What You Eat Matters. Does When You Eat Matter, Too?

While the diet wars may rage on, it's pretty universally accepted that what you eat matters when it comes to weight management. In other words, out-eating our body's energy needs leads to weight gain, and eating less than our body needs to go about its business leads to weight loss. Diets high in sugary drinks, fast food and empty-calorie snacks typically promote weight gain, while those higher in fiber-containing fruits, veggies and other plant based foods typically support a more healthy body weight.

What has traditionally been less clear, however, is to what extent – if any – when we eat plays a role in the weight equation.

At one point during your own weight loss journey, you've probably encountered the old dieter's adage not to eat anything after 6 p.m. (or some other designated time.) Chances are, though, you're equally likely to have heard the opposite message at well: that calories are calories, and it’s the total amount of them you eat that matters – regardless of what time of day you eat them. And the debate has remained unsettled.

Recently, however, evidence has emerged suggesting that when we eat may matter more than previously believed, and as such, that the timing of our eating patterns may indeed be a contributing factor to whether we gain – or lose – weight on any given type of diet.

Take, for example, the small but fascinating 2013 study published in the journal Obesity, in which close to 100 overweight and obese women consumed an identical number of calories – 1,400 per day – for three months. One subset of these women had their calories front-loaded to the early part of day, with 700-calorie breakfasts, 500-calorie lunches and 200-calorie dinners. The remaining group of women followed the opposite pattern, consuming half their daily calories at dinner – which consisted of the identical 700-calorie meal the other participants ate for breakfast. The 700-calorie meal, moreover, always included a sweet dessert, such as a cookie or piece of cake. Both groups of women lost weight, but the big-breakfast eaters lost significantly more weight (10 pounds more on average) and inches off their bellies (1.5 inches more on average) compared to the big dinner eaters – despite consuming the exact same number of total calories daily! They also had greater improvements in metabolic health markers, such as insulin levels and triglycerides, compared to the big dinner eaters. While this study's implications are limited by the fact that it was small, included only women and lasted a relatively short period of time, its findings nonetheless provide some pretty clear support to the hypothesis that the timing of our calorie intake matters in addition to the amount of our calorie intake. Or – that when we eat matters, too.

Recently, the "timing matters" case gained further traction from a mouse study published in the journal Cell Metabolism, in which researchers experimented with feeding lean and obese mice a variety of junky diets. The mice either had access to food around the clock, or had "time restricted feeding" to just nine, 12 or 15 hours per day. All groups of mice, however, still consumed the same total number of calories. The researchers found that even when lean mice were fed high-fat or high-fat and high-sugar diets with access limited to nine or 12 hours daily, they gained only about half the amount of weight their peers who were fed around the clock did – and did not become obese. Furthermore, they had substantially less body fat, improved glucose tolerance and minimal incidence of metabolic diseases like fatty liver compared to their peers fed around the clock. The lean mice fed the same unhealthy diets but who were granted round-the-clock access to food became obese and showed poorer outcomes on these same metabolic parameters. The authors concluded that time restricted feeding for 12 hours daily or less protects against excessive weight gain and certain metabolic disease (in mice), with greater protection related to shorter windows of feeding.

More interesting still was the finding that when lean mice on time restricted feeding schedules were allowed two "cheat days" of unrestricted eating on the weekends, they remained lean. And when obese, eat-round-the-clock mice got switched into the time restricted feeding group, they lost weight. The study's researchers hypothesized that our body's internal clock – our circadian rhythms – may influence genes related to energy metabolism, which in turn influence how efficiently calories are able to be utilized at various times of the day.

On one hand, it's premature to draw concrete "rules" about weight loss based on the nascent state of the scientific data. However, a recent review of time restricted feeding research concluded that research findings were generally consistent among human trials and animal studies – suggesting that there’s indeed some validity to the concept for people, too.

It’s particularly interesting to consider how the research study conditions compare to the changing rhythms of our lives, and to recognize the common patterns in a busy workweek that resemble the research scenarios associated with poorer weight outcomes. Are you someone whose long workday is book-ended by a large coffee at 6 a.m. and a bedtime snack 15 or more hours later, after 9 p.m.?  Do you work long hours, too busy to take time out for a proper meal, and arrive home starving for a large, late-night dinner? Do you skip breakfast altogether? Could the rise in our collective rate of obesity have something to do with the demise of organized meals, replaced by a ubiquitous snacking culture in which we're all at the proverbial trough from dawn until dusk?

I've long counseled my patients seeking weight loss to front-load their calories (and carbs) to the early part of the day – an eating pattern I'd describe as "breakfast like a prince, lunch like a king, dinner like a pauper." This pattern of eating generally serves my patients well, being easy to follow, facilitating greater hunger control throughout the day and ultimately, producing weight loss that doesn't require extreme, unsustainable measures such as low-carb dieting, intermittent fasting or juice-only "cleanses." I came to this approach based on the observation that when my patients skimp on breakfast and get stingy with calories and carbs at lunch, their excessive hunger and feelings of deprivation typically result in feeling starved by mid-afternoon and dinnertime. The consequence? Overeating at dinner by leaps and bounds, and incessant grazing after dinner despite being physiologically full, almost like they've got a sweet itch that just can't get scratched.

Given the terrific results my patients experience, I continue to employ this overall approach to weight loss, though the emerging scientific research has shifted my thinking in terms of why it seems to work so well. By consuming most of one's calories in the early part of the day, well within a 12 -our window of "first bite," do my patients lose weight as the result of minimizing the amount of calories consumed once the 12 hour window has closed? Or is there a separate mechanism at work that would explain why bigger breakfasts are metabolized more efficiently than bigger dinners? I expect that more and more researchers will start riffing on the studies described above, attempting to replicate the findings with more diverse populations and over longer periods of time. Others will no doubt attempt to identify the precise biological mechanism that explains these results and can shed informative light into whether there are optimal eating patterns for weight control. There are still many questions that remain unanswered, but I, for one, am watching this space closely.