Everybody knows that when you go on a diet, consuming a bit more protein
will help you hold on to hard earned muscle. The reason for this is that,
skeletal muscle is your body's main "store" of protein and when food is
scarce the body uses this protein for glucose (sugar) production. You can
blame your brain for this. You see, your brain and central nervous system
rely almost entirely on glucose for energy, fat is out of the question. In
order not to slip into a hypoglycemic stupor, or even worse, a coma, the
body has set up a system that goes around breaking down muscle tissue in
order to feed its glucose manufacturing centers in the liver. A necessary
evil I guess.

So does the type of protein you eat make any difference in how much muscle
you save during a diet? Apparently it does. In a recent study in the
Annals of Nutrition and Metabolism, they compared the effects of a
moderate diet, high-protein diet and resistance training, using two
different protein supplements, or the diet alone on body compositional
changes in overweight police officers (1). It was a randomized 12-week
study. One group was placed on a hypocaloric diet alone (80% of
maintenance). A second group was placed on the hypocaloric diet plus
resistance exercise plus a high-protein intake (1.5 g/kg/day) using a
casein protein hydrolysate. In the third group treatment was identical to
the second, except for the use of a whey protein hydrolysate. They found
no difference in total weight loss between groups (about 5.5 lbs. for all
groups). Mean percent body fat with diet alone decreased from 27 to about
25% at 12 weeks. With diet, exercise and casein the decrease was from 26
to about 18%, and with diet, exercise and whey protein the decrease was
from 27 to about 23%. So the mean fat loss was 2.5 (no protein
supplement), 7.0 (casein supplement) and 4.2 kg (whey supplement) in the
three groups. Lean mass gains in the three groups did not change for diet
alone, versus gains of about 4 kg in the casein group and 2 kg in the whey
group. Mean increase in strength for chest, shoulder and legs was 59% for
casein versus 29% for whey, a statistically significant difference.

I will admit I was really surprised to see such dramatic differences
between the casein and whey groups. It should be noted that the dietary
habits of these police officers were pretty bad before this study even
began. Many weren't eating enough protein, and most were bingeing on carbs
late in the day having not had the time to eat earlier in the day. Just by
improving their baseline diet probably had an impact on their muscle mass
gains. Still, this brings up the "Fast vs. Slow Protein" study that has
gotten so much attention recently (2). You can read a bit more about this
study at Mesomorphosis
(http://www.mesomorphosis.com/exclusive/mcdonald/protein01.htm).

Obviously, whey and casein, although both milk proteins, behave
differently and have different physiological effects. There is a lot to
discuss about these recent findings. Further research is needed to explain
just what peptides in casein are responsible for the anticatabolic effect,
or, as Boirie et al demonstrated, whether it is simply a matter of
absorption rates. Not only that, but what effect do other nutrients like
carbs and fat have on these two protein supplements during a diet? There's
more to come I'm sure...

References:
1. Robert H. Demling, Leslie DeSanti. Effect of a Hypocaloric Diet,
Increased Protein Intake and Resistance Training on Lean Mass Gains and
Fat Mass Loss in Overweight Police Officers. Annals of Nutrition &
Metabolism 44:1:2000, 21-29.