Daytime and nighttime casein supplements similarly increase muscle size and strength in response to resistance training earlier in the day: a preliminary investigation

The results of the present investigation indicate that ingestion of supplemental casein protein at night or earlier in the day similarly influence adaptations to a resistance training program. Resistance training promotes significant increases in muscle strength and mass when the training stimuli is sufficient and protein intake is adequate relative to training [14]. In the current study, participants achieved an average training volume of 39,577 kg per week which provided the mechanical stimulus necessary for a significant increase in muscle strength and size. However, NT and DT groups did not differ in rates of change for these variables or measures of body fat and VJ performance.

Previously, it was reported that protein digestion and absorption occurs during the night just as it does during daytime hours or when most people are awake [7, 8]. One of the first studies to demonstrate the positive anabolic effect of exogenous nighttime protein on whole body protein balance studied older men who were provided 40 g of casein at 0200 via a nasogastric tube versus a volume matched bolus of water [7]. Furthermore, overnight muscle protein fractional synthetic rates were on average 55% greater with the provision of protein. The potential incremental impact of exercise on nighttime whole body and muscle protein balance was evaluated in young males engaged in an evening resistance exercise session and receiving 40 g casein, 30 min before bed (2330) [8]. Whole body protein synthetic rate and net protein balance were increased with the latter adjusted from a potentially net negative to positive balance (61 ± 5 vs − 11 ± 6 μmol/kg during 7.5 h of sleep) [8]. Furthermore, mixed muscle protein synthetic rates were 22% higher in a trial that included 40 g casein achieving a borderline level of significance (p = 0.05) [8].

Acute response studies (single dose) reinforce the idea that net whole body and muscle protein balance can be influenced by protein consumption and that the entire 24 h of a day is available for strategic protein consumption. Specifically, casein protein consumption during the NT period appears to be as responsible for the observed outcomes as casein protein consumption during the DT period. This capability permits greater flexibility during waking hours to achieve a positive protein balance. In theory, this translates to incremental increases in hypertrophy and strength over time. To challenge this concept, a recent study used males engaged in a progressive, 12-week resistance training program in the evening with one group consuming a protein-based supplement (27.5 g protein + 15 g carbohydrate) every night before sleep [6]. Significantly greater muscle strength as well as quadriceps CSA and microscopically greater increases in Type II muscle fiber size were observed in the protein supplemented group. Two important aspects of this experimental design are that the protein supplemented group consumed significantly more total protein daily (1.9 vs 1.3 g/kg) and the exercise stimulus was in the evening followed by low-protein meal (10 g protein, 37 g carbohydrate, 9 g fat), limiting possible conclusions about NT protein consumption opposed to total protein consumption. The current findings do not support localization to the DT period, favoring total daily protein intake as a predictor of adaptation when supplied in recommended quantities [10, 11]. Furthermore, supplements in the current study were consumed at least 3 h removed from exercise, preventing a timing effect relative to exercise that may have been present in the previous research.

Casein is an anecdotally obvious choice for NT protein supplementation, as it is not acid-stable and tends to gel in the stomach and empties in the small intestine at a slower rate than other proteins, resulting in a prolonged period of aminoacidemia [9, 14, 15]. Conceptually, this may be beneficial since sleep would prevent consumption of the next meal (e.g., breakfast) by about 6–8 h. While casein provides a steady release of amino acids for several hours, this may not only support muscle protein synthesis, but also help limit muscle protein breakdown [14]. When comparing whey to casein, whey leads to a faster and greater early hyperaminoacidemia and one potential compensating strategy would be to consume more casein. This could support maximizing MPS after high volume workouts [16]. The present study used 35 g of casein protein which is comparable to the level of casein used in previous nighttime protein studies [68]. Furthermore, nighttime calories, either in the form of protein or carbohydrate, did not lead to increases in fat mass. Most likely this is due to alignment between energy requirements and dietary provision; however, it also supports the notion that eating a smaller caloric load before sleep will not lead to changes in body composition versus ceasing to consume food or supplements past early evening. In a recent study, the provision of 54 g of protein before bed did not influence body composition [12]. The use of casein in the present study may also have influenced the results. Casein is digested over a period of ~ 6–7 h and may not robustly increase MPS when consumed in isolation compared to proteins like soy or whey [17]. The present study, limited by sample size, observed greater, albeit nonsignificant, advantages for DT vs. NT consumption of protein for increasing LST. Moreover, the sterile nature of the study (e.g., diet control, total protein control), in effect, limited total protein consumption due to the fact ~ 50 g of protein were supplemented daily and some quantities of lower quality plant proteins were consumed during participants efforts to meet carbohydrate intake requirements. Therefore, another hypothesis is that it could be possible for DT casein consumption to create an “elevated baseline” for hyperaminoacidemia, thereby reducing the absolute amount of dietary protein necessary to maximize protein synthesis in meals consumed during the 6–7 h postprandial period following casein supplementation [14]. As protein and its constituent amino acid, leucine, are believed to maximally stimulate protein synthesis at a certain threshold (e.g., 2.5-3 g leucine), it is conceivable that DT participants individual meals consumed up to 6–7 h following casein ingestion had a cumulative effect on amino acid and leucine concentrations, which was not possible with NT casein supplementation, and more frequently or robustly stimulated MPS [18, 19].