Breakfast does not make you thinner: seven trials weighed people

1. Seven trials weighed people and it came out the other way round.

In January 2019 Katherine Sievert, a researcher at Monash University, led a meta-analysis in the BMJ, a study that pools comparable results. Her team gathered thirteen controlled trials carried out with adults in high-income countries: seven had weighed the participants and ten had recorded what they ate.

The people assigned to eat breakfast took in, on average, about 260 kilocalories more across the whole day. They also weighed 0.44 kilos more than those who skipped that meal. The difference was small and the trials were short, with an average follow-up of about seven weeks for weight, and all of them carried a high or unclear risk of bias in at least one area. The careful conclusion stops there: adding breakfast has not been shown to work for losing weight.

2. The association had been dressed up as a cause for years.

Many earlier studies were observational, that is, they recorded people's habits without assigning them at random. In those studies, people who ate breakfast tended to weigh less or to show better health indicators. The problem appeared when you looked at the rest of their lives: they also tended to smoke less, move more, sleep better and pay more attention to what they ate.

Those differences are called confounding factors, characteristics that can explain the apparent result. Comparing the breakfasts of two people who sleep, smoke and move in different ways credits the plate with what belongs to the whole pattern. Controlled trials try to separate those variables by assigning participants the behaviour they must follow. When that comparison was made, the supposed slimming effect vanished.

3. A 1944 cereal advertisement put the phrase into circulation.

The idea that breakfast is "the most important meal of the day" was not a conclusion drawn from those trials. In 1944 General Foods launched the Grape-Nuts campaign "Eat a good breakfast, do a better job", which tied a good breakfast to performing better at work, and placed the advertisements in the morning radio programmes, exactly when listeners could turn the advice into a purchase.

The phrase in print is older still. It turns up in 1917 in the magazine Good Health, edited by John Harvey Kellogg, the doctor at the Battle Creek sanatorium, inside an article by Lenna Frances Cooper, a dietitian at the same house: breakfast was there "in many ways the most important meal of the day". It was born inside the house that sold the cereal and took twenty-seven years to reach the radio.

The Library of Congress holds a collection of General Foods radio scripts dated between 1932 and 1949, many of them accompanied by advertisements for the company's products. The campaign built on an older tradition from Battle Creek, the Michigan town turned into a centre of dietary reform, where Kellogg promoted a plain diet and developed cereals with his brother Will Keith Kellogg.

4. Who this does not apply to.

The BMJ review deals with healthy adults trying to control or reduce their weight. Someone who wakes up hungry can eat breakfast and still lose weight if their overall diet keeps an energy deficit. Someone else can wait until later without their metabolism entering any supposed emergency mode.

With children and adolescents it is different: eating breakfast is associated with better school performance, although part of the effect may depend on nutritional status, class attendance and the quality of the food. A review of 45 studies by Alexa Hoyland, Louise Dye and Clare Lawton in 2009 found the cognitive benefits to be clearest in children with nutritional deficiencies.

In people with diabetes treated with insulin, changing a meal alters the relationship between medication, carbohydrates and glucose, and during pregnancy improvising fasts is unwise. In both cases what governs is the individual plan set by the health professional.

5. What to do tomorrow morning.

Breakfast helps you if it stops you reaching the next meal with a hunger that is hard to manage, if it makes it easier to eat nourishing food, or if it fits your timetable. Skipping it suits you if you are never hungry when you get up and that meal only adds energy to the day.

Tomorrow morning, check whether you are hungry before you check the clock: if you are, eat something that fits your plan; if you are not, wait for the next meal.