Faster Muscle Hypertrophy, Lower Visceral & Liver Fat, Trig & Glucose Levels W/ Fish vs. Casein | Plus: Shift From Slow to Fast Twitch Muscle Interesting for Strength Athletes

If casein protein is good for slow and fish protein is good for fast twitch muscle, "fish and cheese" would be the perfect muscle food, no? Read the whole article and find out if that's the case.
As a SuppVersity reader you've read about the "wonders" of fish protein before. If I am not completely mistaken, though, the study at hand which is about to be published in the peer-reviewed journal Bioscience, Biotechnology, and Biochemistry very soon, is the "fish protein study" with the most impressive results.

So impressive, in fact, that I am willing to write about it, although the study was conducted with "hairy" athletes aka rodents. So, let's not make it too exciting and start with the main results, right away: Fuminori Kawabata and his Japanese colleagues found that fish protein intake increases fast-twitch muscle weight, reduces liver triglycerides and serum glucose levels in rats, compared with a casein diet.
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As you can see in Figure 1, the improvements in lipid management were significant specifically for the triglycerides in the blood and liver of the rodents who were fed iso-caloric diets containing either casein or fish protein for 8 weeks.
Figure 1: Changes in serum and liver lipids; * indicates statistical significance (Kawabata. 2014)
In view of the close relationship between triglyceride and glucose management (remember the TRIG has a carbohydrate backbone), it's not really surprising that there was a small, but statistically significant improved (=lower) glucose response to the oral glucose tolerance test, as well (see Figure 2).

Figure 2: Improved glucose clearance during oral glucose tolerance test at the end of the 8 week study period in the fish vs. casein protein groups (Kawabata. 2014)
If we look at the underlying mechanisms, we'd have to name the following findings as potential mechanistic causes of these effects:
  • an increase in muscle mass in the fish vs. casein fed rodents (9.5% soleus muscle, 9.7% gastrocnemius, 10% extensor digitorum longus muscle)
  • reductions in white fat (-3% total; -11% visceral) and non significant increases in brown fat (+1%)
  • increases in muscle GLUT-4 expression (+39%)
With the latter, i.e. the upregulation of genes involved in the fast-twitch muscle and glucose uptake, probably being the most significant change in terms of blood glucose management.
Will you see the same results with eating fish? I doubt they will be identical to the consumption of faster-absorbing fish protein powders (which taste like crap, by the way), but the study at hand should certainly be an incentive to up your fish intake... you won't regret it, although it's not going to make you big and ripped instantaneously. Even if you're planning to have a baby, scientists from the University of Washington have calculated that the benefits in terms of reduced myocardial infarction risk outweigh any potential issues with increased mercury intake from fish (Ponce. 2000) | Learn more about the healthiest fish and "Make the Right Fish Choices"!
In addition, Kawabata et al. found that the consumption of fish vs. casein protein led to a small but significant relative increase of type II (fast twitch; resistance training; see Figure 3 at the bottom) vs. type I (slow twitch; endurance training) fibers:
"Since there were no reported protein sources that affect musclefiber-type changes, and overall, very few foods change muscle fiber type from slow to fast, the possibility of switch to fast-twitch muscle caused by fish protein observed in the present study is thought to be a significant phenomenon in thefield of skeletal muscle physiology." (Kawabata. 2014)
Now, the obvious question is: "We see changes, but what's so different about fish vs. sodium caseinate - is it the amino acid composition?

Table 1: Amino acid component and nutritional analysis of protein sources (Kawabata. 2014).
Table 1 shows the amino acid composition of the fish and casein protein that was added the rodent diets. Among the things that could maybe explain the differences are taurine (no taurine in casein, ~0.5% in fish protein), glycine (+2.4%), cystine (+0.7%), and arginine (+2.58%).

All of the aforementioned amino acids have previously been linked to improvement in blood glucose and / or blood lipid management, but it's hard to believe that the small inter-protein differences could explain the significant differences.

Furthermore, casein protein contains a higher amount of BCAAs which would stand in contrast to its inferior muscle building effects in the non-exercised Sprague Dawley rats in the study at hand.
Figure 3: The effect offish protein on myosin heavy chain gene expressions in the soleus and extensor digitorum longus muscles
Bottom line: I am afraid, I cannot tell you why fish protein favors type II muscle growth. What I can tell you, though, is that this is not the first study to observe that fish vs. casein protein leads to increased muscle gains in type II muscle fibers. The fact that the inter-group differences in this study compared to a previous 4 weeks study by Mizushige et al. were significantly more pronounced do also suggest that the beneficial effects of fish protein on skeletal muscle weight are enhanced by extending the fish protein feeding period (Mizushige. 2010).

A possible explanation for the general increase in fiber size would be a reduction in atrophy-related ubiquitin ligases the scientists observed in the study at hand.

The general change from fat utilization (lipoprotein lipase, an enzyme that breaks down triglycerides, was sign. reduced in the fish oil group) to carbohydrate utilization (GLUT-4 & co were increasd), on the other hand, could explain the improvements in glucose management and reductions in triglycerides. Still, "[f]urther investigations are needed to elucidate whether fish protein intake shifts muscle fiber type from slow to fast." Comment on Facebook!
  • Kawabata, Fuminori, et al. "Fish protein intake induces fast-muscle hypertrophy and reduces liver lipids and serum glucose levels in rats." Bioscience, biotechnology, and biochemistry ahead-of-print (2014): 1-8. 
  • Mizushige, Takafumi, et al. "Fast-twitch muscle hypertrophy partly induces lipid accumulation inhibition with Alaska pollack protein intake in rats." Biomedical Research 31.6 (2010): 347-352.
  • Ponce, Rafael A., et al. "Use of Quality‐Adjusted Life Year Weights with Dose‐Response Models for Public Health Decisions: A Case Study of the Risks and Benefits of Fish Consumption." Risk Analysis 20.4 (2000): 529-542.
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