Most of us have felt the benefits of taking in carbohydrates to improve performance and have probably experimented with some version of carbohydrate periodization. However, questions on carbohydrate intake and dietary practices tend to arise once the competitive season is over.
Maybe you're taking a break from training altogether, doing a bunch of cross training and other physical activity, or started lower-intensity base training for the next season. In any of these scenarios, you might feel like you don't need as many carbohydrates throughout the day or as many carbohydrates before, during, and after training. You also might wonder if you should reduce your intake of "simple" carbs (lower-fiber, faster absorbing carbohydrates like cereals, some breads, rice, pasta, candies, packaged sweet and salty snacks, etc.) overall.
When performance is not the emphasis or goal, the amount and type of carbohydrates in your diet can and should generally change, since full glycogen stores are rarely needed to accomplish lower training intensities and durations; during these periods, high rates of during-exercise carbohydrate intake (greater than 60-90 g carb per hour) are not warranted at lower submaximal exercise intensities when fat can also provide a significant portion of the substrate required to produce ATP.
This often leads to the question as to whether athletes should transition to low-carb, high-fat (LCHF) diets for the purpose of fat "adaptation" or for health when relatively higher amounts of carbohydrates might not be necessary.
One way to support better endurance performance is to have better metabolic flexibility, in which the body can adapt to efficiently use the energy sources (fat and carbohydrates) available. Endurance training in itself leads to some level of metabolic flexibility, but long-term LCHF diets have been shown to increase maximal fat oxidation, partly attributed to decreased insulin levels as a result of the diet. However, a LCHF diet can also reduce activation of enzymes involved in carbohydrate oxidation, which is counter to achieving metabolic flexibility. Following a LCHF diet might impair performance, but it can also have other unwanted side effects; for example, decreased training capacity, diminished well-being, increased fatigue, and poor attention have all been reported in response to dietary changes towards LCHF.
One way for endurance athletes to improve metabolic flexibility and improve nutrition throughout the year, even when performance is not the goal, is to incorporate elements of a low glycemic index (low-GI) carbohydrate diet, or simply one that is based on consumption of high-fiber complex carbs as a main source of carbohydrates.
A recent study examined the effects of a low-GI diet compared to a high-GI and a LCHF diet on metabolic and performance outcomes after a ten-week intervention. In the low-GI diet, 50-60% of daily calories came from carbohydrates and mostly low-GI carbohydrates such as whole grain bread, wild rice, whole grain pasta, and vegetables other than potatoes. Protein comprised 15% of total energy intake and fat 30% of total energy intake. The high-GI diet provided the same relative amount of carbohydrates as low-GI, but with more carbohydrates coming from sources like white bread, pasta, rice, fruit, potatoes, etc. The LCHF group had a maximum carbohydrate intake of 50 grams per day, while fat made up more than 65% of total daily calories. In the LCHF group, foods such as cereals, bread, pasta, rice, and sweet fruits were omitted, and vegetable consumption was restricted to low-calorie, low-starch vegetables (such as cabbage, cucumbers, and peppers).
Now for the dietary intake results. Interestingly, when compared to pre-intervention energy intake, only the low-GI group significantly reduced daily energy intake during the ten-week trial. Compared to pre-intervention, both low-GI and LCHF groups increased protein intake, whereas the high-GI group decreased relative protein intake. Relative fat intake increased in LCHF, decreased in low-GI, and stayed the same in high-GI.
Fat oxidation did increase in LCHF, but at the expense of decreased carbohydrate oxidation compared to the low- and high-GI groups. However, the exercise intensity at which maximal fat oxidation rates occur increased in all groups, independent of diet. Blood lactate after exhaustion (TTE) was not different between LCHF and low-GI, suggesting that even though fat oxidation did not significantly increase as a result of the low-GI diet, substrate oxidation did appear to change in this group, as evidenced by the decreased blood lactate suggesting a shift toward higher fat oxidation.
TTE was only improved in the low-GI group, and peak running speed was improved in the low-GI and high-GI groups but not in LCHF. All groups, independent of diet, improved in the 5km TT, which is attributable to training. However, the low-GI group saw the largest improvement, followed by high-GI and then LCHF.
In muscle, glycogen content was lower in LCHF but was maintained in low-GI. Intramuscular fats, another important fuel source, increased in all groups, independent of diet.
While this is only one study, the takeaway, in my opinion, is important and deserves consideration. The improvements that occurred after ten weeks of endurance training, independent of diet, emphasizes the points that endurance exercise is a key driver of change and that higher-carbohydrate diets do not necessarily impair endurance adaptations. Higher-carbohydrate diets based on complex carbohydrate sources high in micronutrients and fiber, combined with training, can still improve endurance markers associated with fat oxidation without compromising carbohydrate metabolism. Circulating insulin levels can also be decreased when eating this type of diet without restricting carbohydrate in favor of eating a high-fat diet. We care about performance, but we also should care about metabolic flexibility and health. For long-term success in a dynamic endurance sport such as cycling, the most sensible way of eating seems to be a diet that includes plenty of plant-based foods that support energy and macronutrient needs, while also offering numerous other health benefits. If you combine this with adequate protein and dietary fat, along with a good endurance training program, you might find you can attain the best of both worlds in health and performance.
At BaseCamp, we believe that every cyclist has the potential to achieve greatness, no matter where they start. Our mission is to create a community-driven training environment where cyclists and triathletes of all levels can train together, support each other, and grow stronger, faster, and more confident in their abilities. Our cycling training programs are expert driven and tailored to your needs. Whether you're a seasoned pro or just getting started, BaseCamp is where you belong.
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