Mental fatigue, stress and energy needs

Why a meeting can leave you exhausted

You can finish a hard training session feeling satisfied, then leave an apparently undemanding meeting feeling drained. The contrast deserves attention. The physical movement visible to another person does not capture every demand made on attention, emotion or sensory processing.

The tempting next step is to translate that exhaustion directly into calories. But fatigue, physiological stress and energy expenditure are different outcomes. A useful explanation must take the experience seriously while checking what was actually measured.

The brain is already in the energy budget

The adult brain has a substantial ongoing energy requirement. Raichle and Gusnard describe it as accounting for about one fifth of the body’s oxygen consumption despite its small share of body mass. Much of that activity continues when a person is resting; the brain does not switch off between demanding tasks. [1]

That means a resting metabolic measurement already includes brain metabolism. A whole-body resting expenditure equation estimates an aggregate that includes it too. Adding another 20% for the brain would count its baseline demand twice. What remains open is how well a particular estimate captures changes in an individual’s circumstances.

Local changes in brain activity should also not be confused with an equally large change in whole-body expenditure. An imaging signal and a daily calorie total answer different questions. [1]

Mental stress can alter measured energy use

Seematter and colleagues tested acute mental stress in healthy women under short-term feeding conditions. They measured oxygen consumption as an index of energy expenditure and assessed stress-related metabolic responses. Stress increased oxygen consumption; after underfeeding, that increase was smaller while the response of a marker of fat mobilisation was greater. [2]

This supports a real interaction between mental stress and metabolism. It does not establish how many additional calories an office worker needs over a day, or a percentage uplift for autistic people. A brief laboratory challenge and a working week are different exposures. The study also gives no basis for assuming that the most distressed person necessarily burns the most energy.

Food intake and expenditure can diverge

Chaput and Tremblay compared 45 minutes of seated rest with a reading-and-writing task, followed by a buffet. Their experiment found a much larger difference in subsequent energy intake than in expenditure between the conditions. [3]

That distinction matters. Eating more after mental work is not, on its own, evidence that the brain used an equivalent number of extra calories. Equally, a modest measured expenditure difference does not show that the task was comfortable or easy. The appetite response, subjective experience and metabolic measurement need to be interpreted separately.

What this means for EPK

EPK stands for Energy per Kilogram. It is a framework for considering adequate fuelling in relation to body mass, daily demands and a pattern of eating that someone can sustain. The practical ambition is to support function rather than treat hunger and exhaustion as achievements.

These studies give us reasons to investigate mental demands alongside movement. They do not validate a specific EPK target. A wearable can offer useful observations, but its calorie figure remains an estimate; a stress-related rise in heart rate is not automatically equivalent to exercise.

A careful EPK study could record food intake, sleep, perceived stress, mental fatigue and daily movement, with measured expenditure in a validation subgroup. The question would be whether the proposed support improves function and wellbeing, and whether changes in expenditure explain any benefit. We should measure those outcomes rather than build the desired answer into the calorie calculation.

Support the person as well as the calculation

A workable response to demanding days might involve dependable access to food, predictable breaks, quieter surroundings or clearer expectations. These are practical options to explore, not treatments established by the experiments above. More food cannot be assumed to solve an intolerable environment; an apparently low calorie cost cannot make that environment harmless.

This is one of the questions explored in Room to Be Yourself: Autism, Serotonin and the Biology of Stress. Work on the book began in 2018. It brings research into conversation with the conditions that allow people to use their abilities with less distress.

Related research

• Stress, anxiety and mitochondrial pathways
• Body size does not tell the whole energy story
• The energy cost of stress: appetite, food quality and the demands of coping

Related working papers

Read the relevant PDFs: EPK–001 to 007 are the revised version 1.2 papers; EPK–008 to 010 are version 1.0 supplements. Earlier PDFs and DOI records remain on the paper archive.