Protein ingestion acutely inhibits insulin-stimulated muscle carnitine uptake in healthy young men

Abstract

The insulin-mediated increase in forearm carnitine balance with carbohydrate consumption was acutely blunted by a carbohydrate+protein beverage, which suggests that carbohydrate+protein could inhibit chronic muscle carnitine accumulation.

Does skeletal muscle carnitine availability influence fuel selection during exercise?

“…However, the majority of the pertinent studies in healthy human subjects to date have failed to increase skeletal muscle carnitine content via oral or intravenous l -carnitine administration ( 39 ) . For example, neither feeding l -carnitine daily for up to 3 months ( 41 ) , nor intravenously infusing l -carnitine for up to 5 h ( 42 ) , had an effect on muscle total carnitine content, or indeed net uptake of carnitine across the leg ( 43 ) or arm ( 44 ) (Table 1). Furthermore, feeding 2–5 g/d l -carnitine for 1 week to 3 months prior to a bout of exercise had no effect on perceived exertion, exercise performance, VO 2 max or markers of muscle substrate metabolism such as RER, VO 2 , blood lactate, leg NEFA turnover and post-exercise muscle glycogen content ( 39 ) .…”

“…This is likely explained by the finding that carnitine is transported into skeletal muscle against a considerable concentration gradient (>100-fold) that is tightly regulated and saturated under normal conditions, and so it is unlikely that simply increasing plasma carnitine availability per se will increase muscle carnitine transport and storage ( 39 ) . Indeed, recent human and porcine studies of arterio-venous carnitine fluxes confirmed that net muscle carnitine uptake/efflux is negligible under fasting conditions ( 44 , 45 ) , with systemic concentrations of carnitine and acylcarnitines largely governed by gut absorption, hepatic release and renal filtration ( 45 ) . However, insulin appears to stimulate skeletal muscle carnitine transport, and intravenously infusing l -carnitine in the presence of high circulating insulin (>50 mU/l) can increase muscle carnitine content by 15 % ( 36 , 42 , 46 ) .…”

Efeitos ergogênicos da L-carnitina no metabolismo lipídico

“…Esta característica resulta em uma elevada disponibilidade de hidratos de carbono, possivelmente devido a um aumento mediado pela carnitina na oxidação de ácidos graxos (STEPHENS, et al 2006). Paralelamente, um estudo verificou se a ingestão de proteína do soro do leite poderia reduzir a carga de carboidratos necessária para estimular a absorção de carnitina mediada pela insulina em homens jovens saudáveis (SHANNON, et al 2016). No entanto, foi demonstrado que o aumento na absorção de carnitina mediada pela insulina com o consumo de carboidratos foi agudamente atenuado por uma bebida contendo carboidratos e proteína (SHANNON, et al 2016).…”

Beyond the Obvious: Future Innovations in Sports Nutrition

“…As some individuals may not wish to ingest an additional 160 g of a highly insulinemic carbohydrate [ 35 ] as was used above [ 34 ], Greenhaff and colleagues chose an acute dose mixture of 40 g of carbohydrate and 40 g of whey protein, or 80 g of carbohydrate (Vitargo), consumed 1 h after ingestion of 3 g of l -carnitine (containing 0.1 % deuterated l -carnitine) [ 36 ]. The well-characterized insulinemic response that follows dietary protein ingestion, especially with whey protein [ 37 , 38 ], can allow for a reduction in carbohydrate content by fostering a hyperinsulinemic response suffi cient to activate muscle carnitine uptake.…”

“…The well-characterized insulinemic response that follows dietary protein ingestion, especially with whey protein [ 37 , 38 ], can allow for a reduction in carbohydrate content by fostering a hyperinsulinemic response suffi cient to activate muscle carnitine uptake. The use of the (stable) isotopically labeled carnitine permitted the assessment of apparent muscle carnitine uptake when coupled with arteriovenous measurements (across the forearm) of the tracer and unlabeled carnitine and blood fl ow [ 36 ]. The rate of carnitine disposal (presumably into skeletal muscle) was signifi cantly greater with the carbohydrate drink compared to a fl avor-matched, noncaloric control.…”