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Table 2 Dietary Recommendations for elite athletes based on current evidence

From: Exercise-induced stress behavior, gut-microbiota-brain axis and diet: a systematic review for athletes

Nutrient Common recommended intake Claimed benefits Disadvantages Recommendations
Carbohydrates 7 to 12 g/kg per day for athletes who train for more than 2 h/day [145] Restore muscle and liver glycogen stores during intense exercise; Attenuate stress hormone levels and immunosuppression; Reduce fatigue and improve performance and mood [35]. Do not improve immune function nor do prevent decreased plasma glutamine concentrations after intense training [35]; Do not promote a healthy gut microbiota [21] High carb intake from various sources, together with high protein ingestion may increase carbohydrate oxidation rates and attenuate energy depletion during competition [66]. Complex plant carbohydrates, and plant-based protein are recommended during training and resting periods to promote a healthy gut microbiota [16].
Protein 1.2 to 1.6 g/kg per day in the top elite athletes [41, 42] Amino acids are spared for protein synthesis and are not oxidized in order to meet energy needs [42]. Adequate protein intakes enhance host immunity with particularly effects on the T cell system, resulting in decreased incidences of infections. Reduce fatigue and diet-dissatisfaction [35] High-protein, low-carb diets before exercise reduces plasma glutamine concentrations post-exercise [44]. High animal protein intake can produce potentially toxic compounds in the gut [103] Given the existing evidence, it is not recommended that elite athletes consume more than 1.2-1.6 g protein/kg.
Amino Acids
Glutamine There are no determined glutamine recommendations, though acute dosage of >20–30 g seem to be without ill effects in healthy adult humans [47] An acute dose of oral glutamine 2 h before intense exercise may ameliorate stress-induced intestinal permeability, lower plasma endotoxins and be anti-inflammatory [65] Acute glutamine supplementation taken during and after exercise in sufficient amounts to prevent the post-exercise fall in plasma glutamine concentrations have no effect on salivary IgA nor lymphocyte function [48] Glutamine supplementation should depend on symptomatology (i.e. low plasma glutamine levels, leaky gut).
Branched chain amino acids (BCAA) There are no established recommendations for BCAA supplementation, though they supposedly improve exercise performance while increasing muscle protein synthesis. Leucine supplementation can greatly increase leucine and total BCAA concentrations and improve endurance performance [49] and muscle protein synthesis. BCAA may mediate effects of fatigue during exercise by modifying certain brain neurotransmitters [50] While BCAA do compete with free tryptophan to cross the blood–brain barrier, evidence that increased brain 5-HT is driven by an increase in free tryptophan pools in blood is very weak. BCAA supplementation may be effective at reducing fatigue by increasing ammonia production Due to the lack of evidence, no recommendation on the type or amount of BCAA athletes should take can be made.
Tyrosine, 4-hydroxyphenylalanine No supplementation dose has been established. Many athletes may supplement with tyrosine as a way to balance tryptophan: tyrosine ratio as a way to reduce fatigue. The acute consumption of tyrosine increases the ratio of tyrosine to other large neutral amino acids. Tyrosine supplements (150 mg/kg) might reduce adverse effects of acute stress [63]. Tyrosine ingestion does not influence time to exhaustion or several aspects of cognitive function while exercising in heat conditions [99]. Given the inconclusive results, it is not possible to define specific amino acid recommendations that may reduce central fatigue.
Fat and polyunsaturated fats Fat consumption among athletes tends to be quite (15-30% of dietary energy) [56]. Increased fat metabolism during prolonged exercise may improve performance [111]. Fat intake of 30-50% of dietary energy may benefit endurance athletes [112] and improve energy [109] Lipids attenuate intestinal inflammation, bacterial translocation and intestinal injury following intestinal hypoperfusion in athletes with digestive disturbances [113]. Post-exercise lipid ingestion may improve GI function and reduce the flu- like symptoms associated with endotoxemia by improving post-exercise splanchnic flow High-fat diet microbiota can increase anxiety-like behaviour and neuro-inflammation and disrupt intestinal barrier function [114]. High-fat diets could be detrimental to immune function compared to high carbohydrate diets [85]. Omega-6 polyunsaturated fatty acids can negatively alter cell membrane fluidity and immune function during and after exercise [115] The effects high fat diets have on exercise performance are equivocal, and there lacks information regarding stressed individuals. An optimal dosage of omega-3 polyunsaturated fatty acids seems to be approximately 1–2 g/d, at a ratio of EPA to DHA of 2:1 to reduce ROS and inflammation [117]
Vitamins and Antioxidants Vitamins and other antioxidants are not normally increased in athletes, although some are recommended (vitamins C, E, β-carotene and polyphenols), to reduce free radical formation and lipid peroxidation [119] Polyphenol supplementation with blueberry and green tea extracts increased the metabolites characteristic of gut bacteria polyphenol metabolism and ketogenesis in runners during recovery from 3-d heavy exertion [120] Although no negative effects have been reported, athletes´ diets enriched with polyphenol extracts (blueberry and green tea), they do not mitigate the physiological stress of heavy exertion nor do they improve recovery speed [120]. Large doses of simple antioxidant mixtures or individual vitamins are not recommended and may be toxic. Athletes should obtain antioxidants from an increased consumption of fruits and vegetables [95].
Fiber Adequate fiber intake is 14 g total fiber per 1,000 kcal, or 25 g for adult women and 38 g for adult men, based on research demonstrating protection against various diseases [121]. Low dietary fiber consumption is associated with lower microbiota diversity, fewer anti-pathogenic bacteria and less SCFA production [146], which may lead to inflammation [122] and less sympathetic nervous system stimulation [123] Eating a high fiber diet before an intense training or competition could produce GI upset such as distension, gas and bloating [127] Athletes should increase their intake of plant foods (e.g. whole grains, legumes, vegetables, fruits, and nuts) hours prior to or after training and consume less processed foods high in added sugar, refined carbohydrates and fat [121]
Probiotics Probiotic supplementation is highly variable depending on the strain, microbial composition and metagenome. Due to the great diversity of the human microbiome, there have not been specific established dietary recommendations for probiotic supplementation for athletes. Fermented foods enriched with Lactobacillus sp. and Bifidobacteria sp [129] can result in specific changes in gut microbiota activity, improving stress-induced symptoms such as depression, mood disturbance as well as digestive issues [130]. Lactobacillus acidophilus, Lactobacillus casei and Bifidobacterium bifidum had beneficial effects on depression in major depressive patients [132]. Bifidobacterium longum R0175 (PF) can reduce anxiety and free cortisol levels [133]. Lactobacillus helveticus also reduces anxiety [133] and plasma ACTH and corticosterone concentrations in response to stress in rats and can restore hippocampal serotonin (5-HT) and NE levels [74]. Bifidobacterium strains, which is common in the gut flora of many mammals, including humans, have generated the best results [134] though more research is needed to better understand the gut-brain axis.