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Root Cause: The 3 Steps To Understanding Digestion For Runners

cooked pasta on brown wooden bowl

“Understanding and manipulating how your body digests and uses the nutrients in the food that you eat (thereby optimising your digestion and its resident Microbiome) is the key to improved health, energy, fitness, mental wellbeing and long-term weight maintenance”.

You wouldn’t assume you’re at the same standard as every other runner you race in terms of fitness, so why do we assume the way in which we approach our race nutrition should be any different? There is no one size fits all way when it comes to our running diets. Before, during or after a run, how we metabolise the nutrients put into our body will vary based on many elements. Gary Shaw, a nutritionist and successful runner in his own right, breaks down the key to understanding how we can work with our own digestive systems to further excel our running.

1) The Hormonal System & Appetite

If you gave two people a meal, one containing protein and one carbohydrate, but the exact same calorie content, each of them would begin to feel hungry at different timescales during and after their meal. 

This is because our appetite is not controlled by counting calories but the hormonal system, which interprets the different chemical structures of our diets and relays this information to the brain to either suppress or increase the desire to eat.

Peaks and dips in energy levels are not just based on what you eat.
Further reading: Effect of a high-protein breakfast on the postprandial ghrelin response – PubMed (

Even if you gave two people the exact same meal in both calories and macronutrient composition their post meal hunger levels might be slightly different. 

So why is this?

Well although we all share the same physiological basics as a species, each of us is subtly different in terms of firstly our genes, and secondly by the diets, lifestyle, and environment we have exposed ourselves to throughout our lives, and how this has impacted on our digestive capacity over this period.

It’s true you can’t outrun a bad diet, but some are genetically pre-disposed to being more susceptible to the effects than others.

2) How Our Genes Affect Digestion

Our genes instruct us to produce proteins from the nutrients supplied through our diets, which perform various vital health functions (e.g., digestive actions, detoxification, all aspects of metabolism).

Slight differences in this code called SNP’s dictate if these functions are increased or decreased, meaning we may need more nutrients from food to maintain our health or weight than another person might.

So, this partly explains how one diet type works well for one person but not so well for another, and why some are more prone to certain health conditions than others, if their diet & lifestyle has not complimented any underperforming genes (SNP’s).

Further reading: SNPs: impact on gene function and phenotype – PubMed (

3) The ‘Microbiome’ – Possibly Your Best Tool Towards Healthy Digestion

However, the other reason is the composition of what is termed the “Microbiome.”

The Microbiome is the total content of microbial species living both inside and on us (bacteria, yeast) which may not sound very appealing, but has a profound influence on all aspects of our host metabolism, digestive function, and health status.

The foods you crave, might not be you at all! But instead, it could be the bacteria (good or bad) dictating what they need to survive by relaying messages through your vagus nerve to your brain.

The subject area is vast and way beyond the scope of this article, but if we simply note that the type of bacteria in our gut (digestive tract) influence how well we digest the foods we eat, and that the type of foods we eat and the lifestyle we live, affect the composition of the microbiome in our guts, one can see the mutual interaction between the host (us) and our Microbiome and its important relationship with our diet metabolism and health.

Further reading: Gut microbiota functions: metabolism of nutrients and other food components – PMC (

There is also evidence to suggest that sufferers of various past and present eating disorders, and users of chronic restrictive dietary methods, have negatively altered microbiome composition due to their restrictive eating patterns. 

Further reading: The Microbiome and Eating Disorders – PubMed (

So, both the action of restriction and a lack of quality in diet composition (nutrient density and fibre content) negatively affect the Microbiome and its ability to assist with host digestive processes and metabolism, therefore affecting our long-term health, fitness, and wellbeing.

Getting a healthy balance of nutrient dense whole foods can increase the diversity of your microbiome.

Summarising Nutrition

The take home message from of all this being that understanding and manipulating how your body digests and uses the nutrients in the food that you eat (thereby optimising your digestion and its resident Microbiome) is the key to improved health, energy, fitness, mental wellbeing and long-term weight maintenance and NOT intermittent short term calorie restriction. 

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