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  • Dr. Sarah Penney, ND, MSc

What to eat when exercising

Planning on getting fit this summer? Make sure your diet is keeping up! More often than not, we look at nutrition as a dial we can adjust to lose weight, but cutting calories could be detrimental to your new work out plan if you are not careful. When you increase the amount of aerobic exercise (cardio) you are doing on a daily or weekly basis, this also increases your body’s need for certain nutrients to maintain energy levels and facilitate repair and recovery of your muscles. Here is a review of the three most important macronutrients your body needs when you increase your exercise levels, when to get them, and where!


Why: Although dreaded by many who are trying to cut belly fat, carbohydrates are the most important source of energy for athletes. Carbs are chains of sugars that are easily broken down through digestion, starting right in the mouth. These sugars are then absorbed and either used immediately for energy, or packed into a storage unit called glycogen in the liver or muscles. Carbohydrates and glycogen stores provide fuel for our muscles during short bursts of exercise, or approximately the first 30 minutes of any longer exercise session (1). This easily accessible source of energy is essential for our pep at the start of a race.

When: Carbohydrates are a good nutrient to have both before and after working out. If consumed before exercise, they will provide energy for the beginning of your workout, and when consumed after they will help replenish your glycogen (stores in your muscles and liver) for the next exercise session.

What: The type of carbohydrate you chose can be very important, especially if you are worried about weight management. In short, the two types of carbohydrates you need to consider are simple and complex carbohydrates. Simple carbohydrates are short chains of sugar that are easily broken down. They are found in foods made from white flour and high in processed sugar. Simple carbohydrates provide energy, but no additional nutrients. They also have a greater effect on blood sugar levels because the sugars are quickly digested and absorbed, which may lead to greater storage of this energy and greater weight gain when not exercising. Complex carbohydrates are longer sugar chains that are harder for the body to digest. This means that they are absorbed slowly resulting in less of a spike in blood sugar, providing a longer source of fuel. Complex carbohydrates are found in foods that contain whole grains, beans and vegetables. Although the sugar from simple carbohydrates will likely be used quickly if consumed before exercising, complex carbohydrates are the best choice for overall energy levels and weight management.

Where: Whole grains, oatmeal, sweet potatoes, vegetables, bananas and more.


Why: Protein intake is essential for muscle repair and recovery. It also provides a small portion of the energy used once glycogen stores are depleted. Exercise increases our protein requirements – so lets do some basic math. The recommended protein intake for someone with a low to average activity level is 0.8g/kg/d, and regular exercise increases this requirement to 1.0-1.6g/kg/d depending on intensity and duration. So if you weigh 150lbs, which is converted to 68kg, this means you will need between 68-108g of protein/day when active. Protein post workout may also help improve immune function and decrease muscular soreness. (2)

When: Protein can be consumed any time before or after a workout, but it is not easily digested by the body and may be best-consumed post exercise. This is because blood is diverted away from the digestive tract to nourish the muscles during physical activity, which may impair digestion. Protein can also help slow down the absorption of sugars from carbohydrates into the blood stream, which your body could use while exercising.

What: Protein is made out of amino acids, which are the building blocks we need to help repair muscles, support immune function and even produce certain chemicals in our brain. There are 21 amino acids that exist, 12 of which the body can make and 9 we need to get from diet to support biological functions. A complete protein source contains all 9 essential amino acids and is largely found in animal based foods. If you are relying on plant foods as your sole source of proteins, it is important to combine foods carefully to make sure all of your essential amino acid requirements are being met.

Where: Complete protein sources are any animal products including meat, fish, poultry and dairy (including whey protein), in addition to plant based sources like quinoa or soybeans, or a combination of plant products to deliver a complete amino acid profile such as corn and beans, beans and rice or grains and nuts.


Why: Fat is our body’s major form of energy storage, and dietary fat delivers the highest amount of calories of any macronutrient. It is the primary source of energy after carbohydrates are depleted during exercise, and is a virtually unlimited fuel source compared to glycogen (3).

When: Similar to protein, fat is not easily digested by the body and may be best to save for after exercise or away from exercise. Fat can again slow down the absorption of sugars from carbohydrates into the blood stream, which your body could use for fuel while exercising.

What: The fats that are stored in our bodies are made of the fats that we eat – so for many reasons consuming the right type is important. It is well known that we should try and moderate our intake of saturated fats and avoid trans-fats while increasing our intake of certain unsaturated fats. Omega 3 and omega 9 are two unsaturated fats that are thought to help lower levels of inflammation, benefit heart health and brain function, while high levels of omega 6 may promote inflammation. Omega 6 oils are largely found in products containing soybean oil, corn oil, safflower oil, sunflower oil and other vegetable oils.

Where: Foods that include healthy omega 3 fatty acids include fatty fish and some nuts and seeds (like walnuts, almonds and flax seed). The relatively low amounts of omega 6 found in a diet comprised of whole foods is not detrimental to health.

Try to follow these dietary guidelines as often as possible to stay healthy while getting fit!


  1. Rahnama S., Rust, E. Oct 17, 2005. Timing is Everything: Why the Duration and Order of Your Exercise Matters. University of Michigan Medical School: Med Fitness. Accessed June 20, 2013. Available from:

  2. Campbell B., et al. International Society of Sports Nutrition position stand: protein and exercise. Journal of the International Society of Sports Nutrition. 2007. 4(8).

  3. Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Section 30.4, Fuel Choice During Exercise Is Determined by Intensity and Duration of Activity. Available from:

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