The protein component of dairy is a candidate for an important role in its effects on body weight and fat mass loss, possibly related to the greater thermic effects of protein [65,66]. This is supported by the three dairy (or whey) protein supplement studies that showed benefits for body weight and fat mass loss [47,48,49]. All three studies reported higher dietary protein intake compared to the control groups (1.07 vs. 0.61 g/kg [49]; 1.46 vs. 0.78 g/kg [48] and 0.81 vs. 0.61 g/kg [47]). Moreover, findings from Frestedt et al. indicate that even small increases in protein intake of just 0.2 g/kg using whey fraction high in leucine can have beneficial effects for groups who may not otherwise be meeting their RDA for protein [47]. A pilot study where dairy milk was compared to soy milk (protein and calcium content matched between interventions) did not have any additional weight loss effects [45]. The meta-analysis by Wycherley et al. [8] further supports a role for protein by showing modest benefits of high-protein diets from different sources in conjunction with energy-restricted diets for reducing body weight (?0.79 kg [?1.50, ?0.08 kg]) and fat mass (?0.87 kg [?1.26, ?0.48 kg]).
Further accumulating evidence suggests that dairy protein, particularly whey protein, may affect body weight through regulation of food intake and appetite [4]. Indeed, Gilbert showed that milk supplementation induced a smaller increase in desire to eat and hunger, suggesting attenuation of weight loss-related increase in appetite [20]. However, in the context of prescribed energy restriction used in the studies examined, where food and energy intake is strictly controlled therefore not allowing for ad libitum feeding, the appetite regulating mechanism may be less operative. From the current evidence it is not possible to ascertain whether the effects of dairy interventions were directly attributable to dairy protein or to increased intake of total dietary protein.
It is also possible the high calcium content of dairy may have contributed to the weight and fat mass reducing effects. Dairy food intervention groups, where the differences in dairy serves were >2 servings/day, consumed ~582?1862 mg/day more calcium compared to control groups while protein intake in most of these studies were held constant. Using a mixed-model regression analysis including 18 observational studies that examined dietary calcium (with the majority of dietary calcium derived from dairy products), Dougkas et al. [4] showed that an increased calcium intake from 400 to 1200 mg/day was associated with a reduction in body mass index (BMI) from 25.6 to 24.7 kg/m2.
The most cited mechanism for the effect of calcium on weight loss, demonstrated in cell-culture and animal studies, involves the influence that ingested calcium has on intracellular calcium and subsequently adipocyte lipid metabolism by reducing de novo lipogenesis and increasing fat oxidation. However, studies in humans have failed to support this hypothesis [4]. Alternatively, a more plausible mechanism may be that high dairy calcium increases faecal fat excretion. A meta-analysis by Chirstensen et al. [2] indicated that dairy calcium consumption of 1241 mg increased faecal fat excretion by 5.2 g/day (CI 1.6, 8.8 g/day) compared with low dairy calcium intake (<700 mg/day). The authors estimated that this level of fat excretion would translate into 1.9 kg body fat or 2.2 kg body weight loss over a year. As noted by Dougkas et al. [4], other investigators have suggested that the impact of calcium on fat absorption may be protein dependent suggesting that the impact of calcium from dairy may be greater than supplemental calcium.
Furthermore, dairy is a rich source of MCT, C8:0 (caprylic acid) and C10:0 (capric acid), which have been shown to increase energy expenditure, lipid oxidation and satiety [67]. A recent meta-analysis showed dosages ranging from 2 to 54 g/day produced small reductions in body weight (?0.51 kg [?0.80, ?0.23 kg]) compared to long-chain triacylglycerols over study periods of 1?4 months [68]. Dairy is also a source of the fatty acid CLA, which has been shown to affect body weight and fat mass. The meta-analysis by Onakpoya et al. [69] showed small reductions in body weight (?0.70 kg [?1.09, ?0.32 kg]) and fat mass (?1.33 kg [?1.79, ?0.86 kg]) with dosages ranging from 2.4 to 6 g/day over an average period of 8 months. MCT and CLA may therefore contribute to weight reducing effects, but it is unlikely that the small amounts of these fatty acids in dairy will have measurable weight reducing effects. Three servings of dairy/day would supply ~1 g C8:0 + C10:0 (RMIT fatty acid database, FoodWorks, 2009, Xyris Software, Kenmore Hills, Queensland, Australia) and ~0.1 g CLA [70] while the amounts will be negligible in low-fat/skimmed dairy products. Several other dairy constituents such as lactose, protein and their peptide derivatives may also affect body weight through the regulation of food intake and appetite although there is a paucity of evidence from well-designed intervention studies that closely controlled energy intake to confirm the mechanisms [4].
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