Nutritional strategies for promoting fat utilization and delaying the onset of fatigue during prolonged exercise

Carbohydrate ingestion before and during endurance exercise delays the onset of fatigue (reduced power output). Therefore, endurance athletes are recommended to ingest diets high in carbohydrate (70% of total energy) during competition and training. However, increasing the availability of plasma free fatty acids has been shown to slow the rate of muscle and liver glycogen depletion by promoting the utilization of fat. Ingested fat, in the form of long-chain (C 16-22 ) triacylglycerols, is largely unavailable during acute exercise, but medium-chain (C 8-10 ) triacylglycerols are rapidly absorbed and oxidized. We have shown that the ingestion of medium-chain triacylglycerols in combination with carbohydrate spares muscle carbohydrate stores during 2 h of submaximal (< 70% VO 2 peak) cycling exercise, and improves 40 km time-trial performance. These data suggest that by combining carbohydrate and medium-chain triacylglycerols as a pre-exercise supplement and as a nutritional supplement during exercise, fat oxidation will be enhanced, and endogenous carbohydrate will be spared. We have also examined the chronic metabolic adaptations and effects on substrate utilization and endurance performance when athletes ingest a diet that is high in fat (> 70% by energy). Dietary fat adaptation for a period of at least 2-4 weeks has resulted in a nearly two-fold increase in resistance to fatigue during prolonged, low- to moderate-intensity cycling (< 70% VO 2 peak). Moreover, preliminary studies suggest that mean cycling 20 km time-trial performance following prolonged submaximal exercise is enhanced by 80 s after dietary fat adaptation and 3 days of carbohydrate loading. Thus the relative contribution of fuel substrate to prolonged endurance activity may be modified by training, pre-exercise feeding, habitual diet, or by artificially altering the hormonal milieu or the availability of circulating fuels. The time course and dose-response of these effects on maximizing the oxidative contribution of fat for exercise metabolism and in exercise performance have not been systematically studied during moderate- to high-intensity exercise in humans.     

饮食干预运动性月经周期紊乱的研究进展

高强度的训练和日益紧张的竞技比赛易造成女运动员运动性月经周期紊乱（EAMD）。而运动性月经周期紊乱不仅与运动训练有关,更与女运动员的饮食摄入相关。能量摄入不足可直接影响女运动员的可利用能量,并损害生殖系统的功能。利用文献法就国内外学者采用饮食干预运动性月经周期紊乱的研究结果进行整合分析,得出饮食干预能增加女子运动员的可利用能量,并改善能量负平衡,其对运动性月经周期紊乱的防治具有一定的意义。     

Energy and carbohydrate for training and recovery

Soccer players should achieve an energy intake that provides sufficient carbohydrate to fuel the training and competition programme, supplies all nutrient requirements, and allows manipulation of energy or nutrient balance to achieve changes in lean body mass, body fat or growth. Although the traditional culture of soccer has focused on carbohydrate intake for immediate match preparation, top players should adapt their carbohydrate intake on a daily basis to ensure adequate fuel for training and recovery between matches. For players with a mobile playing style, there is sound evidence that dietary programmes that restore and even super-compensate muscle glycogen levels can enhance activity patterns during matches. This will presumably also benefit intensive training, such as twice daily practices. As well as achieving a total intake of carbohydrate commensurate with fuel needs, the everyday diet should promote strategic intake of carbohydrate and protein before and after key training sessions to optimize the adaptations and enhance recovery. The achievement of the ideal physique for soccer is a long-term goal that should be undertaken over successive years, and particularly during the off-season and pre-season. An increase in lean body mass or a decrease in body fat is the product of a targeted training and eating programme. Consultation with a sports nutrition expert can assist soccer players to manipulate energy and nutrient intake to meet such goals. Players should be warned against the accidental or deliberate mismatch of energy intake and energy expenditure, such that energy availability (intake minus the cost of exercise) falls below 125 kJ (30 kcal) per kilogram of fat-free mass per day. Such low energy availability causes disturbances to hormonal, metabolic, and immune function.     

Nutrition for winter sports

Winter sports are played in cold conditions on ice or snow and often at moderate to high altitude. The most important nutritional challenges for winter sport athletes exposed to environmental extremes include increased energy expenditure, accelerated muscle and liver glycogen utilization, exacerbated fluid loss, and increased iron turnover. Winter sports, however, vary greatly regarding their nutritional requirements due to variable physiological and physique characteristics, energy and substrate demands, and environmental training and competition conditions. What most winter sport athletes have in common is a relatively lean physique and high-intensity training periods, thus they require greater energy and nutrient intakes, along with adequate food and fluid before, during, and after training. Event fuelling is most challenging for cross-country skiers competing in long events, ski jumpers aiming to reduce their body weight, and those winter sport athletes incurring repeated qualification rounds and heats. These athletes need to ensure carbohydrate availability throughout competition. Finally, winter sport athletes may benefit from dietary and sport supplements; however, attention should be paid to safety and efficacy if supplementation is considered.     

Pre-exercise carbohydrate and fat ingestion: effects on metabolism and performance

A key goal of pre-exercise nutritional strategies is to maximize carbohydrate stores, thereby minimizing the ergolytic effects of carbohydrate 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.     

Energy balance and dietary habits of America's Cup sailors

This research, which was conducted with crew members of an America's Cup team, had the following objectives: (a) to assess energy expenditure and intake during training; (b) to evaluate the sailors' diet, and (c) to identify any dietary flaws to determine the appropriate intake of nutrients, correct possible dietary mistakes, and improve their food habits. Energy expenditure was estimated on 15 sailors using direct measurements (oxygen consumption) and a 3-day activity questionnaire. Oxygen consumption was measured on sailors during both on-water America's Cup sailing training and dry-land fitness training. Composition of the diet was estimated using a 3-day food record. Average daily energy expenditure of the sailors ranged from 14.95 to 24.4 MJ, depending on body mass and boat role, with the highest values found in grinders and mastmen. Daily energy intake ranged from 15.7 to 23.3 MJ (from +6% to -18% of energy expenditure). The contributions of carbohydrate, protein, and fat to total energy intake were 43%, 18%, and 39% respectively, values that are not in accord with the recommended guidelines for athletes. Our results show the importance of assessing energy balance and food habits for America's Cup sailors performing different roles. The practical outcome of this study was that the sailors were given dietary advice and prescribed a Mediterranean diet, explained in specific nutrition lectures.     

Nutrition for the young athlete

Athletics is a popular sport among young people. To maintain health and optimize growth and athletic performance, young athletes need to consume an appropriate diet. Unfortunately, the dietary intake of many young athletes follows population trends rather than public health or sports nutrition recommendations. To optimize performance in some disciplines, young athletes may strive to achieve a lower body weight or body fat content and this may increase their risk for delayed growth and maturation, amenorrhoea, reduced bone density, and eating disorders. Although many of the sports nutrition principles identified for adults are similar to those for young athletes, there are some important differences. These include a higher metabolic cost of locomotion and preferential fat oxidation in young athletes during exercise. Young athletes, particularity children, are at a thermoregulatory disadvantage due to a higher surface area to weight ratio, a slower acclimatization, and lower sweating rate. An appropriate dietary intake rather than use of supplements (except when clinically indicated) is recommended to ensure young athletes participate fully and safely in athletics.     

Testosterone concentrations in female athletes and ballet dancers with menstrual disorders

Menstrual disorders are common among female athletes and ballet dancers. Endocrine changes, such as high testosterone (HT) levels and high luteinizing hormone (LH)/follicle-stimulating hormone (FSH) ratios, may suggest functional ovarian hyperandrogenism which may induce such dysfunction. The aim of this study was therefore to evaluate endocrine status in female athletes and ballet dancers with menstrual disorders. Their nutritional status and dietary habits were analysed in relation to the testosterone levels. In a cross-sectional approach, 31 female athletes (18.1 ± 2.6 years) and 21 ballerinas (17.1 ± 0.9) with menstrual disorders participated in the study. The levels of serum LH, FSH, progesterone (P), estradiol (E2), prolactin (PRL), thyroid-stimulating hormone, testosterone (T) and sex hormone-binding globulinwere measured to assess hormonal status. In addition, the free androgen index (FAI) was calculated. Nutritional status, total daily energy expenditure and nutritional habits were evaluated. Girls were assigned to one of the following groups: low testosterone (LT) level, normal testosterone level or HT level. There were significant differences between ballerinas and other female athletes in terms of testosterone levels, FAI, age at the beginning of training, length of training period and age at menarche. The PRL level was lowest in the LT group while the FAI index was highest in the HT group. Daily energy and carbohydrate intakes were significantly lower in the HT group. T levels in the study subjects were found to be associated with nutritional factors, energy availability, age at the beginning of training and frequency of training. This is the first report of HT levels being associated with the status of a female ballet dancer, the age of menarche and the length of the training history. Further research is necessary to confirm the results in a larger study group.     

江苏省摔跤队膳食营养状况的调查与评价

研究目的:膳食营养是保证运动员营养素的需要和维持体能的最重要的物质基础,对训练起重要的保证作用,与膳食营养在竞技体育中重要位置不相适应的是运动员膳食的不平衡现象普遍存在,它长期影响训练的效果和训练后的恢复。本研究通过调查备战十运会的江苏省摔跤队的膳食状况,对其做出评估,为平衡膳食提供依据。研究方法:采用食物称重法,称量并记录每个运动员所吃主副食的熟重、剩余重量,计算出所吃食物生重。记录运动员饮水量及营养品服用量,用软件计算出各种营养素摄入量及食物构成。将计算结果与运动员每日膳食营养供给标准,食物摄入供给标准进行比较做出合理评价。研究结果:江苏省摔跤运动员三大营养素的摄入比例多不合理,碳水化合物78%不足,56%的运动员脂肪和蛋白质超过推荐标准。中餐、晚餐的能量摄入比例超标比例较高,早餐不足。运动员各种矿物质摄入都超过推荐值,运动员的维生素摄入相对不足,VA、VC、VB1、VPP不同程度地缺乏。运动员每日的摄入量变化很大,最大幅度可达到83%。运动员进食的多少与运动量、运动强度、饭菜的内容以及个人的喜好有很大的关系。     

运动员非蛋白能量代谢平衡的研究

目的:探讨现代五项和击剑项目运动员非蛋白能量消耗和能量摄入的特点。方法 选取上海现代五项、击剑优秀运动员,共17人。第1天和第3天清晨空腹测定静息能量代谢 （resting energy expenditure,REE）,并进行身体形态检查（身高、体重、体成分）。连续3天填写饮食记录表（dietary record,DR）,采用食物称重法计算膳食能量摄入（energy intake,EI）。结果：（1）上海现代五项和击剑项目运动员每天的饮食摄入低于实际能量消耗。（2）现代五项和击剑运动员的非蛋白能量物质摄入量均低于推荐值。（3）在静息状态下,现代五项和击剑运动员的静息能量代谢无明显差异（P=0.935）,糖脂供能比均接近1.1：1。（4）现代五项和击剑运动员的日均能量摄入无明显差异（P=0.929）,但与推荐糖脂比例相比,现代五项的碳水化合物摄入不足,而击剑运动员的碳水化合物和脂肪摄入比例相对合理。结论：（1）上海现代五项和击剑项目运动员每日的非蛋白能量物质碳水化合物摄入明显不足,需要在饮食中进一步加强补充。（2）现代五项运动员的碳水化合物与脂肪摄入比例需要进一步优化。     

Carbohydrates for training and competition

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.     

运动与补糖研究进展

随着近代体育运动竞技水平的提高,教练员与运动员已开始认识到补糖与提高运动成绩之间关系的重要性。通过补糖,不仅可以提高运动成绩,而且可以推延运动性疲劳的出现,运动后补糖还可以促进运动性疲劳的恢复。近代关于补糖与运动的研究中,涉及了补充不同种类的糖(复合糖、低聚糖、1、6一二磷酸果糖等)与各种类型运动项目及糖的补充时间之间的关系,本文将对这些可行性研究进行分析讨论以供教练员和运动员参考。     

Rethinking fat as a fuel for endurance exercise

Abstract

A key element contributing to deteriorating exercise capacity during physically demanding sport appears to be reduced carbohydrate availability coupled with an inability to effectively utilize alternative lipid fuel sources. Paradoxically, cognitive and physical decline associated with glycogen depletion occurs in the presence of an over-abundance of fuel stored as body fat that the athlete is apparently unable to access effectively. Current fuelling tactics that emphasize high-carbohydrate intakes before and during exercise inhibit fat utilization. The most efficient approach to accelerate the body’s ability to oxidize fat is to lower dietary carbohydrate intake to a level that results in nutritional ketosis (i.e., circulating ketone levels >0.5 mmol/L) while increasing fat intake for a period of several weeks. The coordinated set of metabolic adaptations that ensures proper interorgan fuel supply in the face of low-carbohydrate availability is referred to as keto-adaptation. Beyond simply providing a stable source of fuel for the brain, the major circulating ketone body, beta-hydroxybutyrate, has recently been shown to act as a signalling molecule capable of altering gene expression, eliciting complementary effects of keto-adaptation that could extend human physical and mental performance beyond current expectation. In this paper, we review these new findings and propose that the shift to fatty acids and ketones as primary fuels when dietary carbohydrate is restricted could be of benefit for some athletes.     

Sport-specific nutrition: practical strategies for team sports

Implementation of a nutrition programme for team sports involves application of scientific research together with the social skills necessary to work with a sports medicine and coaching staff. Both field and court team sports are characterized by intermittent activity requiring a heavy reliance on dietary carbohydrate sources to maintain and replenish glycogen. Energy and substrate demands are high during pre-season training and matches, and moderate during training in the competitive season. Dietary planning must include enough carbohydrate on a moderate energy budget, while also meeting protein needs. Strength and power team sports require muscle-building programmes that must be accompanied by adequate nutrition, and simple anthropometric measurements can help the nutrition practitioner monitor and assess body composition periodically. Use of a body mass scale and a urine specific gravity refractometer can help identify athletes prone to dehydration. Sports beverages and caffeine are the most common supplements, while opinion on the practical effectiveness of creatine is divided. Late-maturing adolescent athletes become concerned about gaining size and muscle, and assessment of maturity status can be carried out with anthropometric procedures. An overriding consideration is that an individual approach is needed to meet each athlete's nutritional needs.     

Nutrition for throwers, jumpers, and combined events athletes

Throwers, jumpers, and combined events athletes require speed, strength, power, and a wide variety of technical skills to be successful in their events. Only a handful of studies have assessed the nutritional needs of such athletes. Because of this, recommendations for nutritional requirements to support and enhance training and competition performances for these athletes are made using research findings from sports and exercise protocols similar to their training and competitive events. The goals of the preparation cycle of nutrition periodization for these athletes include attaining desirable body weight, a high ratio of lean body mass to body height, and improving muscular power. Nutritional recommendations for training and competition periods include: (1) meeting energy needs; (2) timing consumption of adequate fluid and electrolyte intakes before, during, and after exercise to promote adequate hydration; (3) timing consumption of carbohydrate intake to provide adequate fuel for energy demands and to spare protein for muscle repair, growth, and maintenance; (4) timing consumption of adequate protein intake to meet protein synthesis and turnover needs; and (5) consuming effective nutritional and dietary supplements. Translating these nutrient and dietary recommendations into guidelines these athletes can apply during training and competition is important for enhancing performance.     

Energy availability in athletes

This review updates and complements the review of energy balance and body composition in the Proceedings of the 2003 IOC Consensus Conference on Sports Nutrition. It argues that the concept of energy availability is more useful than the concept of energy balance for managing the diets of athletes. It then summarizes recent reports of the existence, aetiologies, and clinical consequences of low energy availability in athletes. This is followed by a review of recent research on the failure of appetite to increase ad libitum energy intake in compensation for exercise energy expenditure. The review closes by summarizing the implications of this research for managing the diets of athletes.     

Reduced fat oxidation during high intensity,submaximal exercise: is the availability of carnitine important?

Abstract

The increased energy demand that occurs with incremental exercise intensity is met by increases in the oxidation of both endogenous fat and carbohydrate stores up to an intensity of ~70% V˙O_2max in trained individuals. However, when exercise intensity increases beyond this workload, fat oxidation rates decline, both from a relative and absolute perspective. As endogenous glycogen use is accelerated, glycogen stores can become depleted, ultimately resulting in fatigue and the inability to maintain high intensity, submaximal exercise (>70% V˙O_2max). Despite a considerable accumulation of knowledge that has been gained over the past half century, the precise mechanism(s) regulating muscle fuel selection and underpinning the aforementioned decline in fat oxidation remain largely unclear. A greater understanding would undoubtedly lead to novel strategies to increase fat utilization and, as such, improve exercise capacity. The present review primarily addresses one of the most prominent theories to explain the phenomenon of diminished fat oxidation during high intensity, submaximal exercise; a reduced availability of muscle free carnitine for mitochondrial fat translocation. This is discussed in the light of recent work in this area taking advantage of the discovery that muscle carnitine content can be increased in vivo in humans. Furthermore, the evidence supporting the recently proposed theory that reduced muscle co-enzyme A availability to several key enzymes in the fat oxidation pathway may also exert a degree of control over muscle fuel selection during exercise is also considered. Strong correlational evidence exists that muscle free carnitine availability is likely to be a key limiting factor to fat oxidation during high intensity, submaximal exercise. However, it is concluded that further intervention studies manipulating the muscle carnitine pool in humans are required to establish a direct causal role. In addition, it is concluded that while a depletion of muscle coenzyme A availability during exercise also offers a viable mechanism for impairing fat oxidation, at present, this remains speculative.     

Nutrition guidelines for strength sports: sprinting, weightlifting, throwing events, and bodybuilding

Strength and power athletes are primarily interested in enhancing power relative to body weight and thus almost all undertake some form of resistance training. While athletes may periodically attempt to promote skeletal muscle hypertrophy, key nutritional issues are broader than those pertinent to hypertrophy and include an appreciation of the sports supplement industry, the strategic timing of nutrient intake to maximize fuelling and recovery objectives, plus achievement of pre-competition body mass requirements. Total energy and macronutrient intakes of strength-power athletes are generally high but intakes tend to be unremarkable when expressed relative to body mass. Greater insight into optimization of dietary intake to achieve nutrition-related goals would be achieved from assessment of nutrient distribution over the day, especially intake before, during, and after exercise. This information is not readily available on strength-power athletes and research is warranted. There is a general void of scientific investigation relating specifically to this unique group of athletes. Until this is resolved, sports nutrition recommendations for strength-power athletes should be directed at the individual athlete, focusing on their specific nutrition-related goals, with an emphasis on the nutritional support of training.     

不同项目女子优秀运动员身体成份的实验研究

通过对赛艇、短跑及艺术体操女子优秀运动员身体成份的实验研究,结果表明:不同项目运动员体液指数与遗传及运动训练有关,体液指数可作为评价运动效果和运动选材的参考指标;艺术体操运动员肌肉含量及蛋白质含量相对较小,但无机质含量及体脂比率较高。