The trend of eating only the whites of an egg and tossing the yolk is one of those habits that sounds healthy but actually throws the best part in the bin. Nutritionist Sandra Moñino is blunt: "Taking five whites and one yolk doesn't make much sense." And when celebrities do it too, the habit spreads faster than any doctor's advice.

The logic behind the fad is familiar - the white is pure protein with few calories, while the yolk carries fat, so it gets avoided. The problem is that it's precisely in the yolk that the egg's most nutritious components are hidden: choline for the brain, vitamin B12, folic acid and the fat-soluble vitamins. In other words, people use up more eggs to get less. Moñino recommends instead eating two to three whole eggs - enough protein and all the nutrients in one place.

There's another trap the industry doesn't advertise. Packaged liquid egg whites from the supermarket often come from hens raised in factory conditions rather than free-roaming ones - which means lower quality than an ordinary egg bought from a trusted source. The convenience of the bottle comes at a price that isn't written on the label.

Still, the point isn't that eggs are a problem - quite the opposite. "A very good idea, eggs in the morning are one of the best options", Moñino says of the protein-rich breakfast that keeps you full for a long time. Simply, you don't need to complicate what nature has already packaged well. Sometimes the healthiest choice is also the least fashionable - a whole egg, just the way your grandmothers ate it.