Food dialogue: The case for carbs

Professor Dr Fred Brouns MOP, Lic PE, PhD, FACSM (USA), FECSS (EU), F-ICC acad (Aut), Regis-NAV (NL)

Diet history

Historically humans have been eating diets that were rich in carbohydrates, protein and fibers, along with food source associated lipids. Studies about our early ancestors, going back to 3 million years, show evidence of diets comprised primarily of plant based foods, grasses, sedges and seeds, along with assumed consumption of meat and fish. Depending on regions of living, climate and food availability, changes in diet composition are logical. Extremes have also been observed as for example in the Inuit population consuming foods that are fished, hunted, and gathered locally supplying high levels of lipid (50+) and protein (30+%) and low levels of carbohydrate (<25-30%). Changes in dietary composition have also been created on voluntary terms for reasons of performance being related to heavy labor or athletic performance. As such fat, protein and carbohydrate have all been suggested as “the best energy source” at various stages in time.

Diets for physical performance support

As such I have spoken to top class mountain climbers who felt best on a high protein-lipid diet. One of these climbers stated that he felt that dried meat was the best energy source. Another climber however stated that honey and maple syrup were the best for energy when becoming really tired. We met with professional cyclists in the Tour de France who swore that a lack of sugars consumption induces leg muscle weakness and risks to leave the competition, especially during mountain stages where exercise intensities exceed 80% VO2max. This led them to believe that glucose infusions during the night would help them improve their energy status and recovery. Such statements are based on personal opinions and experience and the question to ourselves has always been whether this is true? Is this supported by any measured data? Is this logical given what we know about the physiology and metabolism of the human body?

In the 1950’s studies from Scandinavian researchers showed a clear relation between the content of muscle and liver glycogen and human endurance performance capacity when being higher than 60% in non-adapted and higher than 70% VO2Max in adapted athletes. These studies were worldwide followed by many other experiments showing similar results.

As a result work was initiated to help unravel the interactions between carbohydrate, lipid and protein metabolism as well as the effects of macronutrients on rates of gastric emptying, digestion and absorption. In addition, experiments were done in which carbohydrate was supplied in high levels and experiments in which carbohydrate was excluded leading to depletion of muscle and liver glycogen.

Evidence The evidence obtained in two directions and described in many reviews was twofold and additive: supplying carbs delays fatigue, improves performance, reduces protein breakdown and amino-acid oxidation. Withdrawal of carbs induces fatigue, reduces power output to <60% of maximal capacity, increases amino-acid oxidation to help shortfalls in energy supply and enhance protein breakdown.

Explanations given for better performance when having available sufficient glucose were that at maximal needs, the energy (ATP) production rate from carbohydrate is higher than that from fatty acids. In addition, it was observed that fast twitch muscle fibers that contribute significantly to almost exclusively to work output at high to supra-maximal intensities lack the ability to oxidize fatty acids and use glucose as prime fuel.

Some key findings of these studies were as follows:

  1. Presence of small amounts of sugars in a beverage stimulate the rate of water uptake in favor of hydration and performance support.
  2. Carbohydrate supply immediately after glycogen depleting exercise improves muscle glycogen recovery as well as mental recovery.
  3. Carbohydrate rich diets prior to endurance competition in sports that have an exercise intensity exceeding 70% VO2 max, while reducing training volume (tapering) helps maximize muscle and liver glycogen depots in favor of appropriate glucose availability during exercise.

Always for all?

The latter has been interpreted by many as a general advice for athletes to follow a high carb diet to support training adaptation and performance. However, in my mind this is not so absolute, since exercise duration, intensity and the number of daily/weekly training sessions plays a determining role. Thus, although this makes sense for professional cyclists who go to the limit on a day to day basis such as Tour de France cyclists, or for speed skaters who perform two training sessions per day, this makes no sense for leisure time sportsmen and women.

Although it has been suggested that the developments outlined above have also led to increased carbohydrates and sugar consumption recommendations for the entire population, there are no data to support this.

Along with these developments hypotheses were made that increasing the fat content of food prior to competition, supplying fat (MCT oils) during exercise and training on an empty stomach to boost fat oxidation might prove to be useful to reduce glucose oxidation during exercise in favor of sparing glycogen and improving for the final stage. Although there was evidence of elevated lipid oxidation, these studies did not show any evidence of improved performance.

High fat?

Recently high fat, ketogenic diets have been suggested to be superior for human performance. Although it has been shown that such diets do boost lipid oxidation rates to higher levels than has been observed in athletes when being on a normal mixed diet with plenty of carbs there has thus far not been any study showing performance benefits.

Suggestions that the best performance in high intensity sports is achieved by athletes who are the best lipid burners is not supported by the available literature. In contrast, it is very clear that the upper elite ultra-endurance athletes who all show high lipid burning capacity, in fact will fail when competing with elite sprint, middle distance and 5000-10000 meters runners.

Yet, high fat diets may be useful for ultra-endurance athletes, high altitude mountain climbers, long distance swimmers and long distance orienteers, who generally perform at intensities of <70% VO2Max and are not dependent on glucose for high energy outputs.

From athlete to public health

Suggestions are being made that a high portion of carbohydrate in the diet leads to diabetes and cardiovascular disease. The weight of the evidence however clearly shows a correlation between sugars consumption and these disease states but not between total carbs and chronic disease. As recently stated by WHO and this summer (2015) by the Scientific Advisory Committee on Nutrition (SACN) in UK: “Total carbohydrate intakes at the current recommended levels (50en%1 ) show no association with the incidence of CVD, type 2 diabetes, glycaemia or colorectal cancer“.

Important is the notification that diabetes is not a sugar disease, nor a disease caused by sugars. In contrast it is a disease in which the normal metabolism of sugars is deranged. In animal studies two models are being applied to induce diabetes. The first is a high sugar diet, the second a high lipid diet. In both cases diabetes develops as a result of becoming overweight and developing an inflammatory state leading to insulin resistance. In Tour de France athletes sugar consumption regularly exceeds 500 g/day and total carbohydrates 1 kg/day. Yet, they are neither overweight/diabetic nor develop diabetes at different rates than the normal population, once stopped with cycling.

Regarding advice to athletes I would like to quote Asker Jeukendrup: “There are people who want to promote these high fat diets for athletes and demonize high carbohydrate diets, but where is the evidence? So far, even though there have been numerous studies, no studies have demonstrated any performance benefits of high fat diets in athletes. On the other hand, many studies have demonstrated positive performance effects of higher carbohydrate intakes for athletes at exercise intensities comparable to race intensities. So what would be the logical advice to athletes, based on the available evidence? I have never argued for carbohydrate in all conditions. The need for carbohydrate is dependent on a number of factors that need to be considered in a context. Statements about diets often lack this context and it is the context that gives meaning to the statement! Who is the athlete? Is it Haile Gebreselassie or is it my overweight neighbor who hopes to complete a half marathon in 3 hours once in her life time? What is their primary goal? Winning? Completing? Surviving? Weight loss? Health? Therefore, I would always ask for context as well as evidence!”

What does this mean for the general population?

Despite the fact that high lipid diets are suggested to be beneficial for all, the general population is still being advised to consume a diet that is composed of 30-40% lipids, 45- 50% carbs and 15-20% protein.

SACN concludes as follows:

  1. Expert nutritionists recommend halving sugar in diet. In particular consumption of sugar-sweetened beverages should be minimized.
  2. SACN advises more fiber in diet by having more fruit and vegetables and wholegrain foods. There is strong evidence to indicate that diets high in fiber are associated with a lower risk of cardiovascular diseases, type 2 diabetes and bowel cancer.
  3. Evidence shows that having more whole grains is associated with a lower incidence of cardiovascular disease, hypertension, type 2 diabetes and bowel cancer. 1 50en% = 50 energy %
  4. Starchy carbohydrates should still form basis of your diet. The Dietary Reference Values for total carbohydrate should be maintained at a population average of approximately 50% of dietary energy.

Principles of the paleo diet and ancestral health fit well with this advice. Consumers should select a significant variety of fresh fruit, vegetables, protein rich plant foods, whole grain, fish and lean meat. This will have a relatively low meal Glycemic index, a low glycemic load, will supply sufficient protein, unsaturated lipids and complex carbohydrates along with abundant dietary fiber.

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