The Right Dietary Fat and Mental Alertness

Does dietary fat affect your mental alertness? Many of us have experienced a mental/energy slump after eating lunch, especially if the quantity is large. The almost irresistible need to lie down and take a nap doesn’t occur immediately. But about 45-60 minutes after we finished eating that delicious lunch, we wish that our daytime job was testing the softness of mattresses.

Known as the postprandial (after a meal) slump or torpor, the lethargy that envelops us has been attributed to the volume of food consumed during the meal, and if alcohol has been consumed, that also.

Lunch and The Siesta

Many cultures that used to designate lunch as the main meal of the day followed this meal by a mandatory rest; stores were closed from 1-4 pm, and workers and children went home for their midday meal and then rested or slept until late afternoon.

Woe unto the tourist who expected to be able to shop in the early afternoon and found every store concealed behind a metal shutter. Presumably, remote work should allow for such a work pattern to be reestablished, but that has not happened.

Postprandial Fatigue and Fat (and Alcohol) Consumption

The commonly accepted reason for this diminution in energy is the increased blood flow to the gastrointestinal tract away from the brain, but it is still not understood why sleepiness should result. The urge to nap after the Thanksgiving meal has erroneously been attributed to consuming turkey, and supposedly linked to increased tryptophan uptake into the brain. Since protein prevents tryptophan from getting into the brain, the real reason may be the consumption of considerably larger than normal amounts of food, and/or foods high in fat, and/or alcohol.

Several years ago, a paper was presented at the 27th annual meeting of the Associated Professional Sleep Societies LLC that suggested daytime sleepiness was affected by nutrient intake at lunch. Non-obese men consuming diets high in fat, protein, or carbohydrate were tested for daytime sleepiness and sleep quality at night.

The Midday Meal and Afternoon Mental Performance 

High-fat food intake was associated with increased daytime sleepiness and decreased alertness. The deterioration in alertness was sufficiently great to be a possible deterrent to the performance of specific jobs, according to the author of the study.

Similar results were described in a large Australian study among more than 1800 Australian men. The men, aged 35-80, participated in a yearlong study during which their food intake, sleep patterns, and daytime alertness were monitored. There was a positive relationship between mealtime fat consumption and daytime sleepiness, suggesting, as found in the previous study, that alertness may be impaired by the consumption of this macronutrient.

The Type of Fat Consumed Matters

However, not all fat needs to be avoided. Another study from Australia on excessive daytime sleepiness found an association with saturated, but not unsaturated fat. Avocado toast, tuna salad tossed in olive oil and vinegar marinade (rather than mayonnaise), a chicken stir-fry heated in sesame oil, or salad tossed with a tahini dressing are foods that contain unsaturated fats.

These studies are useful in planning meals that may precede meetings, lectures, fund-raising speeches, and even auditions. If the audience and the presenters are to be at their best, they should not be served food that may influence their postprandial alertness.

The Serotonin Power Diet and Healthy Fat Consumption

The Serotonin Power Diet has adequate fat for taste and nutrient purposes. However, quantities are limited so as to keep the calorie count low enough for weight loss to occur given a certain number of calories are devoted to serotonin-boosting carbs, adequate protein, and nutrient-dense vegetables and fruits.  Another reason is that fat can make one feel sluggish whereas benefits of the diet include mental alertness in the daytime and calmness and mental clarity in the afternoon and evening. 

Choose Healthy Fats

In keeping with the study mentioned above concluding that unsaturated fats are not associated with postprandial sleepiness, we recommend choosing healthy fats such as avocado, nuts & nut butters, seeds and seed butters, olive and other nut & seed oils, and even dark chocolate. Read about choosing healthy fats here. You can add up to one ounce or one Tablespoon of healthy fats daily in addition to the meal guidelines as written in our book.

Feed Your Brain for Mental Alertness

It is not necessary to limit yourself to a fat free diet, nor is it healthy or tasty to do so.  But choosing to consume lean rather than fatty foods, and focusing on healthy fats rather than saturated and hydrogenated fats, will maintain your mental sharpness and mentally energized.


If you’d like support and guidance to reach your weight loss and wellness goals, we can help: we’re here for you!  We invite you to sign up for a complimentary consultation to learn more.


Join The Serotonin Power Diet Community

Scroll to Top