Sugar & Starch. What and When is Best for Endurance Athletes?

From high-carb, to low-carb, to no-carb. How are we supposed to know how to eat? The fact of the matter is that nobody knows for sure, and anyone claiming to know “the” way to eat is full of it. Dieting may even be similar to training in that it should vary in fairly large magnitudes to force us to adapt. A great friend of ours, Dr. Serrano, sums up the thought with the phrase, “the best diet is the one you are not on.” Programming variability into the diet, just like training, is called diet periodization. This article will cover some fundamental topics related to carbohydrate periodization.

What is the Difference Between Sugar and Starch?

 good preworkout meal

Chemically, there is not a big difference. Sugar comes primarily as three different molecules called monosaccharides, glucose, fructose, and galactose. These can be combined in all sorts of different ways to form other sugar molecules like the disaccharides, sucrose (fructose and glucose), maltose (2 glucose), or lactose (glucose and galactose). When 3 – 10 sugars are combined, they’re no longer considered sugars but oligosaccharides. In excess of 10 combined sugars, it’s called a polysaccharide – either fiber or starch. Yup, that’s all starch is, just lots of sugar linked together.

Biologically, the differences are only slightly more pronounced. The primary biological form of carbohydrate is glucose. Unless it’s fructose, it doesn’t matter if it was eaten as sugar or starch, the body converts the carbohydrate to glucose before it can do anything. Once the glucose molecule is formed, it has two fates – make ATP now or make ATP later. Fructose has basically the same fate. It can be converted to glucose, but it is absorbed from the intestine differently than glucose, and when glucose goes to make ATP, it is actually converted to fructose for a brief time.

Our bodies use (oxidize or “burn”) glucose to generate ATP (fuel/energy). If the glucose is not immediately needed, it can be stored as starch (glycogen) in the muscles and liver for later use. However, the liver and muscles can only hold so much glycogen, so if they’re “full,” the glucose is stored as fat. The human body is a real penny-pincher, and it doesn’t like to let good energy go to waste no matter how badly we want a low body fat percentage!

One of the biggest differences between sugar and starch is digestion rate

Sugar is already in the form needed by the body for metabolism, so it doesn’t need to be digested – it only needs to be absorbed. On the other hand, starch does require digestion, as it must be broken down into sugar in order to be absorbed, oxidized, and/or stored for later. As may be readily apparent, digesting and absorbing the starch takes longer than simply absorbing sugar, so this affects the rate of digestion/absorption and, therefore, blood sugar and the magnitude of available energy. Starches may also be “branched” to accelerate breakdown, such as with glycogen or highly-branched cyclic dextrin.

When to Eat Sugar. When to Eat Starch.

 sugar for endurance athletes

The “when” answers are not as simple as they may seem. Based on the kinetics of the situation just described, it sounds like sugar is always “bad,” but if you think sugar is bad, then you must also take the position that starch is “bad” because it is only many sugars linked together. Some do make the assumption that all carbohydrates are “bad.” Conversely, about the same number of people equate sugar and starch as both being “good.” Those in the middle may ignore my logic and say sugars are “bad” and starch is “good.” All of those people are wrong.
There is no such thing as a “good” or “bad” food. Nutrients may taste good or have bad consequences, but I assure you that the foods have no intentions whatsoever. Sugars and starches are just tools we can use to build the body we want or to fuel runs and rides. Bad consequences arise when the tools are misused - like using a jackhammer to build a birdhouse. So, how do we use carbs?

Remember, sugar and starch are either used to make ATP now or make ATP later. Carbohydrates biological function is to make energy. If we are exercising and need a lot of energy right now, sugars are a good bet. However, sugars are osmotic, which means they draw a lot of water into the stomach, so in practice, eating as many sugars as you need that your stomach can tolerate is the best bet. The remainder of your energy needs can come from starch, preferably a starch with lots of branching so it may deliver quick energy but not waterlog the gut (see our article on highly branched cyclic dextrin).

Conversely while you’re at rest or just lightly moving around, you do not have an immediate energy need. In this case, you can either eat less total carbohydrate, a slow-digesting carbohydrate, or you may just let your body use fat for energy instead.

We’re All Friends Here

 carbohydrates for runners

Sugar and starch can each play a key role in the body. Each are tools, and each can be misused. Scarfing down nothing but brown rice during your marathon is misuse of the tool called starch, and snacking on candy bars while watching television is misuse of sugar. On that note, always eating sugar or starch influences the body to always metabolize sugar, which means it won’t be oxidizing fat and vise versa. Sugar is best used during and after training, and even fructose has some unique advantages. Starch can even be good to use during and after training to provide smooth delivery of energy. Use them to your advantage!



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