These days Instagram influencers have been condemning carbohydrates like the royal establishment during the English Reformation on a 1500s spree. Carbs ruin our way of life. Carbs are heretics. Well, Okay, maybe not that last one. But where did these pervasive ideas about carbs come from, and is there any scientific merit to the claims?

Depending on who you talk to, the standard medical recommendation for carbohydrate intake is 45-65% of calories. However, it is now common practice for those who have written off carbs as the monster in the pantry to follow a low carbohydrate or even the more extreme ketogenic diet, which consists of about 5-10% of the diet calories from carbs or between 20 and 50g of carbohydrates per day. As a reference, an average slice of bread has a little less than 20g of carbs. 

Digesting the carb controversy - 1

So how did this style of eating come to fruition? The ketogenic diet was developed in the 1920’s as a treatment for epilepsy and an alternative to fasting. The ‘keto diet’ itself causes the body to go into a starvation mode in which it uses fat to produce ketones for fuel rather than glucose (1). In the non-medical world, it has evolved from Atkins to the recent resurgence of the keto movement. While still viable as a treatment for epilepsy, general long term effects of a ketogenic diet have yet to be determined. However, the diet itself poses a risk of nutrient deficiencies, poor gut health, and increased risk of cardiovascular disease (2). While seemingly not as extreme, diets like Whole30 and Paleo also contribute to a carb restrictive approach with their rules around grains, refined sugars, and legumes.

Many proponents of these diets make the following claims. Let’s explore the science behind them.

Claim 1: Carbs will make you fat

Surely you’ve seen sensationalized headlines about this. Keto and low carb dieters present proof to support it by citing weight losses of 5-10 lbs a week or two after starting the diet. But there’s a lack of understanding of the science behind this loss. Yes, initially, when you start the ketogenic diet, your body burns through stored carbohydrate in your liver and muscles. For every gram of carbohydrate stored as glycogen, about three grams of water is stored along with it. At first, loss of glycogen leads to loss of water weight. Long term, the weight you lost through water won’t return unless carbs themselves do (and replace lost glycogen stores). But carbohydrates do not cause obesity. In fact, a 2017 analysis of 32 controlled feeding studies done with limitation of calories and protein demonstrated that neither low carb nor high carb contributed to a weight loss benefit (3).

Further down the road, ketogenic diets can reduce appetite, allowing one to eat fewer calories and lose weight, with this mechanism of caloric deficit ultimately being the same as any other diet (4). That said, we would rather eat more mindfully and indulge less on desserts for a while (creating this same caloric deficit) than have some of the possible negative side effects of eating a lot of fat and less carbs: bad breath, lack of energy, nausea, vomiting, constipation, sleep problems, possible vitamin deficiencies and possible hyperlipidemia problems (5). Yuk! 

Digesting the carb controversy - 2

Claim 2: Carbs give you diabetes

While eating excessive amounts of carbohydrates—or any food for that matter—can cause your risk of disease to increase, eating carbs doesn’t magically cause Type 2 diabetes. In fact, in a recent systematic review article, out of 11 studies, the risk of developing Type 2 diabetes was no different on a high carbohydrate vs. low carbohydrate diet (6). So what causes the disease? Overall diet, lifestyle, and genetic factors all appear to be the main contributing factors related to Type 2 diabetes risk and eating whole grains, fruits, legumes and vegetables (sources of carbs) can actually help you maintain healthy eating habits and reduce the incidence of many diseases.

Claim 3: The brain doesn’t run well on carbs

Your brain actually prefers carbs as its main source of energy. When you eat carbohydrates, they are broken down into glucose, which powers the brain cells. In fact, they help ensure that the central nervous center is functioning properly. Lack of carbs can contribute to dizziness, mental and physical weakness.

This claim probably stems from the myth that sugar makes children hyperactive. There is some little evidence that sugar rush and sugar crash can exist when you feed your brain simple carbs like sugar - but not all carbs are created equal (7)! Complex carbs (that excludes simple sugar) provide long-lasting energy and are a better source of carbohydrates. We explain all of this in Parts 3 and 4.

Claim 4: You’ll run faster without carbs

The new low carb, high fat trend in the sports world fails to take into account that at higher intensity exercise (a higher percentage of your VO2max), you use more carbs for fuel. Without them, the body breaks down fat and muscle, which is inefficient and causes you to slow down. This idea is supported by well known researcher and sports dietitian Louise Burke’s recent research aptly named the SUPERNOVA study, which demonstrated that a low carb, high fat diet in long distance race walkers impaired exercise economy and blunted sports performance efforts (8).

But do you need carbs?

While certain fad diets make it seem like carbs are completely optional for our bodies, is that really the case? And what exactly is the difference between simple and complex carbs, anyway? In our next article, we’ll dig deeper into how our bodies process carbohydrates and the science behind what carbs do for us.


Read Introducing: The War on Carbs Series.

Read Part 1: Carbs, Culture, and You

Read Part 3: Do You Actually Need Carbs? 

Read Part 4: How to Harness the Power of Carbs





1) Wheless, J. W. (2008, November). History of the Ketogenic Diet. Epilepsia, 49(8), 3-5. PubMed. 10.1111/j.1528-1167.2008.01821.x

2) Harvard Medical School. (2020, August 31). Should you try the keto diet? Harvard Health Publishing. Retrieved April 28, 2021, from Should you try the keto diet? – Harvard Health Publishing

3) Hall, K. D., & Guo, J. (2017, May 01). Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition. Pathogenesis of Obesity, 152(7), 1718-1727. Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition

4) Gibson, A. A., Seimon, R. V., Lee, C. M., Ayre, J., Franklin, J., Markovic, T. P., Caterson, I. D., & Sainsbury, A. (2015). Do ketogenic diets really suppress appetite? A systematic review and meta-analysis. Obesity reviews : an official journal of the International Association for the Study of Obesity, 16(1), 64–76.

5) Wheless, J. W. (2001). The Ketogenic Diet: An Effective Medical Therapy With Side Effects. Journal of Child Neurology, 16(9), 633–635.

6) Noto, H., Goto, A., Tsujimoto, T., & Noda, M. (2016). Long‐term Low‐carbohydrate Diets and Type 2 Diabetes Risk: A Systematic Review and Meta‐analysis of Observational Studies. Journal of General and Family Medicine, 17(1), 60-70. Long-term Low-carbohydrate Diets and Type 2 Diabetes Risk: A Systematic Review and Meta-analysis of Observational Studies

7) Mantantzis, K., Schlaghecken, F., Sünram-Lea, S. I., & Maylor, E. A. (2019). Sugar rush or sugar crash? A meta-analysis of carbohydrate effects on mood. Neuroscience & Biobehavioral Reviews, 101, 45-67.

8) Burke, L. M., Ross, M. L., Garvican-Lewis, L. A., Welvaert, M., Heikura, I. A., Forbes, S. G., Mirtshin, J. G., Cato, L. E., Strobel, N., Sharma, A. P., & Hawley, J. A. (2017, February). Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. The Journal of Physiology, 595(9), 2785-2807. PubMed. 10.1113/JP273230


Enter your email to receive amazing tips, stories & recipes to help you live a healthy & productive life.

Kylee Van Horn

Kylee Van Horn, RDN is an endurance sports dietitian and freelance nutrition writer who takes a balanced, sustainable approach to fueling for performance.