Breakfast is the most important meal — skipping it makes you gain weight.

Observational studies do find that breakfast-skippers tend to weigh more — but skipping breakfast also correlates with other habits, so the arrow of causation is unclear. The confident public-health message ('eat breakfast to stay slim') runs ahead of that.

A BMJ meta-analysis of randomized controlled trials (Sievert et al., 2019) found that assigning people to eat breakfast did not lead to weight loss and, if anything, slightly increased total daily energy intake compared with skipping it. In other words, adding breakfast did not help control weight in trials. This concerns weight specifically; breakfast may still matter for children's concentration or for people with certain conditions. But the blanket claim that breakfast is essential for a healthy weight is contested and not supported by the trial evidence.