This is a McDonald’s Sausage Biscuit with Egg. The meals supplied in the study were “designed to mimic the contents of various fast-food meals” such as this sandwich.

Our Ability To Focus May Falter After Eating One Meal High In Saturated Fat, Science Daily, 12 May 2020

The study compared how 51 women performed on a test of their attention after they ate either a meal high in saturated fat or the same meal made with sunflower oil, which is high in unsaturated fat.

Their performance on the test was worse after eating the high-saturated-fat meal than after they ate the meal containing a healthier fat.

The loss of focus after a single meal was eye-opening for the researchers. … Most prior work looking at the causative effect of the diet has looked over a period of time.

“If the women had high levels of endotoxemia*, it also wiped out the between-meal differences. They were performing poorly no matter what type of fat they ate,” Madison said.

Possible mechanism:

Previous research has suggested that food high in saturated fat can drive up inflammation throughout the body, and possibly the brain. Fatty acids also can cross the blood-brain barrier.

* I talked about endotoxemia in this post:

One way dietary fat contributes to inflammation is by increasing absorption of endotoxins, to which we launch an inflammatory response. Endotoxins are bits of bacterial membrane that are absorbed along with the fat we eat, especially saturated fat to which endotoxins have an affinity. They’re thought to derive from bacteria (dead or alive) introduced to the intestines from food, primarily meat, eggs, dairy, or fermented food, all of which carry relatively higher levels of microorganisms. The bacteria that colonize our colon are too far along the digestive tract for absorption to take place in any significant quantity.

Here’s the study:

Afternoon Distraction: A High-Saturated-Fat Meal And Endotoxemia Impact Postmeal Attention In A Randomized Crossover Trial, American Journal of Clinical Nutrition, 12 May 2020

It was a good study: double-blind, randomized crossover trial of 51 women. It concluded:

These results suggest that higher LBP, sCD14, and LBP:sCD14 [these are endotoxemia markers] and saturated-fat intake individually and jointly influence attention. Endotoxemia may override the relative cognitive benefit of healthier oil choices.

Both meals were high in fat and both contained animal food (eggs, turkey sausage) which provide the bacterial bits for endotoxemia. Remove the animal food and reduce the fat and you might have seen even better results.

“Because both meals were high-fat and potentially problematic, the high-saturated-fat meal’s cognitive effect could be even greater if it were compared to a lower-fat meal,” [lead author Madison] said.