Discussion:
Mean BMI was significantly decreased (p<0.05) in the
hypocaloric, high-protein diet groups as compared to the control diets
groups, in contrast, no significant changes (p>0.05) were
found in mean LDL-C, HDL-C, TAG, and TC levels among diet groups. This
finding suggests that hypocaloric, high-protein diets have an unclear
effect on blood lipid levels as compared to other non-high-protein
hypocaloric diets. In agreement with our findings, previous studies have
noted that blood lipid levels were only improved within high-protein and
control diet groups individually however no significant changes were
found in lipid levels between diet groups (Azadbakht et al. ,
2013; Amini et al. , 2016). This is in contrast to the stated
hypothesis as well as other studies’ findings that suggest that
higher-protein diets improve blood lipids, including decreasing LDL-C,
TAG, TC and increasing HDL-C levels (Hu, 2005; Layman et al. ,
2003; Layman and Baum, 2004). An indication for a meta-analysis of these
studies is therefore justified to uncover possible combined significant
changes of blood-lipid levels between diets. The results within these
studies are also speculated to be confounded by dietary composition
which was limited in information, for example if a diets was richer in
saturated fatty acids it would likely raise lipid levels, an opposing
effect to diets richer in unsaturated fatty acids (Müller et al. ,
2003). Mean BMI was significantly decreased (p<0.05) in three
of four studies (Abete et al. , 2009; D.A. et al. , 2015;
Johnstone et al. , 2011) between the high-protein and control diet
groups (-2.28 kg/m2 vs -1.85 kg/m2respectively) which is in agreement with other studies involving
hypocaloric, high-protein diets (Halton and Hu, 2004; Wycherley et
al. , 2012; Leidy et al. , 2015). A plausible explanation to the
increased weight loss is that hypocaloric, high-protein diets lessen
decreases in REE as compared to other energy restricted diets (Babaet al. , 1999; Mikkelsen, Toubro and Astrup, 2000; Wycherleyet al. , 2012). This lessened decrease in REE in high-protein
diets may be explained by an increase in mitochondrial oxidation
pathways specific to dietary protein intake as theorized by Abete, Parra
and Martinez, 2009. Additionally, dietary protein intake increases
dietary thermogenesis (Halton and Hu, 2004) and contributes to lessened
REE decreases in high-protein diets (Westerterp, 2004; Tentolouriset al. , 2008). The increased weight loss in the high-protein
diets may also be due to these diets being more satiating than
non-high-protein diets as suggested in previous studies (Stubbs et
al. , 1996). This finding agrees with the decrease in caloric intake
between actual and stated caloric intake values in high-protein diet
groups compared to control diet groups (Table 3). Indicating voluntarily
limited intake of food in the high-protein diet groups as compared to
control. Qualitative questionnaires in the context of satiety are
therefore warranted to explore appetite control in these diets. On the
other hand, increased gluconeogenesis as a result of the
lower-carbohydrate composition (Westerterp-Plantenga et al. ,
2009) in high-protein as compared to control diet groups of all studies
(Table 3) may have increased weight loss through additional energy
expenditure as theorized in the study by Johnstone et al . This
study produced a significantly greater weight loss in the
low-carbohydrate high-protein (LC-HP) diet group compared to the
moderate-carbohydrate, high-protein diet group (MC-HP) (Table 3, Table
2). In disagreement to this explanation however, the high-protein
low-carbohydrate diet group in the study by Petrisko et al did
not produce a statistically significant mean weight loss as compared to
the control diet. This may be because actual dietary carbohydrate values
(19.1%) (Table 3) were higher than stated dietary carbohydrate values
(10%), therefore possibly decreasing gluconeogenesis, and hence less
energy expenditure. Another possibility for the increased weight loss
between diet groups is due to increased mean total body water (TBW)
losses in lower-carbohydrate diet groups compared to the
higher-carbohydrate diet groups as theorized in the study by Johnstone
et al. Even though in this study TBW loss was not significant between
diets (Table 4), there was a significantly greater loss in mean free-fat
mass (FFM) in the LC-HP diet group as compared to the MC-HP diet group.
The increased mean FFM loss may reflect greater mobilisation of hepatic
glycogen stores as a result of the low-carbohydrate content in the diet,
causing an associated water loss as suggested in previous studies (Yang
and Van Itallie, 1976; Bilsborough and Crowe, 2003). Therefore, weight
loss may have been centred around the carbohydrate rather than the
high-protein composition of the diets, hence reducing the clarity of
effects of hypocaloric, high-protein diets on weight loss. Reflecting on
bias in the studies, the only study that had complete control over the
prescribed diets was the study by Johnstone et al. The other
studies’ protocols involved either providing dietary recipes to follow
or food on an outpatient basis, therefore not having as much dietary
input control as in the study by Johnstone et al. Additionally
three-day self-reported food logs to measure compliance were done
towards the end of all studies. In the study by de Luis et al,participants had a 100% retention rate whilst being on a supposed
1193.8 daily caloric restriction for 9 months. The food log was
self-reported as being more or less within dietary guidelines of the
study, however the actual mean weight loss was around -8.4 to -5.0 kg
after 9 months. This weight loss is relatively low considering that
participants had a calculated average BMR of 1747 kcal/day
(Harris-Benedict Equation), which proposes a weight loss of at least
-21.3 kg if the 9 month diet plan was adhered to (-553.2 kcal
deficit/day). The lesser-than-expected weight loss may indicate
participants falsly self-reporting dietary intakes and not completely
adhering to the prescribed diets. In most studies, participants received
a fixed caloric intake based on averages in weight, height, age, and sex
of all participants, hence caloric deficits were not individualized,
leading to possible confounding bias in primary outcome measures. The
study by Abete et al was the only study to measure BMR adjusted
for physical exercise in individual participants via indirect
calorimetry and provided diets with adjusted caloric requirements to
individuals based on these BMR values. Other studies estimated average
physical exercise via activity logs, which predisposes the same biases
as with food logs. Therefore in order to accurately estimate individual
participants BMR as adjusted for physical exercise, dietary studies
should calculate BMR via indirect calorimetry as recommend by other
studies (Abete, Parra and Martinez, 2009; Lam and Ravussin, 2017).
Providing individualized diets through indirect calorimetry may have
also solved the confounding bias present in all studies with a wide age
range of participants (Table 3), the wide age range is deemed a
confounding bias as BMR decreases linearly with age (Shimokata and
Kuzuya, 1993). Furthermore, all participants had voluntarily signed up
to be in all these studies, which meant that these were obese adults who
were motivated enough to start dieting, which may prove less external
validity of studies as obese adults in the general population may not be
as motivated to diet and therefore possibily have different retention
rates to the obese participants in these studies. Finally, the internal
validity of the studies presented in this systematic review is low as a
result of non-individualized caloric intakes, high risk of biases within
studies (Figure 2), different dietary compositions and other
aforementioned confounders. Studies with larger sample sizes and
different study designs such as longitudinal studies are warranted to
increase external validity, as well as studies with a particular focus
on other cardiovascular risk factors such as hypertension and asking for
complete dietary nutrient composition in these studies would provide a
more complete outlook on the effects that hypocaloric, high-protein
diets have on cardiometabolic health.