“It's not what you eat, it's when you eat", claimed a headline in The Independent today. It added that "people who dine later struggle to lose weight”– a claim we also looked at back in September 2012 (though that was a study in mice).
This Spanish study enrolled overweight and obese adults on a 20-week weight-loss programme and found that those who ate their lunch before 3pm lost an average of 2.2kg more weight than those eating lunch after 3pm. In Spain lunch is the main meal of the day, often eaten in mid to late afternoon. The researchers did not find a link between the timing of breakfast or dinner and weight loss.
As energy intake and expenditure were similar between early and late lunchers, these two factors could not explain the differences in weight loss. But both groups were on a weight-loss programme, so claims that “it's not what you eat” are simply untrue.
Neither does this study prove that eating an early lunch makes you slimmer, or that eating a late lunch makes you fat; it shows only that the timing of lunch may be related to weight loss in some way. What this link may be is not entirely clear, but it will no doubt be the subject of further research.
Where did the story come from?
The study was carried out by researchers based in various Universities in Spain and was funded by numerous grants and contracts from Spanish and US government agencies and National Institutes in Spain.
The study was published in the peer-reviewed International Journal of Obesity.
The media reporting of the study was generally accurate although conclusions along the lines of “eating lunch too late may make you fat” is a misrepresentation of the study in question.
This study looked at overweight people who were actively engaging in weight-loss programmes – not whether people in general lost or gained weight.
What kind of research was this?
This was a longitudinal study to investigate how timing of food intake relates to weight-loss in overweight or obese adults.
The researchers indicated that a relationship between the timing of eating and weight had been demonstrated in animals, but research in humans was lacking.
In an observational study such as this, it is not possible to explain why people who were eating earlier lost more weight. Because all people in the study were following a weight loss programme, it is likely they followed similar nutritional intake and activity advice, although people chose what time of the day they ate, rather than being randomly allocated to an eating time. Therefore, there may be other biological or behavioural factors that differ between those who eat earlier and those who eat later which may explain the differences in weight loss.
What did the research involve?
Researchers recruited 510 overweight or obese adults from Murcia, south east Spain, who had enrolled in weight-loss clinics. People on a special diet, under treatment with weight-loss medication, or who had a diagnosis of diabetes, chronic kidney failure, liver diseases, or cancer were excluded from the study. This left 420 people to take part.
All participants underwent a dietary intake assessment before beginning a 20-week group weight-loss programme based on following a Mediterranean diet. The programme included behavioural and cognitive techniques. Participants were subject to a range of tests and questionnaires that gathered key information on:
- body fat
- blood tests relating to obesity
- blood pressure
- energy intake before and during treatment (through 24-hour dietary recall and 7-day food diary)
- energy expenditure
- sleep duration
- appetite hormone levels (appetite hormones are produced by the body when it is low on energy and requires food)
Other information gathered included whether people were generally ‘early birds’ or ‘night owls’ in relation to sleep patterns and when people thought they felt or performed best (collected via a questionnaire).
DNA tests were also used to determine genetic variations relating to the aptly named CLOCK gene that is thought to be related to the timing of natural biological cycles.
Participants were grouped into early and late eaters for breakfast, lunch and dinner using the average (median) values as cut off points. Differences between early and late eaters were analysed for differences relating to weight loss over the 20-week (five-month) period. Further analysis was done for lunch only as it was discovered during initial analysis this was the only mealtime to be related to weight loss. “Early eaters” were defined as those that ate before 3pm and late eaters any time after this.
The analysis was appropriate and took into account differences in gender, age, nutritional clinic attended and body mass index (BMI).
What were the basic results?
Those who took part in the study were 49.5% female, had an average age of 42 years and a BMI of 31.4kg/m2 (classified as obese – a healthy BMI is between 18.5 and 25kg/m2).
‘Late lunch eaters’ (who dined after 3pm) lost significantly less weight and displayed significantly slower weight-loss during the 20 weeks of treatment than early lunchers.
Early lunchers lost an average of 9.9kg of body weight over the 20-week programme compared to 7.7Kg in the late lunchers, a difference of 2.2kg.
Weight loss in the first five weeks was similar, but from week five onwards, the early lunchers started to lose more weight than their late lunching counterparts and this difference continued to grow until the end of the 20-week weight loss programme.
Interestingly, other important measures were similar between late and early lunchers, including:
- energy intake, such as dietary consumption
- estimated energy expenditure
- appetite hormones
- sleep duration
This means these factors cannot explain the differences in weight loss. Nevertheless, late eaters were more likely to be evening types (night owls), consumed less energy at breakfast, and skipped breakfast more frequently than early eaters (all p<0.05).
Some genetic variations relating to the CLOCK gene were different in early and late lunchers but these were not related to weight loss (p>0.05).
How did the researchers interpret the results?
The researchers concluded that, “this is the ﬁrst prospective longitudinal study to show that timing of food intake relates to weight-loss effectiveness in humans”. They added that those who ate their lunch later lost significantly more weight after 20-weeks than those who ate earlier, and crucially, “this difference in weight-loss success was not explained by differences in caloric intake, macronutrient distribution or energy expenditure”.
This observational study on overweight and obese Spanish adults showed that those who ate before 3pm lost significantly more weight (2.2kg on average) while on a 20-week weight loss programme than those who ate lunch after 3pm. This difference could not be explained by variations in calorie intake or energy expenditure, which were similar in the two groups.
It is worth noting that only early and late lunchtime habits were associated with differences in weight loss, not breakfast or dinner.
The authors’ noted that around 40% of the total daily calories of the Mediterranean diet the participants consumed were at lunch, so this was the main meal of the day. This may be different from other countries, where dinner may be the main meal of the day. As such, the results may not be directly applicable to people in other countries, including the UK.
As this was an observational study, we cannot say that eating an early lunch caused those people to lose weight, only that the two appear linked in some way. As other behavioural or biological factors may influence when a person chooses to eat their lunch, there may be other factors involved in the link between mealtime and weight loss.
This well designed study raises some intriguing questions about how the timing of a meal relates to weight loss success.
The first question to ask is how did one group lose significantly more weight than the other if their average energy intake and expenditure were similar in the two groups?
Many theories spring to mind and three potential explanations are outlined below that may warrant further research:
- First – were there errors in the measurement of energy intake and expenditure that if measured more accurately, would show differences in one or both variables that would explain the weight loss differences?
- Second – is it the case that the timing of the meal affects how the food is metabolised in the body, with those eating later more inclined to convert their food to weight? The researchers highlight that currently, the biological mechanism linking meal timing to weight loss is not known.
- Third – is it the case that although energy expenditure was similar, the timing of it was not measured. The timing of energy expenditure, such as going for a morning run or an evening run, may be equally important to weight loss as eating times.
None of these hypotheses were tested in the current research, but they may be answered by future research on the topic. If a link between meal timing and weight loss were verified, it could help many overweight or obese people optimise their weight loss efforts, which in turn could help them reduce their risk of obesity related disease.