The results of this 12-week, randomized, double blind clinical
trial in free-living overweight and obese men indicate that
WPC exerts stronger effects on appetite, CI, anthropometry,
and body composition than SPI. Accordingly, we confirmed
the research hypothesis that supplemental preloads of WPC
and SPI at 30 minutes prior to the largest meal of overweight
and obese men would decrease appetite, CI, and anthropometry,
although WPC had stronger effects than SPI. Moreover,
although WPC modified body composition, SPI did not
increase LM. Whey protein concentrate and SPI preloads
suppressed appetite and CI significantly, but the magnitude of
the decrease by WPC was higher. This confirms previous
findings [9,26]. However, Bowen et al [8] found that the
amount of appetite and CI-lowering effects of soy were
similar to whey protein. This could be attributed to the
consumption of similar doses of soy protein in the 2 studies
(57 g SPI in that study compared with 54 g). In addition,
because in the present study, supplements were in the form
of beverages and were consumed exactly 30 minutes prior to
the largest meal, WPC exerted stronger effects on appetite
and CI. Anthropometric measurements and BFM decreased in
both treatments, with a higher magnitude in the WPC group.
Similarly, Baer et al [5] reported the same findings; however,
in that study, BW, BMI, and BFM did not differ between WPC
and SPI treatments. Low doses of supplements (~41.6 g WPC
and 46.8 g SPI compared with 54 g) and the consumption of
test proteins in divided doses, either before and after or
during breakfast and dinner, could contribute to this disagreement.
Furthermore, the mean value of BW in our study
was higher (94.55 kg in contrast with 90.86 kg) which could
result in more weight loss. In addition, the study by Baer et al
was conducted on men and women, and because the
standard of WC is different between 2 sexes [27], this could
also contribute to the conflicting results. In contrast, in the
study by Denysschen et al [17], the magnitude of WC decrease
was higher in the soy protein group compared with the whey
protein group. This may result from the isoflavone contents of
soy protein (56.2 mg isoflavone) [28] because the SPI of this
study lacked isoflavone. In addition, the study by Denysschen
et al reported that BW and BMI did not change and BFM
decrease did not differ between the groups. This could be due
to low doses of supplements (whey protein 26.2 g compared
with 54 g WPC in the Denysschen study) and consumption of
supplemental beverages immediately after exercise, because
whey protein's effects on BW and composition depend on its
preloading time to increase blood concentration of amino
acids (particularly branched chain amino acids) [8]. At study
completion, WPC increased LM significantly when compared
with SPI. This was in disagreement with the findings of Baer
et al [5] in which lean body mass did not differ between whey
and soy protein treatments. Lower doses of supplements (52 g
WPC equals to 41.6 g pure whey compared with 54 g pure
whey in this study), not offering them as liquid preloads
before large meals, and the short-term intervention could
explain this disagreement. In another study, a4-week supplementation
of 60 g of whey protein did not alter BW but
decreased BFM and increased fat-free mass [16]. This may be
due to a sufficient dose, although the duration of the
intervention was one third that of the present study.
The major strengths of the present study include the
randomized, double-blinded nature and that it took place in
the participants' work place. This provided precise monitoring
of the intervention. After determining the largest meal, we
administered complete doses of 65 g WPC and 60 g SPI (equal
to 54 g effective compound as protein) 30 minutes before
lunch for 12 weeks. Unlike previous studies that evaluated the
effects of substituting dietary proteins from mixed protein
sources [29–32], this study revealed beneficial effects of WPC
and SPI preloads on appetite, CI, and BW and composition
management. The protein concentration of treatment beverages
(13% WPC and 12% SPI) and the amount of increase in
protein intake were shown to be sufficient in lowering
appetite and CI [33]. However, it is clear that different protein
sources have different gastric-emptying rates (ie, fast or slow
proteins) [19] and, thereby, affect appetite and CI differently.
Hence, unlike previous studies that compare slow proteins
(eg, casein) with fast ones (eg, whey) [6,11,18], making it
difficult to separate gastric-emptying rate effects of proteins
from their physiologic characteristics, the present study
compared 2 proteins with approximate similar rates of
digestion (ie, fast proteins) [34]. This helped us exclude the
digestion rate of proteins as a bias. We provided treatment
proteins in intact form, which may induce greater decreases
in CI by involving incretin hormones because different protein
types have different effects on these hormones [35]. Moreover,
satiety is closely influenced by postdigestion and
absorption processes, which are related to information sent
from the gastrointestinal