Brain Fuel: How Does A Lack of Food Affect Your Mental Energy?


It’s 3:00pm and you’re experiencing the late afternoon ‘slump’. Home time is a still few hours away and you’ve got a pile of work to get through, but despite your best efforts, you can’t seem to get your brain into gear.

“I know; I’ll have a snack! That will give me the mental energy I need. After all, I haven’t eaten since lunch and that’s probably affecting my concentration.”

Hmmm… not likely.

While caloric-dense foods do contain potential energy, your brain doesn’t really require it in vast amounts like the rest of your body.

Don’t get me wrong, your brain certainly needs its share. It’s estimated that some 20% of your Basal Metabolic Rate (energy required for bodily functions), is apportioned to keeping the lights on.

However, unlike your muscles that require increased energy intake to fuel longer workouts, your brain’s energy demands don’t significantly increase for tasks that require greater concentration.

There’s actually ample evidence your brain can continue to function at full capacity for both short and long periods of time without much food at all. In fact, some studies have shown your brain performs better when you’re hungry as opposed to after you’ve eaten!

This all makes perfect sense when you think about it from an evolutionary perspective. If a lack of food availability caused a steep decline in cognitive function, it would’ve been very difficult for our hunter-gather ancestors to remain sharp enough to find their next meal.

Furthermore, it’s been well-acknowledged in research (and for many of us, anecdotally) that there’s a strong causal relationship between eating (especially larger meals) and the feeling of lethargy that often follows. This sensation is referred to as postprandial somnolence, more commonly known as ‘food coma’.

This is likely because our central nervous system’s state is greatly influenced by our eating patterns. When hungry, we’re more motivated to be active and search for food. This is the sympathetic ‘fight or flight’ state. Satiety (feeling full) on the other hand, activates the parasympathetic ‘rest and digest’ state.

So, if your cognitive ability isn’t improved with increased energy (in fact the opposite), how do you deal with that 3:00pm slump?

The first and most obvious answer is caffeine – a powerful psychoactive stimulant, capable of raising mental alertness for hours. Now, depending on your predisposition, having a coffee late in the afternoon might not the best idea if you’re looking to get to bed early. The typical half-life of 180mg of caffeine (a double shot of espresso) is around 5-6 hours, depending on your size and activity levels. However, some people feel the effects for much longer, disrupting their sleep patterns and leaving them in an even worse predicament the next day.

The second, and more preferable option in my opinion, is walking. Try getting up from your desk and going for a short stroll outside, or if you don’t have that much time, then at least around the office or your home.

We’ve known for some time how regular, low-intensity aerobic exercise can help preserve our brain’s health into old age as well as reverse some of the brain’s deterioration in ageing populations.

More recent discoveries have shed light on how only 10 minutes of low intensity exercise can immediately alter how certain parts of the brain communicate and coordinate with one another, potentialy improving our memory function. The findings suggest that exercise does not need to be prolonged or intense to benefit the brain and that effects can begin far more quickly than many of us might expect.

This could be in-part due to increases in cerebral blood flow, which until recently was thought to be a tightly regulated bodily process separate from increases in exercise-induced blood pressure elevation. As it turns out, our brain blood flow is quite dynamic and depends on cyclic aortic pressures that interact with pulses from foot impacts when walking. Our stride rates are within the range of our normal heart rates (about 120 per minute) when we’re walking at a brisk pace.

As Ernest Greene, researcher from the New Mexico Highlands University, put it, “These activities may optimise brain perfusion, function, and an overall sense of wellbeing during exercise.” Pretty cool, huh?

So, the next time you start to feel the afternoon dip coming on, spare yourself the unnecessary calories-in and burn some instead with a short walk. It will boost your cognitive function almost instantly, and you’ll be forming a habit that will help preserve your brain’s health well into old age.

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References: Green MW, Elliman NA, Rogers PJ. (1997) ’The effects of food deprivation and incentive motivation on blood glucose levels and cognitive function’ Journal of Psychopharmacology Nov;134(1) Pages 88-94  Kim, S.W., Lee, B.I. (2009) ‘Metabolic state, neurohormones, and vagal stimulation, not increased serotonin, orchestrate postprandial drowsiness’ Bioscience Hypothesis Volume 2, Issue 6, 2009, Pages 422-427  Lieberman H.R. et al. (2008) ‘A double-blind, placebo-controlled test of 2 d of calorie deprivation: effects on cognition, activity, sleep, and interstitial glucose concentrations’ The American Journal of Clinical Nutrition, Volume 88, Issue 3, September, Pages 667–676  Reynolds, G. (2018) ‘Even a 10-Minute Walk May Be Good for the Brain’ The New York Times  Richardson, M.W. (2019) ‘How Much Energy Does the Brain Use?’  Shimada, H. Et al.(2017) Journal of NeuroEngineering and Rehabilitation ‘Effects of exercise on brain activity during walking in older adults: a randomized controlled trial’ Volume 14 p. 50. Shukitt-Haleab, B. et al.(1997) ‘Effects of 30 Days of Undernutrition on Reaction Time, Moods, and Symptoms’ Physiology & Behavior Volume 62, Issue 4, October 1997, Pages 783-789  Vanitallie, T.B. (2006) ’Sleep and energy balance: Interactive homeostatic systems’ Metabolism October 55(10 Suppl 2):S30-5.  Experimental Biology (2017). ‘How walking benefits the brain: Researchers show that foot’s impact helps control, increase the amount of blood sent to the brain’ ScienceDaily. Retrieved from  ‘Espresso Shot’ Caffeine Informer