What’s Happens When You Massively Overeat Carbohydrates?

Apr 05, 2024

As cyclists, carbohydrates are a critical fuel source for performance. Any moderate to high-intensity exercise relies heavily on carbohydrates as a fuel source, and without sufficient carbohydrate availability, we fatigue.

As such, ensuring sufficient carbohydrates are available when they're needed is a key component of any sports nutrition strategy.

 Whilst fuelling is important, cycling performance is also often influenced by an athlete's weight-to-weight ratio. As such, a key concern for many is balancing sufficient fuel intake to perform without this, leading to unwanted accumulation of excess body fat that might negatively impact their power to weight.

This concern often drives a strong phobia of dietary carbohydrates. It inadvertently leads to significant under fuelling in many athletes, as they fear any meaningful gain in fat that would negatively impact their body weight.

But, looking at this fear objectively, using the extensive scientific data we have available to us, what actually happens if we consume too much carbohydrates in our diet? Is this something we should be concerned about, and do carbohydrates make any meaningful contribution to accumulating body fat stores?

A study dating back to 1988 (Carbohydrate Overfeeding Study) looked at the impact of massively overfeeding carbohydrates on man, and the results will surprise you.

They took three healthy young men and studied them over 14 consecutive days. During the first three days, they consumed a highly restricted diet with only 1600 kcal that was high in fat and low in carbohydrates. They also exercised a lot to deplete their glycogen stores, much in the same way many cyclists do on heavy training days whilst under fuelling.

Halfway through the initial three days, they were put into a metabolic ward. This is a sealed room that allows the accurate measurement of their energy expenditure and fuel use.

At the end of the third day, they then started the massive carbohydrate overfeeding part of the study. From day 4-10, they consumed around 3600 kcal each day, of which 86% of the diet was carbohydrate, which increased to around 5000 kcal by the end of the 7 days of overfeeding. During this time, they maintained an energy excess of around 1500 kcal per day.

This diet meant that, while their physical activity was low, they consumed a massive 750-1000 grams of carbohydrates per day—equivalent to around 6-8, 160-gram bags of family sharing bags of Haribo each day!

What happened with this excess carbohydrate?

When we eat a lot of carbohydrates, we also burn a lot of carbohydrates. On the first days of a low kcal diet with lots of activity, participants oxidised (i.e. burned off) around 70 grams of carbohydrates per day. When their carbohydrate intake was massively increased on day 4, this then increased to around 400 grams a day, which further rose to around 1000 grams a day by day 10. Some of this was burned off as energy during the day, but some were used to convert carbohydrates to fat.

 When we eat a lot of carbohydrates, we also store a lot of glycogen, particularly when we start eating a lot of carbohydrates with low glycogen stores. Any cyclist with a heavy training load is likely to have low glycogen stores many times during periods of heavy training.

 In the study, on day 1 of carbohydrate overfeeding, they saw all of the excess carbohydrates that weren’t used for energy that day stored as glycogen. This resulted in the gain of around 340 grams of glycogen. As the days of carbohydrate overfeeding progressed, the amount of extra glycogen stored daily decreased. Around 192 grams of glycogen was stored on day 2, 166 grams on day 3, and 76 grams on day 4. On day 4, glycogen stores topped out/saturated at around 770 grams more than at the start of the trial.

Once glycogen stores were fully topped up after four days, any excess carbohydrate went either towards the participant's daily energy expenditure or towards a process called de Novo lipogenesis, the technical name for converting carbohydrates into fat.

How much fat was stored? When we consider that the diet the participants were consuming contained around 60-100 grams of fat each day alongside the excess carbohydrate. They saw only 7 grams of fat gained on day 1 of carbohydrate excess, around 130grams on day 2 & 3, 160 grams on day 4. As glycogen stores saturated on day 4, the remaining 3 days of massive carbohydrate intake saw around 240grams a day of fat stored.

What happened to energy expenditure? Despite doing no extra physical activity while confined to the metabolic chamber, The excess energy led to a 35% increase in the participant's daily energy expenditure; this meant that a huge amount of the extra energy during the carbohydrate overfeeding period was simply burned off as fuel.

What happened to body weight? For every gram of glycogen we store, we also store 3 grams of water. As such, with participants gaining around 700 grams of glycogen in the first few days and also storing around 1.1kg of fat, it’s not surprising to see that by the end of the overfeeding period, participants had gained around 4.5kg. However, the study finished with two days of low calories, by the end of these two days, body weights had dropped by 4.2kg (as the stored glycogen was used up) and another 2 days later, had returned to the body weights recorded pre trial.

So, what does this mean for me as a cyclist?

 In effect, excess carbohydrates will, in the short term, go towards maximising glycogen stores, which can positively impact subsequent performance in sessions that are long and hard enough.

 It takes days of massive overeating of carbohydrates to result in carbohydrates being converted to and stored as fat, but fat consumed alongside high carbohydrates will be stored as fat.

When we eat more than we need, there is often an increases energy expenditure to help burn off some of the excess.

As glycogen stores increase, body weight will increase, but the additional fuel will likely support performance, particularly for longer, harder sessions and body weight will likely return to close to baseline when this glycogen is used up.

As such, short-term overfeeding of carbohydrates as an athlete is not something that we should fear, as if you’re training regularly and turning over a lot of glycogen, it’s likely to simply be stored as glycogen and not fat.

Thanks for reading!

Coach Ben






If you're a road, mountain bike, gravel or track cyclist and want to take your performance and physique to the next level...let the FTR coaches show you exactly how to achieve this inside the Fuel The Ride Academy.

Join The Academy