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EmergingScience
Impactofnutrientintaketimingonthemetabolic
responsetoexercise
Brooke R Stephens and Barry Braun
Effects of nutrient intake timing and exercise on carbohydrate (e.g. insulin
sensitivity), protein (muscle protein synthesis), and fat (circulating triacylglycerols) Downloaded from https://academic.oup.com/nutritionreviews/article/66/8/473/1833172 by guest on 09 January 2023
metabolism are reviewed in this paper. Altered nutrient intake timing relative to
exercise can modulate the metabolic response, which is relevant for individuals
seeking to use exercise to enhance health.
©2008International Life Sciences Institute
INTRODUCTION nutritional environment proximate to an exercise bout
affects the duration and (or) magnitude of the metabolic
Tweakingthetimingofnutrientintakerelativetoexercise response (Figure 1). For instance, post-exercise carbohy-
to optimize recovery and enhance subsequent perfor- drate availability has potent effects on glycogen repletion
mance has been an obsession of sport nutritionists for 7
and mediates insulin action in rodents. Improvements
1 2
decades. In a seminal paper by Ivy et al., highly trained inpost-exerciseinsulinactionpersistconsiderablylonger
subjects who consumed a carbohydrate supplement whencarbohydrateis restricted and glycogen resynthesis
immediatelyafter exercise were able to synthesize muscle is delayed compared to conditions in which carbohydrate
glycogen significantly faster than when the same amount is readily available and glycogen is rapidly resynthesized.7
of carbohydrate was consumed 2 h post-exercise. Thus,
immediate feeding of carbohydrate after exercise is
widely perceived as an effective strategy to maximize METABOLICEFFECTSOFTHETIMINGOFFOODINTAKE
recovery and optimize performance in athletes.1,2 INRELATIONTOABOUTOFEXERCISE
In contrast, less attention has been given to under-
standing how the timing of nutrient intake relative to an Carbohydrate/energyintakeandinsulinaction
exercise bout modulates the health-related benefits of
exercise in the general population.The effects of exercise Because the timing of carbohydrate intake post-exercise
for improving metabolic health and reducing the risk of affects the time-course of early glycogen repletion in
2
cardiometabolic disease have been clearly established humans, we recently performed a study to examine
andincludeenhancinginsulin-stimulatedglucosemetabo- whether delaying re-feeding of energy and carbohydrate
3 lowering circulating triacylglycerols,4 and stimulat-
lism, several hours after exercise would maximize the impact
5 8
ing muscle protein synthesis. Many of these metabolic of a single exercise bout on insulin action. Insulin action
effects are transient and quickly “lost” without regular was assessed using stable isotope tracers during a con-
6
application of the exercise stimulus. Thus, each exercise tinuous glucose infusion 12 h after exercise (63 min of
bout is analogous to a pharmaceutical dose; taken in suf- cycle ergometry at 65% VO2peak,followed by ten,30-sec
ficientquantity,exerciseinducesdesiredmetaboliceffects sprints). A standardized meal containing 3808 kjoules
for a limited period of time and subsequent doses are and 144 grams of carbohydrate was given immediately
requiredinordertomaintaintheeffect.Likeanydrug,the before (Pre), immediately after (ImmPost), or 3 h after
Affiliations: BR Stephens and B Braun are with the Energy Metabolism Laboratory, Department of Kinesiology, School of Public Health and
Health Sciences, University of Massachusetts, Amherst, Massachusetts, USA.
Correspondence: B Braun, Department of Kinesiology, 110Totman Building, University of Massachusetts, Amherst, MA 01003, USA. E-mail:
bbraun@kin.umass.edu, Phone: +1-413-545-0331, Fax: +1-413-545-2906.
Keywords:carbohydrate,exercise, fat, nutrient intake timing, protein
doi:10.1111/j.1753-4887.2008.00079.x
Nutrition Reviews® Vol. 66(8):473–476 473
Protein/aminoacidintakeandmuscle
protein dynamics
Carbohydrateandmuscleglycogenhasgarneredmuchof
theattentioninthemainstreampress,buthowthetiming
of protein ingestion impacts the anabolic response to
exercisehasnotoftenbeenconsidered.Proteinintakeand
exercise has a synergistic effect on increasing the rate of
muscle protein synthesis, leading to a more positive
protein balance.10 In several studies,upregulation of ana-
bolic pathwaysinmusclevariedaccordingtowhenwhole
proteins or amino acids were consumed in relation to an Downloaded from https://academic.oup.com/nutritionreviews/article/66/8/473/1833172 by guest on 09 January 2023
11–13
exercise bout. Inonestudy,theanabolicresponsewas
Figure 1 Interactions between the exercise “drug” and enhanced when free amino acids and carbohydrate were
nutritional factors to modulate the magnitude of the provided before resistance exercise compared to a condi-
effect on metabolic health. The total energy in the meal tion in which the same nutrients were consumed imme-
(relative to the energy expended during exercise), the 11
macronutrient composition of the meal (carbohydrate, diately,1 h,or 3 h after exercise. Toevaluatewhetherthe
protein, fat) and the timing of the meal relative to the exer- sameresults held true for whole proteins,the same group
cise bout (pre-exercise, immediately after, or several hours evaluated muscle protein dynamics after provision of 20
post-exercise) all play complex and potentially important grams of whey protein consumed either immediately
roles in determining the effectiveness of exercise on regula- before (PRE) or 1 h after (POST) 80 leg-extension exer-
tion of metabolism. cises at 80% of maximal force. Subjects were healthy,
young males and females unaccustomed to resistance
exercise. Net phenylalanine balance across the leg was
th determinedusingthedirectlimbbalance(arterio-venous
(Delayed)exercise.A4 conditionservedasano-exercise
control. Relative to the control, insulin action increased differencecoupledwithmeasureoflimbbloodflow)tech-
by 22% in Pre, 44% in ImmPost, and 19% in Delayed nique at rest and for 5 h following exercise. Uptake of
administration of the carbohydrate meal. phenylalanine, indicative of the anabolic response to the
Based on results from the animal literature, we had ingestion of whey proteins,was not different between the
hypothesized that immediate re-feeding of carbohydrate two conditions (although the authors noted that mean
after exercise would cause rapid resynthesis of muscle phenylalanine uptake was considerably higher, though
glycogen and thus minimize improvements to insulin not statistically significant, at the 5-h timepoint in PRE
action.7 In stark contrast, however, the largest improve- comparedtoPOSTsubjects).14Overall,thestudysuggests
ment to insulin action occurred in the condition when that the timing of ingestion of whole proteins in relation
energy/carbohydrate was consumed immediately post- to an exercise bout is not as important for protein resyn-
exercise. These results suggest that the availability of thesis and repair as is the timing of free amino acids.14
carbohydrate immediately post-exercise does not blunt In contrast with these findings, data from several
post-exercise insulin action in humans and may actually other studies on muscle protein metabolism suggest
enhance it. In accordance with our observations, a timing does modulate the muscle anabolic response. A
9 studyconductedinelderlysubjectssuggeststhatconsum-
recent paper by Venables and Jeukendrup suggested
that the process of glycogen resynthesis plays a role in ing protein immediately after resistance exercise is more
enhanced insulin action after exercise. Without direct effective for stimulating muscle fiber hypertrophy than
measures of glycogen depletion, repletion, and insulin when the protein intake is delayed for 2 h.12 In addition,
action in the same subjects in the same study, it is not using a crossover design, Levenhagen et al.13 evaluated
possible to draw definitive conclusions. However, the the effects of consuming a supplement containing whole
available data imply that, unlike in the rodent studies, protein(casein),carbohydrate,andfateitherimmediately
the relationship between muscle glycogen and insulin after or 3 h following 60 minof cyclingat60%VO maxin
2
action in humans is not straightforward. Although the young,healthy men and women.Consuming the supple-
relationship is still murky, it seems clear that the timing ment immediately after exercise resulted in greater leg
of food intake in relation to a bout of exercise modu- protein synthesis and greater net whole-body protein
13
lates the effectiveness of exercise to increase insulin deposition compared to the delayed condition.
action. These findings may hold particular importance In summary, the available evidence suggests that
for individuals with insulin resistance or diabetes who nutrient timing in relation to exercise alters protein
most need those benefits. dynamics. Consuming free amino acids immediately
474 Nutrition Reviews® Vol. 66(8):473–476
prior to exercise is more effective for increasing muscle CONCLUSIONS
protein accretion compared to consuming a supplement
after exercise. In contrast, the timing of whey protein The timing of nutrient intake relative to exercise is rel-
ingestion does not appear to have much influence on the evant for maximizing performance in athletes,but it may
muscleanabolicresponsetoresistanceexercise.However, also be an important component of the exercise “drug”
immediate consumption of a nutrient supplement con- for men and women seeking to enhance cardiometabolic
taining an intact protein source (casein) in addition to health. There is emerging evidence that consuming a
carbohydrate and fat after an endurance exercise bout mixed meal immediately after exercise may prolong the
may be more effective for stimulating protein synthesis post-exercise increase in insulin action, but these results
and deposition. These findings have implications for the need to be confirmed in the insulin-resistant and/or
prevention of age-related muscle loss and preservation of diabetic individuals who could gain the largest poten-
lean muscle mass. tial benefit. For individuals with muscle atrophy or Downloaded from https://academic.oup.com/nutritionreviews/article/66/8/473/1833172 by guest on 09 January 2023
age-related muscle loss, consuming an amino acid
supplement immediately prior to resistance exercise may
Fat intake and postprandial lipemia optimize muscle protein accretion. With respect to
endurance exercise, consuming intact proteins, carbohy-
The timing of a meal high in fat proximate to a bout drate,and fat immediately after exercise may be advisable
of exercise may alter postprandial lipemia. Exercise to enhance muscle protein resynthesis. To counter the
increases plasma and skeletal muscle lipoprotein lipase effects of a high-fat meal to promote atherosclerosis or
(LPL) activity,the rate-limiting enzyme in triacylglycerol cardiovascular disease, exercise either before or after the
storage.15 Peak LPL activity occurs several hours post- meal may reduce post-meal lipemia, regardless of nutri-
15 ent timing.
exercise. Thetimecourseof theincrease in LPL activity
could impact the postprandial triacylglycerol response to
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476 Nutrition Reviews® Vol. 66(8):473–476
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