The performance of prolonged endurance exercise is limited by body carbohydrate stores. Therefore, exercise physiologist have proposed a number of diet-training strategies that have the potential to increase the reliance on fatty acid burning and therefore attenuating the rate of glycogen use during exercise. Because of limited ability of carbohydrate ingestion during exercise and because of limited amount of carbohydrate stores in body, quite a lot of studies have focused on 1-2 week short-term diet interventions that alter the pattern of substrate utilization during exercise, favoring fat as primary fuel. Most commonly used strategy is “fat adaptation”, where athletes consume a low carb high fat diet for up to 2 weeks, and then immediately follow this by carbohydrate restoration (i.e. carbo-loading) for 1-3 days before race. By implementing such strategy, the rate of whole body and muscle fat oxidation is increased and carbohydrate use by muscles is attenuated during sub maximal exercise.
In such studies, the athletes are typically randomized into one of following groups:
- Traditional, high carb diet and carbo-loading
- Low carb diet without carbo-loading
- Low carb diet with carbo-loading
Subjects on traditional high carb diet usually consume anywhere between 7 – 10 g of carbs per kg of body weight (500 – 700 g per day), while subject on low carb diet ingest around 2 – 2,5 g of carbs per kg (140 – 180 g per day) in preceding days before the performance test. Most commonly used carbo-loading protocol lasts 1 day, with subjects consuming 7 – 10 g of carbs per kg (500 – 700 g per day), as numerous studies have shown, that only 1 day of carbohydrate restoration is necessary to fully restore muscle glycogen in endurance athletes. Performance is then measured by cycling time trial, as it is easier for controlling variables and taking measurements than with running. It can include preload (i.e. 2 hours of 70% of VO2 max), with final maximum time trials efforts lasting around 30 – 60 minutes. Differences in study designs and in performance measurements make direct comparisons between trials impossible, but most of the studies reach the same conclusions, that can be summarized as:
- LC does in fact spare muscle glycogen and promotes higher rates of fat burning during exercise, regardless of pre-test carbo-loading.
- Even though the glycogen is spared, the performance on LC diet is at best equal to that of HC diet.
- The shortest recommended period for fat-adaptation is around five days.
- When on LC diet (and hence having lower glycogen stores), athlete’s Ratings of Perceived Exertion (RPE) are higher while exercising at the same intensity as athletes exercising on HC diet.
- Even though the exercise intensities are lower when on LC diet, the training effects are higher than while training in HC state.
To summarize, there is no final consensus regarding “train low race high” strategy in endurance training. The main objection when translating the results of these studies into Ironman distance triathlon training and racing is the applicability of used performance test. While it would certainly be tremendously hard to implement the final test in the form of ironman triathlon, the results do suggest, that performance on LC diet is not impaired (when done properly) and that glycogen stores are in fact spared. This could be of bigger benefit when the exercise duration increases to over 9 hours. Adding that to lower reliance to carb ingestion during exercise and concurrently lowering of possible gastrointestinal disorders, I believe LC is an appropriate approach to maximizing performance.
Below is presented relevant scientific review literature on the topic of fat adaptation. For greater details on specific studies follow the referenced literature in articles bellow.
Strategies of dietary carbohydrate manipulation and their effects on performance in cycling time trialsCorreia-Oliveira CR, Bertuzzi R, Dal’Molin Kiss MA, Lima-Silva AE. Sports Med. 2013 Aug;43(8):707-19
The relationship between carbohydrate (CHO) availability and exercise performance has been thoroughly discussed. CHO improvesperformance in both prolonged, low-intensity and short, high-intensity exercises. Most studies have focused on the effects of CHO supplementation on the performance of constant-load, time-to-exhaustion exercises. Nevertheless, in the last 20 years, there has been a consistent increase in research on the effects of different forms of CHO supplementation (e.g., diet manipulation, CHO supplementation before or during exercise) on performance during closed-loop exercises, such as cycling time trials (TTs). A TT is a highly reproducible exercise and reflects a more realistic scenario of competition compared with the time-to-exhaustion test. CHO manipulation has been performed in various time periods, such as days before, minutes before, during a TT or in a matched manner (e.g. before and during a TT). The purpose of this review is to address the possible effects of these different forms of CHO manipulation on the performance during acycling TT. Previous data suggest that when a high-CHO diet (~70% of CHO) is consumed before a TT (24-72 h before), the mean power output increases and reduces the TT time. When participants are supplemented with CHO (from 45 to 400 g) prior to a TT (from 2 min to 6 h before the TT), mean power output and time seem to improve due to an increase in CHO oxidation. Similarly, this performance also seems to increase when participants ingest CHO during a TT because such consumption maintains plasma glucose levels. A CHO mouth rinse also improves performance by activating several brain areas related to reward and motor control through CHO receptors in the oral cavity. However, some studies reported controversial results concerning the benefits of CHO on TT performance. Methodological issues such astime of supplementation, quantity, concentration and type of CHO ingested, as well as the TT duration and intensity, should be considered in future studies because small variations in any of these factors may have beneficial or adverse effects on TT performance.
An athlete’s carbohydrate intake can be judged by whether total daily intake and the timing of consumption in relation to exercise maintain adequate carbohydrate substrate for the muscle and central nervous system (“high carbohydrate availability”) or whether carbohydrate fuel sources are limiting for the daily exercise programme (“low carbohydrate availability”). Carbohydrate availability is increased by consuming carbohydrate in the hours or days prior to the session, intake during exercise, and refuelling during recovery between sessions. This is important for the competition setting or for high-intensity training where optimal performance is desired. Carbohydrate intake during exercise should be scaled according to the characteristics of the event. During sustained high-intensity sports lasting ~1 h, small amounts of carbohydrate, including even mouth-rinsing, enhance performance via central nervous system effects. While 30-60 g • h(-1) is an appropriate target for sports of longer duration, events >2.5 h may benefit from higher intakes of up to 90 g • h(-1). Products containing special blends of different carbohydrates may maximize absorption of carbohydrate at such high rates. In real life, athletes undertake training sessions with varying carbohydrate availability. Whether implementing additional “train-low” strategies to increase the training adaptation leads to enhanced performance in well-trained individuals is unclear.
Glycogen was first identified in muscle over a century and a half ago. Even though we have known of its existence and its role in metabolism for a long time, recognition of its ability to directly and indirectly modulate signaling and the adaptation to exercise is far more recent. Acute exercise induces a number of changes within the body (i.e. sympathetic nervous system activation and elevation of plasma free fatty acids) and muscle (increased AMP-activated protein kinase activity and fat metabolism) that may underlie the long- termadaptation to training. These changes are also affected by glycogen depletion. This review discusses the effect of exercise in a glycogen-depleted state on metabolism and signaling and how this affects the adaptation to exercise. Although ‘training low’ may increase cellular markers associated with training and enhance functions such as fat oxidation at sub- maximal exercise intensities, how this translates to performance is unclear. Further research is warranted to identify situations both in health and athletic performance where training with low glycogen levels may be beneficial. In the meantime, athletes and coaches need to weigh the pros and cons of training with low carbohydrate within a periodized training program.
The performance of prolonged (>90 min), continuous, endurance exercise is limited by endogenous carbohydrate (CHO) stores. Accordingly, for many decades, sports nutritionists and exercise physiologists have proposed a number of diet-training strategies that have the potential to increase fatty acid availability and rates of lipid oxidation and thereby attenuate the rate of glycogen utilization during exercise. Because the acute ingestion of exogenous substrates (primarily CHO) during exercise has little effect on the rates of muscle glycogenolysis, recent studies have focused on short-term (<1-2 weeks) diet-training interventions that increase endogenous substrate stores (i.e., muscle glycogen and lipids) and alter patterns of substrate utilization during exercise. One such strategy is “fat adaptation”, an intervention in which well-trained endurance athletes consume a high-fat, low-CHO diet for up to 2 weeks while undertaking their normal training and then immediately follow this by CHO restoration (consuming a high-CHO diet and tapering for 1-3 days before a majorendurance event). Compared with an isoenergetic CHO diet for the same intervention period, this “dietary periodization” protocol increases the rate of whole-body and muscle fat oxidation while attenuating the rate of muscle glycogenolysis during submaximal exercise. Of note is that these metabolic perturbations favouring the oxidation of fat persist even in the face of restored endogenous CHO stores and increased exogenous CHO availability. Here we review the current knowledge of some of the potential mechanisms by which skeletal muscle sustains high rates of fat oxidation in the face of high exogenous and endogenous CHO availability.
Several markers of endurance training adaptation are enhanced to a greater extent when individuals undertake selected training sessions with low compared with normal muscle glycogen content or with low exogenous carbohydrate availability. The potential mechanisms underlying the cellular responses arising from such nutrient-exercise interactions are discussed in the context of promoting training adaptation.
Availability of carbohydrate as a substrate for the muscle and central nervous system is critical for the performance of both intermittent high-intensity work and prolonged aerobic exercise. Therefore, strategies that promote carbohydrate availability, such as ingesting carbohydrate before, during and after exercise, are critical for the performance of many sports and a key component of current sports nutrition guidelines. Guidelines for daily carbohydrate intakes have evolved from the “one size fits all” recommendation for a high-carbohydrate diets to an individualized approach to fuel needs based on the athlete’s body size and exercise program. More recently, it has been suggested that athletes should train with low carbohydrate stores but restore fuel availability for competition (“train low, compete high”), based on observations that the intracellular signaling pathways underpinning adaptations to training are enhanced when exercise is undertaken withlow glycogen stores. The present literature is limited to studies of “twice a day” training (low glycogen for the second session) or withholding carbohydrate intake during training sessions. Despite increasing the muscle adaptive response and reducing the reliance on carbohydrate utilization during exercise, there is no clear evidence that these strategies enhance exercise performance. Further studies on dietary periodization strategies, especially those mimicking real-life athletic practices, are needed.
For decades, glycogen has been recognized as a storage form of glucose within the liver and muscles. Only recently has a greater role for glycogen as a regulator of metabolic signalling been suggested. Glycogen either directly or indirectly regulates a number of signalling proteins, including the adenosine-5′-phosphate- (AMP-) activated protein kinase (AMPK) and p38 mitogen-activated protein kinase (MAPK). AMPK and p38 MAPK play a significant role in controlling the expression and activity of the peroxisome proliferator activated receptor γ coactivators (PGCs), respectively. The PGCs can directly increase muscle mitochondrial mass and endurance exercise performance. As low muscle glycogen is generally associated with greater activation of these pathways, the concept of training with low glycogen to maximize the physiological adaptations to endurance exercise is gaining acceptance in the scientific community. In this review, we evaluate the scientific basis for this philosophy and propose some practical applications of this philosophy for the general population as well as elite endurance athletes.
A key goal of pre-exercise nutritional strategies is to maximize carbohydrate stores, thereby minimizing the ergolytic effects ofcarbohydrate depletion. Increased dietary carbohydrate intake in the days before competition increases muscle glycogen levels and enhances exercise performance in endurance events lasting 90 min or more. Ingestion of carbohydrate 3-4 h before exercise increases liver and muscle glycogen and enhances subsequent endurance exercise performance. The effects of carbohydrate ingestion on blood glucose and free fatty acid concentrations and carbohydrate oxidation during exercise persist for at least 6 h. Although an increase in plasma insulin following carbohydrate ingestion in the hour before exercise inhibits lipolysis and liver glucose output, and can lead to transient hypoglycaemia during subsequent exercise in susceptible individuals, there is no convincing evidence that this is always associated with impaired exercise performance. However, individual experience should inform individual practice. Interventions to increase fat availability before exercise have been shown to reduce carbohydrate utilization during exercise, but do not appear to have ergogenic benefits.
High-carbohydrate diets have been commonplace in the athletic population for many years but more recently high-fat diets have gained popularity. It is becoming more and more clear that in trained athletes extreme dietary and training regimes are not necessary to achieve optimal muscle glycogen stores. With adequate carbohydrate intake (7–10 g•kg–1 body mass per day) muscle glycogen stores can be returned to normal resting levels (to 350– 800 mmol•kg–1 dry weight muscle) within 24 hours. Normalized stores appear adequate for the fuel needs of events of less than 90 minutes in duration and supercompensated glycogen levels do not seem to enhance the performance of these events. Optimal glycogen stores will typically postpone fatigue and extend the duration of steady state exercise by ~20%, and improve performance over a set distance or workload by 2–3%. Extremely high-carbohydrate intakes are not necessary to achieve this and may result in increased risk factors for cardiovascular diseases. High-fat diets can result in increased fat oxidation after 5 days on a high-fat diet which is only partly explained by substrate availability. Adaptations at the muscular level that result in changes in substrate utilization in response to a diet may also occur after 5 days. However, high-fat diets will reduce muscle and liver glycogen concentrations and therefore performance may be reduced. The potential benefits of an adaptation period to a high-fat diet followed by a period of carbo-loading are not clear but the vast majority of studies reports no effect on performance. Therefore, there is currently very little or no evidence to support the use of high-fat diets and long term health effects of such diets in athletes are unknown. A diet containing 7–10 g•kg–1 body mass per day of carbohydrate, 10–15 energy percent from protein and the remainder from fat seems a good compromise that will allow endurance athletes to train hard and perform well.
It is well known that adaptation to a fat-rich carbohydrate-poor diet results in lower resting muscle glycogen content and a higher rate of fat oxidation during exercise when compared with a carbohydrate-rich diet. The net effect of such an adaptation could potentially be a sparing of muscle glycogen, and because muscle glycogen storage is coupled to endurance performance, it is possible that adaptation to a high-fat diet potentially could enhance endurance performance. Therefore, the first issue in this review is to critically evaluate the available evidence for a potential endurance performance enhancement after long-term fat-rich diet adaptation. Attainment of optimal performance is among other factors dependent also on the quality and quantity of the training performed. When exercise intensity is increased, there is an increased need for carbohydrates. On the other hand, consumption of a fat-rich diet decreases the storage of glycogen in both muscle and liver. Therefore, training intensity may be compromised in individuals while consuming a fat-rich diet. During submaximal exercise, fat for oxidation in muscle is recruited from plasma fatty acids, plasma triacylglycerol, and muscle triacylglycerol: the final question addressed in this review is which of these source(s) of fat contributes to the increased oxidation of fat during submaximal exercise after long-term fat diet adaptation.
Several procedures have been utilized to elevate plasma free fatty acid (FFA) concentration and increase fatty acid (FA) delivery to skeletal muscle during exercise. These include fasting, caffeine ingestion, L-carnitine supplementation, ingestion of medium-chain and long-chain triglyceride (LCT) solutions, and intravenous infusion of intralipid emulsions. Studies in which both untrained and well-trained subjects have ingested LCT solutions or received an infusion of intralipid (in combination with an injection of heparin) before exercise have reported significant reductions in whole-body carbohydrate oxidation and decreased muscle glycogen utilization during both moderate and intense dynamic exercise lasting 15-60 min. The effects of increased FA provision on rates of muscle glucose uptake during exercise are, however, equivocal. Despite substantial muscle glycogen sparing (15-48% compared with control), exercise capacity is not systematically improved in the face of increased FA availability.
The concept of manipulating an individuals habitual diet before an exercise bout in an attempt to modify patterns of fuel substrate utilization andenhance subsequent exercise capacity is not new. Modern studies have focused on nutritional and training strategies aimed to optimize endogenous carbohydrate (CHO) stores while simultaneously maximizing the capacity for fat oxidation during continuous, submaximal (60-70% of maximal O(2) uptake [(.)VO(2max)] exercise. Such “nutritional periodization” typically encompasses 5-6 d of a high-fat diet (60-70% E) followed by 1-2 d of high-CHO intake (70-80% E; CHO restoration). Despite the brevity of the adaptation period, ingestion of a high-fat diet by endurance-trained athletes results in substantially higher rates of fat oxidation and concomitant muscle glycogen sparing during submaximal exercise compared with an isoenergetic high-CHO diet. Higher rates of fat oxidation during exercise persist even under conditions in which CHO availability is increased, either by having athletes consume a high-CHO meal before exercise and/or ingest glucose solutions during exercise. Yet, despite marked changes in the patterns of fuel utilization that favor fat oxidation, fat-adaptation/CHO restoration strategies do not provide clear benefits to the performance of prolonged endurance exercise.
The focus of this review is on studies where dietary fat content was manipulated to investigate the potential ergogenic effect of fat loading on endurance exercise performance. Adaptation to a fat-rich diet is influenced by several factors, of which the duration of the adaptation period, the exercise intensity of the performance test and the content of fat and carbohydrate in the experimental diet are the most important. Evidence is presented that short term adaptation, < 6 days, to a fat-rich diet is detrimental to exercise performance. When adaptation to a fat-rich diet was performed over longer periods, studies where performance was tested at moderate intensity, 60 to 80% of maximal oxygen uptake, demonstrate either no difference or an attenuated performance after consumption of a fat-rich compared with a carbohydrate-rich diet. When performance was measured at high intensity after a longer period of adaptation, it was at best maintained, but in most cases attenuated, compared with consuming a carbohydrate-rich diet. Furthermore, evidence is presented that adaptation to a fat-rich diet leads to an increased capacity of the fat oxidative system and an enhancement of the fat supply and subsequently the amount of fat oxidised during exercise. However, in most cases muscle glycogen storage is compromised, and although muscle glycogen breakdown is diminished to a certain extent, this is probably part of the explanation for the lack of performance enhancement after adaptation to a fat-rich diet.
This is the first part in a series of three articles about fat metabolism during exercise. In this part the mobilization of fatty acids and their metabolismwill be discussed as well as the possible limiting steps of fat oxidation. It is known for a long time that fatty acids are an important fuel for contracting muscle. After lipolysis, fatty acids from adipose tissue have to be transported through the blood to the muscle. Fatty acids derived from circulating TG may also be used as a fuel but are believed to be less important during exercise. In the muscle the IMTG stores may also provide fatty acids for oxidation after stimulation of hormone sensitive lipase. In the muscle cell, fatty acids will be transported by carrier proteins (FABP), and after activation, fatty acyl CoA have to cross the mitochondrial membrane through the carnitine palmytoyl transferase system, after which the acyl CoA will be degraded to acetyl CoA for oxidation. The two steps that are most likely to limit fat oxidation are fatty acid mobilization from adipose tissue and transport of fatty acids into the mitochondria along with mitochondrial density and the muscles capacity to oxidize fatty acids.
Fat metabolism during exercise: a review–part II: regulation of metabolism and the effects of trainingJeukendrup AE, Saris WH, Wagenmakers AJ. Int J Sports Med. 1998 Jul;19(5):293-302.
This part discusses the complex regulation of fat metabolism. Catecholamines as a stimulator of lipolysis and insulin as a suppressor play very important roles in the regulation of fat oxidation. The interaction of carbohydrate and fat metabolism has been extensively studied in the past decennia but the understanding of this multifactorial regulation is complex and still incompletely understood. In 1963, Randle et al. proposed the glucose-fatty acid cycle as a possible mechanism, and more recently, regulation through malonyl-CoA has been put forward as a possible way to explain shifts in carbohydrate and fat metabolism at rest and during exercise. The exercise intensity affects fatoxidation mainly by increasing lipolysis and fatty acid availability during exercise of low to moderate intensity. At high exercise intensities, both a reduction in fatty acid availability (decreased RaFa) and intramuscular factors reduce fat oxidation. These intramuscular factors are largely unknown. The increased mitochondrial density after training and increased oxidative enzymes may partly explain the increased fatty acid oxidation during exercise as observed after training. However, also supply of fatty acids to the mitochondria may be important. The available evidence suggests that the additional fatty acids oxidized after training are primarily derived from intramuscular triacylglycerols and not from adipose tissue derived fatty acids or circulating triacylglycerols.
By changes in nutrition it is possible to manipulate fat oxidation. It is often theorized that increasing fat oxidation may reduce glycogen breakdown and thus enhance performance. Therefore, the effects of acute, short-term and long-term fat feeding have been subjects of investigation for many years. Ingestion of long-chain triacylglycerols (LCT) during exercise may reduce the gastric emptying rate and LCT will appear in the plasma only slowly. Medium-chain triacylglycerols (MCT) do not have these disadvantages and they are rapidly oxidized. However, the contribution of MCT to energy expenditure is only small because they can only be ingested in small amounts without causing gastrointestinal distress. So at present, fat supplementation in the hours preceding to or during exercise (either long chain or medium chain triacylglycerols) cannot be recommended. High-fat diets and fasting have been suggested to increase fatty acid availability and spare muscle glycogen resulting in improved performance. Both fasting and short term high-fat diets will decrease muscle glycogen content and reduce fatigue resistance. Chronic high-fat diets may provoke adaptive responses preventing the decremental effects on exerciseperformance. However, at present, there is little evidence to support this hypothesis. Also from a health perspective, caution should be exercised when recommending high-fat diets to athletes.
Compared with the limited capacity of the human body to store carbohydrate (CHO), endogenous fat depots are large and represent a vast source of fuel for exercise. However, fatty acid (FA) oxidation is limited, especially during intense exercise, and CHO remains the major fuel for oxidative metabolism. In the search for strategies to improve athletic performance, recent interest has focused on several nutritional procedures which may theoretically promote FA oxidation, attenuate the rate of muscle glycogen depletion and improve exercise capacity. In some individuals the ingestion of caffeine improves endurance capacity, but L-carnitine supplementation has no effect on either rates of FA oxidation, muscle glycogen utilisation or performance. Likewise, the ingestion of small amounts of medium-chain triglyceride (MCT) has no major effect on either fat metabolism or exerciseperformance. On the other hand, in endurance-trained individuals, substrate utilisation during submaximal [60% of peak oxygen uptake (VO2peak)]exercise can be altered substantially by the ingestion of a high fat (60 to 70% of energy intake), low CHO (15 to 20% of energy intake) diet for 7 to 10 days. Adaptation to such a diet, however, does not appear to alter the rate of working muscle glycogen utilisation during prolonged, moderate intensity exercise, nor consistently improve performance. At present, there is insufficient scientific evidence to recommend that athletes either ingestfat, in the form of MCTs, during exercise, or “fat-adapt” in the weeks prior to a major endurance event to improve athletic performance.