Breakfast and weight: what trials show about eating or skipping the 'most important meal of the day'

“Breakfast is the most important meal of the day” is one of those sayings that sounds like a finding. Generations were raised on it, cereal boxes repeated it, and weight-loss advice still often includes a line about never skipping the morning meal, on the theory that people who do will overeat later.

When researchers at Monash University in Australia went looking for the trials behind that theory, they found thirteen, most of them small and short. Pooled together in The BMJ, the trials pointed the other way: adults who were told to eat breakfast took in more calories over the day and ended up slightly heavier than those told to skip it. The advice had been built on a different kind of evidence, studies that observe what breakfast eaters are like, and those studies continue to link skipping breakfast with diabetes and heart disease. The two kinds of evidence do not agree.

Where the advice to eat breakfast for weight control came from

Surveys have long shown that people who eat breakfast tend to be slimmer than people who do not. From there it is a short step to telling people to eat breakfast in order to be slim. A trial team at the University of Alabama at Birmingham described the result in 2014: public health authorities commonly recommend breakfast consumption to reduce obesity, though the effect of following that advice had never been tested.

The step from pattern to advice skips something. Breakfast eaters differ from skippers in ways that have nothing to do with the meal. They may sleep more regularly, smoke less, work different hours or eat better in general.

Andrew Brown and David Allison, also at Alabama, studied how the literature had handled that gap. Their 2013 paper found that study after study had re-confirmed the same pattern, in a body of work that has gratuitously established the association, but not the causal relation, between skipping breakfast and obesity. They also found that authors regularly described those studies in causal terms the designs did not support. The Monash reviewers noted how old the idea is: breakfast has been advocated as the most important meal of the day in the media since 1917.

What happened to weight in breakfast trials?

A randomized trial assigns people to eat breakfast or skip it, whatever their habit, and so removes the differences between the kinds of people who choose each.

Katherine Sievert, Flavia Cicuttini and colleagues collected every such trial in adults from high-income countries. Of 13 included trials, seven examined the effect of eating breakfast on weight change, and 10 examined the effect on energy intake. On weight, the pooled result favored skipping, by a little under half a kilogram, or about a pound. The authors summarized that they found no evidence to support the notion that breakfast consumption promotes weight loss or that skipping breakfast leads to weight gain.

The largest single trial is the Alabama one. A total of 309 participants were randomly assigned to be told to eat breakfast, told to skip it, or placed in a comparison group, and were followed for 16 weeks. Nearly everyone reported doing as asked. Weight change did not differ between the groups, whether people had been breakfast eaters or skippers to begin with.

A British experiment looked more closely at where the calories went. In the Bath Breakfast Project, James Betts and colleagues assigned 33 lean adults to a substantial breakfast or to nothing before noon for six weeks, and measured everything they could. Those eating breakfast did not eat less later. In the breakfast group, energy intake remained 539 kcal/d greater than after fasting, kcal/d being calories per day. But they also moved more: physical activity thermogenesis was markedly higher with breakfast than with fasting, meaning they burned more energy through activity. The two roughly canceled out, and body mass and adiposity did not differ between treatments at baseline or follow-up. Adiposity is body fat.

Do breakfast skippers make up the calories later?

The belief that they do is the heart of the traditional advice, and the pooled trials did not bear it out. Participants assigned to breakfast had a higher total daily energy intake than those assigned to skip breakfast, by about 260 calories a day. Whatever skippers added later in the day did not close the gap.

Independent scientists asked to comment when the review appeared were divided on how far to trust that figure. Kevin Murphy, an endocrinologist at Imperial College London, called it “a high quality paper, but there does need to be some caution because, as the authors note, the evidence that they assess is inconsistent across different studies.”

Tom Sanders, a nutrition scientist at King’s College London, was blunter. A deficit of that size, if it were real and sustained, should produce far more weight loss than the trials recorded, and Sanders pointed to the unreliability of the food diaries used to measure intake. “The difference in weight reported 0.44 kg is not meaningful and could just reflect differences in large bowel content,” Sanders said.

Both points stand. The direction of the trial evidence is against breakfast as a weight-control tool. The size of any effect in favor of skipping is small and uncertain.

Breakfast beyond weight: diabetes, heart disease and cholesterol

Weight is only one outcome, and on others the picture is less comfortable for skippers.

Long-term observational studies consistently find more disease among people who go without breakfast. A German team pooled the diabetes studies: in total 6 studies, based on 96,175 participants and 4935 cases, were included. Skippers were about a third more likely to develop type 2 diabetes. When the analysis took body mass index (BMI) into account the excess fell to about a fifth, and the authors concluded that the association is partly mediated by BMI, meaning part of the link runs through skippers being heavier.

For heart disease the best-known study used a nationally representative sample of 6,550 adults 40 to 75 years of age in the United States, followed for about two decades. The small group who said they never ate breakfast, about 1 in 20, died of cardiovascular causes at nearly twice the rate of daily breakfast eaters.

These are the same kind of studies that produced the weight advice, with the same weakness. Someone who never eats breakfast may also be someone who works nights, sleeps badly or smokes, and statistical adjustment can only partly allow for that.

The trials offer a few hints that something real may lie underneath. A meta-analysis of randomized trials from Harvard concluded that breakfast skipping may have a modest impact on weight loss and may increase LDL in the short term. LDL is the form of cholesterol tied to heart disease. In the Bath experiment, blood sugar was more erratic in the afternoon and evening among those who fasted through the morning. Both results come from a handful of people over a few weeks.

How breakfast figures in heart guidance

The American Heart Association took up the subject in a 2017 scientific statement on meal timing. It observed that skipping meals and snacking have become more prevalent, reviewed the research on breakfast alongside fasting and meal frequency, and came down on the side of regularity over any single meal. Its summary was that irregular eating patterns appear less favorable for achieving a healthy cardiometabolic profile, cardiometabolic referring to the heart, blood vessels and blood sugar together.

The Monash reviewers addressed weight-loss advice specifically. They wrote that caution is needed when recommending breakfast for weight loss in adults, as it could have the opposite effect. They added that breakfast may matter for other reasons, noting possible effects such as improved concentration and attentiveness levels in childhood, which their review did not examine.

For people who take medicines that lower blood sugar, when they eat interacts with their treatment, and that is a matter for their doctor.

Short trials, and no agreed definition of breakfast

The trial evidence is thin. Follow-up was brief (mean period seven weeks for weight, two weeks for energy intake), and the trials included in this review lasted from 24 hours to 16 weeks. Every one had design weaknesses, and the reviewers’ own verdict was that the findings should be interpreted with caution. Nobody can hide from participants whether they are eating breakfast.

There is also a problem of definition. A breakfast can be a bowl of cereal or a cooked meal more than twice its size, and the trials did not share one definition. Betts raised this when commenting on the review: “the very concepts of breakfast consumption versus skipping are not well-defined”. Betts also declared, alongside that comment, research grants and consultancies with companies including Kellogg’s, Nestle and PepsiCo, a reminder that the food industry has long had a stake in the question.

None of the trials tested what the breakfast was made of, and none ran long enough to count heart attacks or new cases of diabetes. The observational studies run for decades but cannot isolate the meal.

On weight, the trials favor neither eating breakfast nor skipping it by any margin that matters.