Fuel for the Active Body

Firstly, to get the most of this blog you need to be sure you understand the basics of nutrition.  I encourage you to re-visit my previous blogs on basics of nutrition, sugar, and fats and the Q&A on high protein diets.  For the most part, the same basic nutrition principles apply to those who do high levels of activity and athletes. Eat a varied diet of fruits and vegetables, whole grains, healthy fats, and lean protein.  If you do this, you should meet the daily recommendations for all your macro and micronutrients.

In this blog, I will run through some of the principles of exercise metabolism and how this impacts nutritional needs.  I want you to better understand what is fuelling your body during exercise (mainly carbs!).  I will then go through how you can assess your own nutritional needs based on your activity.

Before I begin, it’s important to point out there are various levels of physically activity ranging from casual exercisers, weekend warriors, fitness buffs, and up to high performance athletes.   Therefore, sports nutrition is not a ‘one size fits all’ prescription and needs must be adapted to your particular situation.   For example, it’s highly unlikely you need energy gels/drinks during your daily pilates class, however, a marathon runner benefits greatly from these products while training intensely.  Recommendations vary greatly because sports and/or physical activities vary so much, making it difficult to encompass the needs of everyone into a generic formula.

I will try to summarise general guidelines but feel free to send through specific question if you want more clarity on individual recommendations.

Energy Systems

For all those who hate science, I warn you. STOP HERE!  Skip to the end (and no, it won’t count as your daily exercise) and look at some of the practical advice.  For you science geeks out there, I want to run through exactly what is happening to your metabolism during and after exercise.  Once you understand this, you can better decide what type of nutrition will benefit YOU.

Your body has 3 sources of energy to fuel your exercise (called energy systems).  Energy in the body is stored in the form of adenosine triphosphate (ATP).  For simplicity’s sake, I won’t go into any more detail except to say that breaking this down provides energy.  The amount of energy stored in your muscles is very limited; essentially enough to fuel you for 1-3 seconds.  Therefore, the body needs to generate energy from other sources. 

The first energy system is the ATP-creatine phosphate (ATP-CP) system.  In this system, CP acts as a shuttle within the cell providing substrate for ATP to be regenerated after its been broken down thus creating more energy.  However, this system is very limited and only can last from 3-15 seconds.  It is termed an anaerobic reaction (without oxygen) and is used most during very intense, short burst of energy like lifting, jumping, or sprinting

The next energy system is glycolysis, which is a breaking down of glucose.  This glucose comes from both blood glucose and muscle glycogen (stored glucose in the muscle).  This is also an anaerobic process and can fuel activities lasting from 30 seconds to 3 minutes. After that, the cells need oxygen to continue to fuel the exercising muscle. 

This brings us to the last energy system, the aerobic system.  This system supplies the most energy to the body but it is not fast and the energy is not readily available unlike the other 2 systems.  Here, the end product of glycolysis (pyruvate) is able to go through additional metabolic pathways in the presence of oxygen (hence the name aerobic system) yielding 18 times more energy than glycolysis.  In addition, triglycerides (coming from stored fat) can be broken down and metabolized resulting in an (almost) endless supply of energy. 

Now, your body does not use each of these system exclusively, but sources the energy from wherever is more appropriate.  For example, a quick sprint for the ball playing football is likely fuelled by ATP-CP or glycolytic system while the jog back is fuelled by the aerobic system.

You note that in general, you are using glucose (carbohydrate) and to a lesser extent fat to fuel exercise.  The lower the intensity of the exercise, the more the body is able to rely on fats as there is time for it to be broken down for energy.  Interestingly, training actually increases your ability to breakdown fat and use it as energy at higher intensities. However, don’t be fooled by the ‘fat burning zone’; the idea that you don’t want to work too hard because you will stop metabolizing fat and rely on glucose.  In the end, it’s the total intensity (i.e. calories burned) which will make a difference, not the energy system used to burn them.  In fact, you burn the greatest proportion of fats sitting still and as great as it would be, sitting is not an effective form of exercise ;)!

So, what do we need to be eating if we are exercising? 

Well, it depends.  If you are participating in moderate intensity exercise (eg. working out once a day, 5 days a week with a mix of cardio and weights) your macro and micro nutrient requirements will likely be met by following the USDA dietary recommendations discussed previously.   The ranges are: carbohydrate 45-65% of total energy, fats 20-35% of total energy, protein 10-35% of total energy.

Unless you are trying to lose weight, total daily caloric intake will need to increase to support the energy demands of exercise.  Your total energy needs can be assessed using an equation which takes into account you reported level of activity.  There are a few different versions out there but this one gives you a good idea:

BMI calculator

As your total daily calories go up so can the absolute numbers (grams) of carbs, fats, and proteins.  There are some more specific recommendations for these absolute numbers depending on your type of physical activity.

Dietary protein is very important for those who are exercise training.  It is a building block for contractile and metabolic proteins.  In addition, it plays a role in stimulating muscle protein synthesis and preventing muscle degradation.  For those participating in strength training, protein intake is recommended to be between 1.3-2.0g/kg of protein.  Higher levels (over 2g/kg/day) have been shown to be advantageous to prevent loss of lean mass during energy restriction and inactivity due to injury.

However, it’s also important that energy needs are met (generally through carbohydrate intake) to prevent muscle protein breakdown.  Particularly, those doing endurance training need to ensure they have adequate carbohydrate availability to perform. Remember, your body is highly dependent on carbohydrate for fuel during endurance sport and you cannot store a large amount in the muscle (as compared to fat). Carbohydrate needs increase with more intense and longer duration activity to prevent you from ‘hitting the wall’ which is essentially when carbohydrate stores are diminished.

Below is a summary of the recommended levels of carbohydrates for athletes based on recommendations by the American College of Sports Medicine.

Summary of carbohydrate needs for athletes

Light (Low intensity or skill-based activities)  = 3–5 g/kg of athlete’s body weight/day

Moderate (Moderate exercise program (eg, ~1 hour per day))  = 5–7 g/kg/day

High (Endurance program (eg, 1–3 h/d mod-high-intensity exercise)) =  6–10 g/kg/day

Very High (Extreme commitment (eg, 4–5 hour/day mod-high intensity exercise)) = 8–12 g/kg/day

 While these guidelines are based around ‘athletes’, here’s how I would apply them to ‘Joe’, a 30 year old male, 80 kg, 180 cm, moderate exercise program (1 hr per day, 5 days/week including resistance training).

 Sports Nutrition graphic.png

The large caloric needs due to high levels of physical activity allow ‘Joe’ to eat the required amount of carbs and protein to support his activity and still fall within the recommended ranges for macro nutrients.  His intake needs to be fluid and change according to his daily needs and how he responds to the meals.  As he is not doing solely cardiovascular training (i.e. it’s mixed with resistance training) I selected a carbohydrate intake on the lower end of the range and a protein intake on the higher end.

So, what does he eat?  Its right back to what I am always saying. Lots of colourful fruits, vegetables and whole grains, a variety of protein (nuts, seeds, fish, lean meat, eggs), healthy fats (avocado, olive oil, salmon), and low fat dairy to support strong bones.  Eat a small mixed meal (protein, carbs and fats) a few hours before exercise and be sure the replenish following a session.

Following the guidelines above, you should have sufficient micronutrients to support your needs.  Note: as the B vitamins (Vit B6, Thiamin, and riboflavin) are used in energy metabolism, you may need 1-2 times the RDA, however this should be covered in a higher calorie, nutrient dense diet described above.

We have covered a lot of information, and in actuality, we barely skimmed the surface of nutrition and exercise.  Please use this as a guide but DO send in questions around your specific needs.  It all seems so complicated and I often get asked: Do I need to be calculating my ‘macros’ and analysing everything I put into your mouth?  Most likely no.  Just follow the guidelines above, space out your meals, and be aware of works for YOU in terms of your exercise performance.






Topics: Science, Nutrition, Food, Fitness, Exercise, Education, Running

Dr. Tara Coletta, PhD

Written by Dr. Tara Coletta, PhD

Tara's research focused on obesity and metabolism. She studied exercise science (MS, UMass Amherst) before earning a PhD in nutritional biochemistry (Tufts University). Wellness remains an integral part of Tara’s life as she works to balance being a mother of three.