There are obvious reasons to avoid a heavy meal right before a workout. | Simon kenion Shears

Back in October I competed in the Challenge Paguera triathlon. In the months building up to the race, I got into a productive training regimen of running one day, swimming the next, cycling on the third, and then repeat.

On one particular morning, however, writing an article kept my wrists handcuffed to my laptop. When I finally glanced at my Garmin 245 runner’s smart watch, it was nearly 2pm. If I ran now, I calculated, then it would be at least half past three before I managed to shower, cook, and sit down for lunch. Delaying my first meal of the day wasn’t sensible training either and once again I was caught in the familiar dilemma of ‘eat and then exercise?’ or ‘exercise and then eat?’

There are obvious reasons to avoid a heavy meal right before a workout: painful stomach cramps, stitches, the increased likelihood of painting the gym floor with vomit. There’s logic in eating a few hours before working out, too. According to Dr. Nancy Cohen, head of the department of nutrition at the University of Massachusetts, “you’ll want to eat a meal high in carbs and protein and low in fat roughly three to four hours before you exercise”. If we want to get the most out of our workouts, then what we eat matters. Carbohydrates provide the glucose that is burned, producing the energy we’ll need to exert our muscles and break a sweat. Proteins support muscle growth, ensuring our beach bodies are primed for the summer sun. What we eat affects how our bodies perform and grow.

However, regardless of meal composition, when we eat has a crucial say in whether we actually get up off of the sofa in the first place. In an ideal world, we’d take Dr. Cohen’s advice, confident that three hours after eating a well-balanced, healthy meal, exercise will still be on the day’s menu. Unfortunately, we don’t live in an ideal world. We’re not professional athletes- with coaches who sensibly schedule and enforce exercise. Most of us hear no voices urging us off the sofa apart from the ones in our own head. These internal voices reflect our instincts and, after eating, our instinct is to rest.

During strenuous exercise our bodies burn a lot of energy as contracting muscles generate force, power and heat. There is also a tremendous increase in metabolism as blood and nutrients are supplied to our limbs. This reduces the energy stocks available to our internal organs and brain, putting the safe operation of our vital bodily functions at risk.

Throughout our evolutionary history, intense bodily exertion was only ever justified when we were running from a predator or chasing prey. In a ‘fight or flight’ state of emergency, a flash flood of hormones boosts the body’s alertness. Our heart beats faster, sending extra blood to our extremities. In this excited state, our sympathetic nervous system willingly allocates countless joules of precious energy to our skeletal muscles.

We don’t dread exercise because we’re lazy. Laziness is the emotion we evolved to keep us from wasting precious energy on unnecessary tasks. We dread exercise simply because our muscles aren’t in a sympathetic tone.

One way we can take advantage of our unconscious ‘fight or flight’ willingness to expend energy, and do more exercise, is by fasting.

We evolved in environments where food was relatively scarce and have developed numerous adaptations that enable us to function at a high level, both physically and cognitively, when in a food-deprived state. During fasting the body switches to using up the body’s fat stores as fuel. These fats are broken down into shorter molecules known as ketones. The brain uses them more efficiently than glucose as they provide more energy per unit of oxygen used. When we are in a state of ketosis, we are more focused and alert, which would be useful if we urgently needed to find food.

Whilst we sleep, we partake in one of natures enforced periods of fasting. Hence why gym participation data shows that we’re more likely to stick with exercise scheduled for first thing in the morning.

We will not pass out, starve to death or run out of energy if we engage in moderate exercise on an empty stomach. Fat, stored around our midriff and on our bum, can also be converted to glucose and burnt as fuel.

When we do eat and break the fast, our willingness to expend energy on exercise dwindles. In response to the arrival of food in the stomach and small intestine, the activity of the ‘fight or flight’ sympathetic nervous system decreases and the activity of the parasympathetic nervous system- sometimes called the ‘rest and digest’ system- increases. The heart rate slows to conserve energy for digestion. There is also an increase in intestinal activity and the sphincters along our digestive tract relax.

How much can we eat without dampening our willingness to exercise? As long as you keep your carbohydrate intake below 50 grams per day during a fast, ketosis and a sympathetic tone can be sustained.

When considering my morning run, did I take advantage of my extended fast? No. The hormone ghrelin, released from my empty stomach and intestines, made its way up to my brain, stimulating my appetite. I stood up and, like a mindless zombie, wondered into the kitchen, gorging my face on last night’s pasta.

Dropping the Tupperware into the sink, the thought of running was now unbearable. In a severe postprandial somnolence or ‘food coma’, I lay on the bed, grateful to live in Spain where a post-lunch snooze, or ‘siesta’, is still something of a cultural norm. The 8km jog, so pivotal to my training, would be postponed until morning.
Or so I thought.

Before I could drift off into dreamland, my conscience reminded me that a black coffee might solve my dilemma. According to a study by the Department of Psychology of St John’s University, ‘coffee acutely increases sympathetic nerve activity.’ I got up and put the kettle on. Forty minutes later I was bouncing my bloated gut past a forest of boat masts along the Paseo Maritimo.