Insulin, sympathetic nervous system, and nutrient timing.

Insulin secretion is attenuated by sympathetic nervous system activity; eg, via exercise.  Theoretically, exercising after a meal should blunt insulin secretion and I don’t think this will lessen the benefits of exercise, but rather enhance nutrient partitioning.   And this isn’t about the [mythical?] post-workout “anabolic window.”

Sympathetic innervation of pancreas: norepinephrine –> adrenergic receptor activation = decreased insulin secretion & increased lipolysis (Stich et al., 1999):

Stich insulin

Stich CAS

note how quickly catecholamines are cleared upon exercise cessation

Stich NEFA

To further demonstrate the insulin-suppressive effects of exercise, Heesch and colleagues gave 60 grams of maltodextrin during a 2-hour exercise session; either during the first half, second half, or all along (evenly spaced out) (2013): 

Heesch metabolitesThe 4 conditions:
PP: 250 mL flavored water every 15 minutes for 2 hours (placebo group).
CP: 15 grams maltodextrin every 15 minutes for 1st hour, then water for 2nd hour.
PC: water for first hour, 15 grams maltodextrin every 15 minutes during 2nd hour.
CC: 7.5 g maltodextrin every 15 minutes for 2 hours.

Results and interpretation: if you go into this believing in that the exercise-induced sympathetic nervous system activation suppresses insulin secretion, then the results were relatively predictable.

PP: blood glucose is low and goes lower.  Insulin steadily declines because: 1) glucose is low; and 2) sympathetic nervous system suppresses insulin secretion.

CP: blood glucose increases during first half while the participants are getting maltodextrin, then plummets during second half.  Insulin starts to climb a little in the beginning (due to maltodextrin), then plummets rapidly as soon as the maltodextrin is done.

PC: glucose starts low when during first half, then climbs during maltodextrin treatment.  Insulin levels decline during first half, but don’t increase during the second half when the participants are getting maltodextrin because sympathetic nervous system is fully activated, suppressing insulin secretion.

CC: glucose only increases in the first half, then starts to decline in the second half; the dose of maltodextrin isn’t high enough to overpower exercise-induced insulin suppression and insulin levels just decline throughout the entire trial.

Exercise blunts insulin secretion; meal timing matters (Tipton et al., 2001).

6 grams essential amino acids and 35 grams sucrose either immediately before (PRE) or after (POST) a resistance exercise session consisting of: 10 sets of 8 reps leg press at 80% 1RM, then 8 sets of 8 reps leg extensions at 80% of 1 RM (2 minutes rest between sets, 45 minutes total).

Tipton

In PRE, insulin increased 3x; in POST, insulin increased almost 6x.  Exercising after the meal nearly cut the insulin spike in half.

In other words, if exercise is performed after the meal, all bets are off; sympathetic nervous system activity is drastically lower… (Costa et al., 2009).  Also, see how quickly catecholamines declined when participants stop exercising in the Stich study (above).

Costa 2009: 86 grams maltodextrin and 29 grams soy protein ingested either immediately after or 1 hour after exercise.  Full insulin response in both cases.Costa

Dietary protein sans exercise can be quite insulinogenic in some contexts (Morifuji et al., 2010)

Morifuji insulin

Morifuji glucose

…which is why you might consider exercising afterword.  But aren’t carbs (and insulin) necessary to fully reap the benefits of exercise?  Magic 8-Ball says: “My sources say no.”  I tend to agree.  So do Staples and colleagues (2011):  25 g protein with or without 50 grams carbs, immediately post-workout.  Calories weren’t controlled (on purpose?).  Modest insulin spike with protein alone, and it doesn’t appear as though the carb-induced insulin is blunted very much (in relative agreement with Tipton and Costa):

Staples insulin

BUT, interestingly, more insulin & calories didn’t translate to more muscle protein synthesis or less degradation in this study:Staples muscle

This is not an isolated finding:

Coingestion of carbohydrate with protein does not further augment postexercise muscle protein synthesis (Koopman et al., 2007)

Addition of carbohydrates or alanine to an essential amino acid mixture does not enhance human skeletal muscle protein anabolism (Glynn et al., 2013)

Even meals primarily comprised of protein-rich foods stimulate insulin secretion.  To attenuate this and facilitate nutrient partitioning, I propose exercising afterword.

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calories proper

calories proper


Further reading: Nutrient timing, Op. 101

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