Close X

Eat early or eat later?

11th October 2023

Intermittent fasting (IF) has gained popularity as a way of losing weight and improving cardiometabolic health. A debate on whether the window for eating within an IF regime is best timed early or late in the day took place at the American Diabetes Association’s 83rd Scientific Sessions. Dr Susan Aldridge reports. 

Time-restricted eating (TRE) is a form of IF in which eating takes place only within a specific time window during a 24-hour period. Outside of that time, the individual fasts. The ADA debate was over what impact the timing of this window – early or late – has upon outcomes. Research findings in this area are sure to be of interest to healthcare professionals advising people with, or at risk of, type 2 diabetes and/or obesity on lifestyle interventions. It became clear as the debate progressed that there is still much to be learned, but studies so far provide evidence for both early TRE and for a more flexible approach. 

Breakfast like a king?

Courtney Peterson, Associate Professor at the University of Alabama at Birmingham, US, opened the debate by noting that there is strong evidence suggesting that it is better for cardiometabolic health to eat earlier in the day. This became apparent from the 1960s when the oral glucose tolerance test was developed and it was found that glucose tolerance was always better in the morning. By the mid-1990s, researchers were beginning to work out some of the mechanisms of the circadian clock of eating and experiments on mixed meal tests and glucose infusion found glucose spikes were higher later in the day. 

“It looks as if insulin sensitivity is highest in mid to late morning,” said Professor Peterson. “In my lab, we’ve done work with people who have prediabetes and found delayed release of insulin in the evening, which we can trace back to insulin sensitivity and beta cell function, both of which are lower in the evening. We also now know that melatonin induces insulin resistance.” Added to this, there is also the thermic effect of food, which means that slightly more calories are burned when digesting food if you eat in the morning.  

There are two approaches to early TRE. Mealtimes can be fixed, but the calorie distribution can be changed with more calories at breakfast, as with the saying, “Breakfast like a king, lunch like a prince and dine like a pauper”. Professor Peterson cited a 12-week randomised controlled trial (RCT) in which overweight women who followed this approach lost more weight and were less hungry than controls. 

The second approach involves changing mealtimes so that you do not eat within three hours of bedtime and start eating very early in the day. In Professor Peterson’s lab, they studied the impact of 14 weeks of this early TRE on weight loss in adults with obesity who ate between the hours of 7am and 3pm compared with those who ate within a 12-hour window and found that the former had 50% more weight loss. 

Her team has also carried out a five-week crossover study in people with prediabetes of TRE between 8am and 2pm versus eating in a 12-hour window and found that the TRE improved beta cell responsiveness, blood pressure and insulin resistance. “There is also evidence that if you eliminate weight loss as a potential confounder in a one-week crossover feeding study, breakfasting like a king improves glycaemic control in just one week, which I find pretty staggering,” Professor Peterson said.

There are now seven studies comparing early TRE with mid-morning or late TRE, in which you match the fasting duration but shift the window for eating to earlier or later in the day. Six of these have shown benefits to cardiometabolic health in terms of fasting glucose, insulin resistance and systolic blood pressure. 

Professor Peterson pointed to another study that looked at three mechanisms behind TRE, namely hunger, energy expenditure and adipose tissue gene expression and found that time of day affects all three, with genes involved in fat storage being upregulated later in the day.  

Meanwhile, the first meta-analysis of 12 RCTs comparing early versus late TRE shows a significant benefit of the former on weight loss, fasting glucose, insulin resistance and LDL-cholesterol. “We are beginning to see evidence that these effects are not just due to fasting, but more to do with time of day because late TRE, where you skip breakfast and lunch, may worsen cardiometabolic health, with worse glucose tolerance and postprandial glucose,” she added. 

In summary, there are daily rhythms in metabolism, insulin sensitivity, secretion of appetite hormones and adipose tissue gene expression, such that breakfasting like a king will lead to weight loss and better glycaemic control. “Early TRE improves blood pressure and glycaemic control more than mid-day TRE, but the data are mixed on weight loss,” Professor Peterson said. “Many of these effects are independent of weight loss, but we need more and longer studies.” 

The case for eating later

Kelsey Gabel, Associate Professor of Nutrition at the University of Illinois Chicago, took to the stage to argue for later eating and being more relaxed over the TRE window because, put simply, a calorie is a calorie, regardless of timing.  

“In TRE, individuals just shorten their eating window,” she explained. “There is no calorie counting, no tracking or monitoring intake and no dietary restrictions – you just watch the clock.” These characteristics make this eating style popular for its simplicity. In early TRE, you might eat from 7am to 3pm and in late TRE from 12 noon to 8pm. “I would argue that the timing does not matter and that TRE produces a calorie deficit that leads to weight loss and cardiometabolic benefit,” she said. 

She cited a 16-week RCT with 147 participants who ate breakfast and 162 who did not, and this was the first study to show that skipping breakfast did not lead to weight gain, as has been previously reported. Clearly, further research into ‘breakfasting like a king’ is warranted. Another trial looked at calories loaded towards either morning or evening consumption and found the same amount of weight loss – eating earlier did not produce more weight loss. 

Studies on late or self-selected TRE show a 1-5% weight loss and blood pressure decrease, but no consistent change in lipids. Glucose does not change, but insulin levels and insulin resistance seem to decrease.

Meanwhile, there has also been a number of trials on early versus late TRE showing similar decreases in body weight, waist circumference and quality of life.    

“Regardless of whether it is early or late, TRE results in unintentional calorie restriction of 20-30% and cardiometabolic benefits due to the weight loss resulting from the calorie restriction,” explained Professor Gabel. “Individuals should choose the TRE window that best suits their lifestyle. Evening dinner with family and friends is a cultural norm, so late TRE allows people to live their normal life.”

Work, family and social activity are the main barriers to earlier TRE, which will hinder long-term adherence. Later TRE offers the same calorie restriction as earlier TRE, while maintaining social and family obligations. When it comes to personal preferences, a recent study of 800,000 people using a fasting app showed that the most popular time window for TRE was from 12 noon for the following 16 hours.  

Professor Gabel is currently involved in three studies of late TRE and found that people were adherent for six to seven days in the week, even over a one-year period. “Late TRE addresses barriers on adherence, making it viable for the wider population,” she said. “For maximum benefit, you need to meet people where they already are.” 

Final thoughts

Both speakers were given a chance to respond to one another’s arguments to wrap up the debate. In her rebuttal, Professor Peterson pointed to research that suggests that there are two different phenotypes when it comes to the effects of skipping breakfast. It may be that introducing ‘behavioural phenotyping’ could identify those who have most to gain from early TRE. She has also noted from her own studies that, although people didn’t want to do early TRE at the start, once they tried it, they did tend to stick with it.

In her response, Professor Gabel said that one way forward might be to have a calorie-loaded meal later in the day, but make the evening meal the smallest of the day. “While I don’t think late TRE will solve all obesity problems, it is a good place to start,” she said.  

All forms of TRE are associated with calorie restriction, by definition, but larger studies are needed to prove the impact of timing. While there may be some evidence that shifting to earlier in the day will improve outcomes, there may still be issues with adherence. “We need to get creative on shifting the eating window,” Professor Gabel concluded. 

In summary, then, it looks as if there needs to be a balance between the optimum window for TRE, individual factors and adherence to gain maximum benefit in managing or preventing type 2 diabetes and obesity. 

To learn more about diet for people with type 2 diabetes, enrol on the EASD e-Learning course ‘Lifestyle intervention’.

Any opinions expressed in this article are the responsibility of the EASD e-Learning Programme Director, Dr Eleanor D Kennedy.