While many experts agree that eating 3 healthy meals a day is good for ones mental and physical health, there is no definitive data or statement that directly defines breakfast as the most important meal of the day. Many other factors need to be taken into account when defining the importance of a certain aspect of ones dietary health.
In a paper published in the National Institute of Health (NIH), we can read that, "Breakfast is often referred to as the most important meal of the day and in recent years has been implicated in weight control, cardio-metabolic risk factors and cognitive performance although, at present, the literature remains inconclusive as to the precise health benefits of breakfast." This means that there has not been a definitive answer as to whether or not breakfast is THE most important meal of the day. In the same article it also states that "significant variation exists in the definitions of breakfast and breakfast skippers, and in methods used to relate breakfast nutrient intakes to overall diet quality". This points to the conclusion that different definitions exist for different people and that different results can be observed within different diets
In the same source cited in the claim, it is stated that "Breakfast kick-starts your metabolism, helping you burn calories throughout the day. It also gives you the energy you need to get things done and helps you focus at work or at school. Those are just a few reasons why it’s the most important meal of the day.
Many studies have linked eating breakfast to good health, including better memory and concentration, lower levels of “bad” LDL cholesterol, and lower chances of getting diabetes, heart disease, and being overweight. It’s hard to know, though, if breakfast causes these healthy habits or if people who eat it have healthier lifestyles", supporting the point that different dietary lifestyles cannot be used to define breakfast as the most important meal of the day.
https://pmc.ncbi.nlm.nih.gov/articles/PMC5986439/#sec1-nutrients-10-00559
https://www.webmd.com/food-recipes/breakfast-lose-weight