Does fasting or eating less help your memory and concentration?
Fasting can improve memory and concentration in older adults with insulin resistance, but the effect disappears when fasting alone without exercising, and in children skipping breakfast is actually harmful to concentration that morning.
Short-term fasting on the 5:2 schedule (eating far less on two days per week) improved both memory and so-called executive functions -- such as planning and switching between tasks -- in older adults with insulin resistance. At the same time, the biological rate of brain ageing, measured by MRI, decreased. On a few cognitive measures the 5:2 schedule performed slightly better than an ordinary healthy diet. That comparison matters: part of the benefit can already be achieved simply by eating more healthily, without strict fasting.
In postmenopausal women with obesity, however, the effect disappeared. Two fasting days per week over three months produced no measurable cognitive improvement, unless participants also started exercising at the same time. That combination did work well. This shows that fasting alone is no miracle cure, and that physical exercise does a large part of the work.
Animal research adds a word of caution. In old male mice, spatial memory actually worsened with fasting, while younger mice benefited from it. Whether this pattern also occurs in humans is unknown. Biochemically, the fasting body produces ketone bodies (an alternative fuel for the brain) and levels of a brain protein that promotes the formation of new nerve cells rise. These mechanisms, however, have largely been described in animal and laboratory studies and are far from proven in humans.
For children the story is different. Multiple intervention studies show that skipping breakfast that morning impairs attention, working memory and concentration, with the strongest effect in undernourished children. In students, skipping breakfast is associated with lower grades and slower reaction times, although an observational study cannot demonstrate cause and effect. One large review study on breakfast was partly funded by a breakfast cereal manufacturer, which may have biased the outcomes in a positive direction; the individual intervention studies included in it nonetheless point consistently in the same direction.
Based on one small RCT in older adults (n=40), one RCT in postmenopausal women with obesity (n=92), one mouse study, one review article on mechanisms, a systematic review of 45 breakfast studies in children, and one observational study in students (n=298). Human RCTs are scarce and small; animal research is not directly transferable to humans.