The world is currently facing a growing obesity epidemic, with approximately 1 in 8 individuals living with obesity worldwide (Phelps et al., 2024). While researchers continue to explore potential solutions via medical treatments, a nutrition discussion has persisted in the background: are carbohydrates to blame? From traditional media personalities writing bestselling books to social media influencers swearing by low-carbohydrate diets, this macronutrient is often portrayed as a main driver of weight gain. But does science support these claims about the detrimental aspects of carbohydrates?
Carbohydrates vs sugars: the label distinction that matters
Beyond promotion of low-carbohydrate diets through social and traditional media, supermarket choices reflect an approach of carbohydrate concern. Products are often marketed with labels such as “low-carb” or “no added sugar” and thus perceived as a healthier choice. Yet here, we need to make the first important distinction within this blog: the difference on a nutritional label between carbohydrates and sugars.
They are both usually presented on food labels, with sugars as a subpoint of carbohydrates. That is because simple carbohydrates (two or fewer glucose, fructose, or galactose molecules chained together) are deemed sugars based on a World Health Organization classification (Te Morenga et al., 2012). The amounts of simple carbohydrates (sugars) and complex carbohydrates make up the total amount of carbohydrate in a product.
Both sugars and complex carbohydrates provide energy. While simple sugars provide quick energy boosts, complex carbohydrates provide a stream of energy that is more balanced throughout the day. Excess energy can indeed lead to increases in fat mass and therefore, potentially obesity. Does that make carbohydrates the main culprit? As always, human nutrition is more nuanced than that.
The fuel argument: it depends on what you’re doing
During a normal day, where most of the time is spent in a resting state, the requirement for carbohydrates is not very high, with most of the energy being obtained via fat metabolism. The reason carbohydrates still play an essential role in human nutrition is that they become the primary fuel source for the exercising muscle at high intensities (Van Loon et al., 2001). Thus, there is a significant increase in the need for carbohydrate intake.
In specific sports, such as cycling, during races and hard training days, you might see more than 1,000 grams of carbohydrate consumed a day (Morton et al., 2026) — meaning that more than 4,000 calories come in the form of this one macronutrient. Telling a high-level cyclist to reduce their carbohydrate intake is akin to telling them to give up their career altogether, because high performance in sports is dependent on eating enough carbohydrates to fuel your work.
On the other hand, for an individual who has a desk job and is not very active outside working hours, advising to eat more than 1,000 g of carbohydrate a day would be a sure-fire way to lead them to weight gain. Even so, an approach of moderation is likely a wiser strategy than jumping to the extreme of eliminating carbohydrates altogether.
Why your brain needs them too
At the end of the day, the brain relies on glucose as its primary fuel source under normal living conditions (Jensen et al., 2020), and there is a relatively stable concentration of glucose circulating in the bloodstream (known as blood glucose or blood sugar). For these reasons, common nutritional recommendations suggest that about 45–65% of your energy intake — the calories consumed in a day — should be in the form of carbohydrates (Jeukendrup & Gleeson, 2023).
In order to feel more energetic throughout the day, and avoid the dreaded sugar rushes and crashes, it would be ideal to focus mostly on complex carbohydrates, with simple forms having their role around exercise and as a sweet treat when desired.
The bottom line
So, are carbohydrates the culprit for the obesity epidemic? Based on current evidence, you cannot isolate this macronutrient as being to blame. Carbohydrates in excess can contribute to weight gain — much like any other nutrient. The nutrients we consume are tools; it is the way we incorporate them into our routines that ultimately determines the outcome.
📚 Want to see how carbohydrates fit alongside protein? Read the previous pieces in this series — including How much protein should we consume? and Animal vs plant-based protein.
References
Jensen, N. J., Wodschow, H. Z., Nilsson, M., & Rungby, J. (2020). Effects of ketone bodies on brain metabolism and function in neurodegenerative diseases. International Journal of Molecular Sciences, 21(22), 8767.
Jeukendrup, A., & Gleeson, M. (2023). Sport nutrition (4th ed.). Human Kinetics.
Morton, J. P., Areta, J. L., Casey, A., Hawley, J. A., & Phillips, S. M. (2026). High-carbohydrate availability in elite endurance training: updates from recent ultra-endurance and Grand Tour data. Sports Medicine.
Phelps, N. H., Singleton, R. K., Zhou, B., Heap, R. A., Mishra, A., Bennett, J. E., et al. (2024). Worldwide trends in underweight and obesity from 1990 to 2022: A pooled analysis of 3663 population-representative studies with 222 million children, adolescents, and adults. The Lancet, 403(10431), 1027–1050.
Te Morenga, L., Mallard, S., & Mann, J. (2012). Dietary sugars and body weight: systematic review and meta-analyses of randomised controlled trials and cohort studies. BMJ, 346, e7492.
Van Loon, L. J. C., Greenhaff, P. L., Constantin-Teodosiu, D., Saris, W. H. M., & Wagenmakers, A. J. M. (2001). The effects of increasing exercise intensity on muscle fuel utilisation in humans. The Journal of Physiology, 536(1), 295–304.