeared that eating more frequent but smaller meals was the only pattern of eating associated with a reduction in weight.

That said, there are some caveats we should add. Firstly, although comprehensive and including a large number of participants, this is only one study, so more data would be needed before we can draw rock solid conclusions. Secondly, the researchers did not determine the intentionality of weight loss among study participants prior to their enrolment; it could have been that a similar study with subjects actively seeking to lose weight might have observed rather different eating patterns and findings. Thirdly, while many of the participants were healthy, the results could have been skewed by those with existing health conditions. Nevertheless, this is the first long-term study into the real world effects of meal timing and weight loss, and it seems to suggest that TRF may not be quite the magic bullet many had hoped for.

Silver lining

Before we get too despondent however, there is a silver lining here. A key finding from this study was that when those who consumed more but smaller meals and snacks were the only category of participants who experienced significant weight loss. In other words, all other things being equal, consuming your calories in smaller and more frequent portion sizes could be a powerful tool to lose or manage weight.

This finding is not without support from other studies. In a 2016 study published in the British Journal of Nutrition, researchers investigated the relationship between meal frequency, weight and body mass index (BMI) in over 7,700 men and women⁽¹⁷⁾. The main finding was that that increased eating frequency was associated with a lower energy density (ie fewer sources of concentrated calories) and better diet quality. Moreover, smaller more frequent meals were inversely associated with BMI and waist circumference – ie more frequent meals equated to lower weight and BMI. In addition, a number of other studies have found an inverse and significant relationship between eating frequency and body weight/BMI^(18,19).

Why could smaller, more frequent meals and snacks result in better diet quality and reduced weight/BMI. This effect is quite poorly understood. One hypothesis is that increasing eating frequency may aid weight management through an effect on appetite control⁽²⁰⁾. Another hypothesis is that increasing eating frequency increases dietary-induced thermogenesis (in plain English, the body’s base metabolism runs at a faster rate, burning more energy). However, there is no data to support this second hypothesis as of yet.

In summary

Regardless of the reasons, the finding that simply splitting your calorie intake over more but smaller meals could help with weight management is extremely intriguing. This is especially the case given that even the most dedicated athlete will likely struggle to adhere to a strict TRF regime. Not only is it difficult to accommodate pre or post-training nutrition requirements into a TR regime in terms of timing, a strict TRF regime can be very hard to fit around the family and a social life. However, by switching from say three square meals per day to four or maybe five smaller meals per day, it seems that athletes seeking weight control might be able to have their cake and eat it (albeit in smaller portions)!

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