How Does Junk Food Affect Your Fitness?

Human beings are remarkable. Each day people complete amazing tasks, smash goals, conquer fears, push boundaries, make life-changing discoveries or just make it through the day without falling asleep at work.

This ability comes from a combination of mental and physical strength, but has a common denominator – your diet and wellbeing! Think of it this way, you can’t concentrate on an empty stomach and definitely won’t perform at your best if you haven’t eaten properly.

It may be cliché, but think of your body as a car – you can’t expect to get from A-Z on an empty tank and you certainly won’t get anywhere if you’ve put the wrong fuel in it.

Happy stomach = happy mind – these combined will keep you performing at your best. You’ll be able to push yourself on the final stretch, even when every part of your body wants to give up. 

However, what happens when perhaps our dietary choices are not appropriate for the activity we are doing? Can we eat greasy fast food every day and still expect to be on top of our fitness game?

Firstly, let’s take things back to basics – why is nutrition so important when it comes to fitness?

Physical activity is demanding on your body. Therefore, you must ensure you are fuelling correctly both beforehand, to support performance, and afterwards, to aid with the recovery process.

Nutrition also plays a central role in optimal health and performance. As you may know, our diet can be broken down simply into being made up of macronutrients and micronutrients.

Macronutrients are components in our diet required in relatively large quantities (hence the name – macro), to keep the body functioning optimally. They include protein, carbohydrates and fat.

Protein, carbohydrates and fat all have individual responsibilities within the body and specific importance when it comes to performing at your best. 


Proteins are primarily functional and structural components within each cell of the body as so are required for growth and repair, as well as the maintenance of optimal health.  

Protein is made up of essential and non-essential amino acids, which are the building blocks of your muscles. Essential amino acids are those which cannot be synthesised by the body and therefore must be obtained from food. In their absence, it would be impossible to build, repair or maintain muscle mass.


Carbohydrates are the body’s main source of energy, needed for brain and organ function, as well as physical activity. They also play a role in the structure and function of cells, tissues and organs.

Excess glucose is stored in the liver and muscles as glycogen and can be quickly converted back into glucose if required  Glycogen stores within the body are limited and depletion has been found to negatively impact exercise performance, namely through feelings of fatigue and exhaustion. When levels are extremely low in muscle stores, the body is forced to revert to utilising protein to produce glucose, which can lead to muscle damage and even overtraining if sustained for a period of time.


Dietary fat is needed as an energy source, to provide the body with essential fatty acids (dietary fats that are vital for growth and cell functions but cannot be synthesised by the body), to allow for optimal functioning of nerves and the brain, assist in the production of hormones and are essential for the absorption of vitamins A, D, E and K.

What is junk food?

Although definitions of junk food have varied over the years, available definitions include:

  • Food that is high in calories but low in nutritional content (1).
  • Food that is unhealthy but is quick and easy to eat (2).

Therefore to summarise, junk food is food items that are often highly processed, often containing a whole host of artificial colours and flavourings. It often includes things like crisps, sweets and carbonated, sugary drinks.

These food items are often calorie dense (high in calories/ energy, compared to food volume), highly palatable and often not very satiating (filling) – therefore often easy to consume in large amounts. 

Due to the ingredients (or lack of), they are made up of, junk food is often low in many beneficial nutrients, such as vitamins, minerals and fibre – all of which play important roles in general health.

What are the consequences of eating ‘too much’ junk food?

Firstly, ‘too much’ will vary from person to person and we are by no means saying you should not enjoy a chocolate bar from time to time! As discussed in previous blogs, it is all about moderation and building a dietary approach which works for you.

Consequences of consuming large quantities of junk food can include:

  • Unstable blood sugar levels due to the consumption of a large amount of foods high in sugar and minimal fibre

Our bodies perform at their best when blood sugar levels are kept relatively constant and so it’s important to understand the effect that these sorts of food can have.

  • Bloating and excess thirst due to a high intake of salt. High salt intake can lead to heightened levels of water retention over time.
  • In more extreme cases, micronutrient deficiencies, as junk food is often void of these nutrients.

Micronutrients are only required by the body in small amounts and are comprised of essential vitamins and minerals e.g vitamin A, C and D, iron and zinc. They play vital roles in the functioning of our body’s systems, and deficiencies can lead to detrimental effects on our health.

Does junk food affect your fitness?

It can do – yes!

A packet of crisps here and there will be unlikely to impair your sporting performance but should be avoided as the bulk of your diet if you want to stay on your A-game.

How does junk food affect your fitness?

The consequences mentioned earlier can in turn begin to impact your energy levels and general day to day life, which can then undoubtedly begin to affect your fitness levels. 

There are a number of other ways the overconsumption of junk food can impact your fitness levels:

  • Can impact your ability to recover and adapt to your training stimulus – prioritising processed junk food over good quality sources of protein, carbohydrates and fat can hinder not only your intra-session performance but also the recovery process.

For example, Omega-3 fatty acids are a type of polyunsaturated fat, thought to be essential for exercise recovery, due to their anti-inflammatory properties. 

  • Can lead to an increased risk of illness and then time off from training – appropriate nutrition is required for all cells to function optimally, including the immune system (3). 

Being ill can lead to detrimental effects on your training, ultimately resulting in periods of time off.

To conclude, as with many aspects of optimal nutrition, it often comes down to moderation. Including the odd chocolate bar or packet of sweets in your diet is very unlikely to have an impact on your training and fitness levels. 

However, if poor dietary choices lead to overconsumption or over-reliance on junk food type foods, over time, this may start to have detrimental impacts on your sporting capabilities and you may not get the most out of your sessions!

Appropriate nutrition should form part of a multifaceted approach to supporting sporting performance and your overall fitness levels.

If you would like to discuss your current fitness goals and discuss if your nutrition could be tailored to support it, please contact or book in for a call here to speak to a Nutritionist.

Fresh Fitness Food provides personalised meal plans delivered straight to your door, ensuring not only that you have the nutrients you need to manage your stress levels, but also that you have the time usually spent shopping, cooking and washing up, to engage in your favourite stress-reducing activity. To discuss which nutrition plan is right for you, book a call with our in-house nutrition team here.

Order today and start smashing your goals with personalised nutrition!

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  1. 2021. Definition of JUNK FOOD. [online] Available at: <> [Accessed 27 June 2021].
  2. 2021. JUNK FOOD (noun) definition and synonyms | Macmillan Dictionary. [online] Available at: <> [Accessed 27 June 2021].
  3. Childs, Calder and Miles, 2019. Diet and Immune Function. Nutrients, 11(8), p.1933.
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Published by Georgia Chilton

In her teenage years, a love of food and rowing led Georgia into this field as she wanted to know how to optimise performance through nutrition. With a BSc in Nutrition and an MSc in Sports and Exercise Nutrition, she has the skill set to help you track towards your goals and maximise your potential.

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