What to Eat Before & After Exercise: The Science of Glucose/Fat Burn and Carbs

When it comes to fueling our body for exercise, the right nutrition can make all the difference in performance, endurance, and recovery. Whether you're doing a casual workout, interval training, endurance sports, or strength training, your body relies on different energy sources. So, what should you eat to optimize your workout? Let's first have a look at carbs, and in the next episode I’ll talk about protein.

Understanding energy sources: Glucose vs. Fat

Your body can burn two primary fuels for energy: fat and glucose. The type of workout you're doing determines which fuel source your body prioritizes. 

• Fat comes from your body’s fat cell reserves or your muscle’s fat reserves.
  
  
• Glucose can come from: what you just ate (blood glucose), your liver’s glycogen stores (that’s one of the forms by which glucose is stored in your body), or your muscle’s glycogen stores.

Low intensity vs High intensity workouts

Let’s first look at VO2 max. It’s a key metric of fitness, a way of measuring how well your heart, lungs, and muscles work together to use oxygen. It can be measured with different smart devices or when you do a test on a treadmill.

We can also estimate VO2 max by using our max heart rate (maxHR). Your maxHR is roughly 220 minus your age (for example if you’re 40 years old, your max heart rate is 220 - 40 = 180bpm).

To estimate what fuel you’re using, you can measure your heart rate as a percentage of your VO2 max (or maxHR). For example, if you’re 40 years old and your heart rate during a walk is about 90bpm, this is 50% of your maxHR (again, which is calculated as 220 minus your age = 180bpm). 

So what counts as low vs high intensity workouts?

• Lower intensity workouts (like walking, hiking, light jogging, cycling with a friend) are generally at about 50% of our VO2 max. At this intensity, up to 60% of the fuel that we’re using comes from fat. While it takes a bit longer to kick in, it has a huge reserve, perfect for long, low intensity workouts. (ready study)
  
  
• Higher intensity workouts (like HIIT, sprinting, long-distance cycling at race pace) are at about 85% of our VO2 max. Your body needs more glucose for quick energy. (read study)

What to eat before a workout?

1. For Low-Intensity Workouts (Walking, Easy Jogging, Light Cycling - 50% maxHR)

• Your body relies mostly on fat stores and blood glucose, so you don’t need to eat extra carbs before exercise unless you're already low on energy.
  
  
• If you’re working out for less than 2 hours, your liver will release enough glycogen naturally. 
  
  
• Fasted workouts? Okay for low intensity, but may be a stressor on the body for women (just check in on how you feel and do what works best for you)
  
  

2. For Moderate-Intensity Endurance Workouts (Long Hikes, Steady-State Running - 60-70% maxHR)

• Your body gradually shifts to burning more fat over time.
  
  
• If your goal is fat burning, this is an effective zone.

• If you’re not metabolically flexible (struggle with long walks, feel exhausted if you don’t eat, need to snack all the time), your body is not efficient at burning fat. Training your body to use fat as fuel (by reducing your glucose spikes) can improve endurance. My glucose hacks can help you.

3. For High-Intensity Training (HIIT, Sprinting, Weightlifting, Competitive Endurance - 85% maxHR)

• Your body primarily burns glycogen (stored glucose).
  
  
• To perform well, your muscles should be topped up with glycogen.
  
  
• What to eat before? Carbs the day before and in the morning of your workout. Avoid fasted workouts if you want peak performance.

Do you need to eat during a workout?

• For workouts under 2 hours at high intensity, no need to eat during—just ensure your glycogen reserves are full by eating carbs before.
  
  💡 Hack: Did you know you can trick your brain into boosting endurance without consuming extra sugar?
  Swishing a glucose solution or fruit juice in your mouth for about 5-10 seconds (and spitting it out) sends a signal to your brain that energy is available, helping improve endurance by 3-7%. (ready study)
  This is ideal for short bursts of high-intensity exercise (HIIT, weightlifting, sprinting).
  
  
• For endurance exercise over 3 hours (like a marathon or long cycling events):
  Your glycogen reserves will run out after 2-3 hours, leading to fatigue ("hitting the wall").
  
  
• To keep going, consume 30-60g of glucose per hour.
  
  💡 Hack: Since your gut can only absorb 60g of glucose per hour, adding 30-40g of fructose (which uses a different absorption pathway) allows your body to process up to 100g of carbs per hour. (read study)

Best carb sources for long endurance exercise? Glucose-based drinks, energy gels, bananas, or carb-rich foods like energy bars. 

Note: outside of the context of high intensity endurance exercise, having too much fructose in our diet is detrimental. Check out my sugar episode for details on that.

What to eat after working out? Post-workout refueling

After your workout, replenishing glycogen is key, especially after high intensity or endurance workout. But that doesn’t mean reaching for processed sugar.

✅ Best options: Starches (sweet potatoes, rice, quinoa) + whole fruits.

⛔️ Avoid: High-fructose foods like fruit juice, cookies, cakes, which can overload your liver when you’re not exercising.

💡 Hack: Use my veggie starter hack before eating starches to reduce glucose spikes and prevent energy crashes post-workout. Download free veggie starter recipes to get started.

Final takeaways

• Casual workouts? You don’t need special carb loading—your body has plenty of energy stored.
  
  
• Endurance training? Carb fueling becomes important after 2+ hours of high-intensity exercise.
  
  
• Want to improve fat burning? Train your body by reducing your glucose spikes and building metabolic flexibility.
  
  
• Post-workout refueling: Choose whole-food starches instead of sugar-loaded snacks, and do the hacks to avoid big glucose spikes.

The scientific studies mentioned in this episode

Jeukendrup A. “A step towards personalized sports nutrition: carbohydrate intake during exercise.” Sports medicine (Auckland, N.Z.) 44, Suppl 1 (2014): S25-33. https://pubmed.ncbi.nlm.nih.gov/24791914/

Rodrigues Oliveira-Silva I G et al., “Effect of carbohydrate mouth rinse on muscle strength and muscular endurance: A systematic review with meta-analysis.” Critical reviews in food science and nutrition 63, no. 27 (2023): 8796-8807. https://pubmed.ncbi.nlm.nih.gov/35373671/

Spriet L L, and Randell RK, "Regulation of Fat Metabolism During Exercise." Gatorade Sports Science Institute, July 2020, https://www.gssiweb.org/sports-science-exchange/article/regulation-of-fat-metabolism-during-exercise

Watt M J et al., “Intramuscular triacylglycerol, glycogen and acetyl group metabolism during 4 h of moderate exercise in man.” The Journal of physiology 541, Pt 3 (2002): 969-78. https://pubmed.ncbi.nlm.nih.gov/12068055/