The Science of Carbohydrates

I’m going to guess that most people’s favorite macronutrient is the carbohydrate. Oatmeal, sweet potatoes, rice cakes, fruit… I can go on forever (dare me). Even if they’re the most popular macronutrient, many don’t know much about the science behind carbohydrates and how they work for us metabolically. In this article, I’ll be giving you a rundown on carbohydrate metabolism and their application in performance.

Some Background: The Monosaccharides

The main goal of our metabolism is to make energy in the form of ATP. Our bodies’ primary fuel source for this comes from carbohydrates. When we consume carbohydrates, the final end products of digestion are almost entirely glucose, fructose, and galactose – these are the monosaccharides. Glucose, however, makes up about 80% of these and is the final common form for the transport of carbohydrates to our cells.

Pathways to performance

Carbohydrate metabolism can be narrowed down primarily to a few things:

  • Blood glucose regulation

  • Glycogen storage

  • Glycogen synthesis

  • Glycolysis (glucose breakdown)

If you saw our previous newsletter on insulin sensitivity, we went over fasting blood glucose a bit. In short, when we ingest carbs, our blood sugar rises, which stimulates the secretion of insulin from the pancreas. In contrast, when our blood sugar drops (during exercise, for example), we break down liver glycogen (preferentially, as gluconeogenesis can occur as well) in order to bring those levels back up. This keeps us at equilibrium and allows our levels to stay constant during activity.

Stated previously regarding carb ingestion and performance, glucose is king. Fructose is considered to be better than nothing, although it is still inferior when comparing it to glucose. Lastly, galactose (found in milk products) has not shown us any benefits to performance when taken pre- or intra-workout. Ideally, intra-workout carbohydrates are worth using if you train for longer bouts of at least 2-3 hours at a time (endurance training rather than resistance training). This helps maintain high rates of carb oxidation during intense activity. There is a threshold, however, for the number of carbs we can consume and their guaranteed absorption. Typically 60 grams/hour is where this cut off lies.

However, there are benefits outside of the 2-3 hour mark of endurance exercise for intra-workout carbohydrates. The goal of resistance training is to stimulate the muscle as much as possible so we can break it down (and then grow). Having fuel available to tap into immediately (i.e. an intra-workout carbohydrate powder) can be very helpful for performance. If you are training with physique goals in mind, you might not have a huge bolus of carbohydrates, but this will, of course, depend on caloric intake, goals and your size. For females, anywhere from 10-20g can be useful for females and 20-40g for males.

While glucose is superior, having a mixture of glucose and fructose is an optimal combination if you want more carbohydrates available for transport into the blood stream. Since glucose and fructose use different transporters (SGLT-1 and GLUT-5, respectively) more carbs can be used for energy if both transporters are fully utilized with enough glucose and fructose being ingested. If you are only using a small amount of carbohydrates intra-workout, you will not necessarily need the combination and instead should look for a combination of glucose and cyclic dextrin (which won’t spike blood sugar too high).

Endurance training has a general recommendation of 6-10 grams/kg/day, but resistance training does not have an established guideline for carbs. As with any macronutrient target, it will mostly come down to the individual and their activity level, body composition, genetics, and so on. Guidelines are an okay starting point, but it takes some trial and error to find what works for you exclusively.

A general rule of thumb is:

Glucose > Fructose/galactose > Placebo (nothing)

Post-training considerations

After endurance exercise, our muscle glycogen is significantly reduced. Consuming carbohydrates immediately after training is very important for athletes training at high intensities for long periods of time or those who need to train more than once a day. Ingesting glucose, sucrose, or any glucose polymers/maltodextrins will resynthesize muscle glycogen at equal capacities. Fructose is not the choice to go with for resynthesizing muscle glycogen (it is better for liver glycogen).

While skeletal muscle glycogen only drops ~30% after resistance training, replenishing glycogen is still important for the recovery process. Generally, 1 gram/kg/hour is good for resynthesizing glycogen after resistance exercise. In practical instances, this means can again mean anywhere from 10-20g of carbohydrates for females and 20-40g of carbohydrates for males post RT. This amounts can and will vary depending on goals and metabolism, but these are rough estimates we’ve found helpful with clients who are primarily focused on resistance training.

This is also important to consider because of our net muscle protein balance. Combining carbohydrates and protein about 1-hour post-training can aid muscle protein synthesis, giving us a more positive net muscle protein balance (muscle protein synthesis > muscle protein breakdown).

There is so much more we can explore with how our bodies work and respond to carbs, but next time in part 2 we’ll be going over our second macronutrient (and probably the most important!) – protein.  We’ll discuss protein metabolism and its central role in recovery and progress.