We demonstrated that the inclusion of 3-4 servings/d of dairy foods in a moderate energy-restricted diet for overweight and obese individuals, who were low-dairy consumers at initiation of the study, did not result in significantly greater amounts of central fat and weight loss compared to an energy-restricted diet with ≤1 serving of dairy/d. Additionally, we did not observe greater IAAT loss for the subjects consuming the AD diet, nor a greater reduction in adipose tissue gene expression of inflammatory markers or in circulating inflammatory cytokines. The findings of the present study are in agreement with results reported by some investigators [5
] but in contrast with other studies that demonstrated a greater reduction in fat mass with dairy intervention [6
]. Researchers in Australia [24
] compared 3 servings of dairy foods per day to 5 servings/d, within the context of a reduced calorie diet, on changes in body weight and fat in overweight individuals during a 12-week intervention. They reported significantly greater weight and fat loss in the group receiving 5
servings of dairy/d. Kadooka et al. [25
] reported a significant decrease in abdominal visceral fat in obese adults who received fermented milk (e.g., yogurt with probiotics), and Faghih et al. [26
] compared the effect of cow's milk, fortified soy milk, and calcium supplementation on weight and fat loss, reporting a significant treatment effect with the cow milk group having the greatest weight loss. Differences in outcomes across clinical studies with dairy and calcium interventions might be explained by the lack of control for individual differences in food self-selection during dietary intervention with different amounts of dairy and/or calcium (e.g., changes in intakes of fats or other calorie sources due to experimental interventions). However, by providing all the food for the present study and matching subjects on adiposity, our study allowed for a direct assessment of the effects of increasing dairy and calcium consumption on metabolic outcomes without confounding effects of other dietary changes that may occur in free-living research volunteers.
Another possibility for our discrepant results compared to prior studies may be the “dairy effect” requires a longer period of time to see an impact on weight gain and fat accumulation over time rather than an effect on weight and fat loss. Specific to this point is the research by Gunther et al. [21
] and Eagan et al. [22
]. Gunther and colleagues documented no benefit of dairy products to modulate body weight or fat mass in young free-living women over a 1-year intervention. However, a six month followup by Eagan et al. [22
] with the same group of women found a significant effect of dietary calcium to reduce body fat accumulation. Research by Bush et al. [23
] also found that dietary calcium intake was significantly associated with less gain in IAAT over 1
yr. in premenopausal women. These studies suggest that any effect of dairy intake may be cumulative over a longer period of time, perhaps as part of a satiety/appetite regulation mechanism. More recently research, reported by Gilbert et al., [27
] provided more evidence of an appetite-suppression effect of dairy foods. Specifically, in a 6 mo. milk supplementation single-blind placebo-controlled weight loss trial, there was a significant treatment by time interaction that resulted in a smaller increase in desire to eat and hunger, suggesting that incorporation of milk into a weight loss diet modulates the orexigenic effect of weight loss. The controlled feeding design of the present study, allowing for the control of all food intake, prevented us from being able to observe a similar influence of dairy foods on appetite and hunger and a voluntary reduction of food and energy intake. Regardless, an important point to make is that while increasing dairy consumption did not increase fat or weight loss, inclusion of dairy foods in a weight loss paradigm did not compromise weight loss.
Furthermore, we did not see an adverse increase in serum lipids as a result of the AD diet which is in contrast to the findings of Wennersberg et al. [28
]. The difference between our results and those of Wennersberg et al. can be explained by the difference in the dairy foods incorporated into our menu. We used 1 or 2% fat milk (depending on caloric needs), low-fat yogurt, and full-fat cheese with saturated, mono- and polyunsaturated fats balanced between the LD and AD menus. Wennersberg et al. used a higher fat diet and included more saturated fats, for example, cream 12–40% fat, cheese 15–30% fat, and butter or butter spreads 40–80% fat.
There have been mixed reports on the effect of calcium intake to promote fat substrate utilization [4
], which could be a potential mechanism by which increased calcium or dairy intake impacts weight and fat loss. Melanson et al. [31
] examined 24-h energy expenditure, using a room calorimeter, in a group of healthy, young adult, non-obese men and women and reported that an acute intake of calcium was positively correlated with fat oxidation over 24
hrs, during sleep, and during light physical activity. Also, acute calcium intake was inversely related to 24-h RER. Conversely, habitual calcium intake was not associated with increased fat oxidation. In contrast, Jacobsen and colleagues [11
] reported that in young normal-weight adults under isocaloric conditions, short-term calcium intake, as high as 1800
mg/d, had no effect on total energy expenditure or fat oxidation, but fecal fat excretion increased about 2.5-fold. Teegarden and coworkers [12
] examined energy expenditure and fat oxidation in 24 young adult overweight women and found (after controlling for fat-free mass) that those taking 900
mg/d as calcium supplements had a significantly higher (P
= 0.02) rate of fat oxidation after a test meal challenge (1.5
g/hr) compared to the control group or those consuming 900
mg/Ca via dairy foods. Cumming et al. [29
] reported significantly higher rates of fat oxidation in a group of middle-aged overweight and obese men and women following a high-calcium breakfast meal, either dairy or supplements, compared to the low-calcium breakfast. Although our subject characteristics were similar to those of Teegarden et al. and Cumming et al., we did not observe significant differences in the fasting RER, a measure of substrate utilization. However, we did observe that our AD group had a significantly higher postprandial RER compared to the LD group, but this difference was observed prior to initiation of the intervention diets, suggesting that there were inherent differences in substrate utilization between the groups.
The work of Zemel et al. [3
], Shi et al. [30
], and Sun and Zemel [32
] in animal models and in vitro
studies suggest that calcium and or dairy food may be a significant contributor to greater weight and fat loss. Sun and Zemel [32
] demonstrated that high-calcium diets shift energy partitionin, thereby reducing weight gain during refeeding in obese mice. Based on the work of these investigators, vitamin D status (calcitriol, specifically) and its relationship to metabolic status are instrumental to the underlying hypothesis that the inclusion of dairy foods in a moderate energy-restricted diet will result in greater weight and fat loss. In our study, the overall values for serum vitamin D remained relatively stable in both groups (LD: 38.7 and 39.6
nmol/L; AD: 30.8 and 35.1
nmol/L), and serum calcitriol concentrations were not different between the LD and AD treatments (). Furthermore, the stability of the vitamin D levels explains the lack of a significant change in serum PTH. Therefore, we cannot discount the possibility that under our experimental conditions, the absence of dairy treatment effects on fat loss and inflammatory parameters may be explained in part by the lack of treatment-related changes in blood calcitriol levels. Although the AD diet provided greater dietary vitamin D intake, there was not a sufficient increase in circulating vitamin D to have a suppression effect. Furthermore, the observed increase in vitamin D was statistically significant; the absolute values was substantially lower than the recommended level. Our findings are in contrast to those reported by Cann et al. [33
] from Women's Health Initiative (WHI) which demonstrated a small, but statistically significant effect of calcium and vitamin D to reduce weight gain over three years. Postmenopausal women with less than 1200
mg/d Ca intake at baseline and randomized to supplements were 11% less likely to experience small (1–3
kg) and modest (>3
kg) gains in weight. The exceptionally large sample size and the multiple years of followup for this study enabled the investigators to detect small changes in weight gain. Unfortunately, Cann and colleagues do not report serum levels for either 25-OH vitamin D or 1,25(OH)2
vitamin D at baseline or for 3-year followup that we might be able to compare results based on actual serum levels of vitamin D. Additionally, the WHI research volunteers were postmenopausal women with a different endocrine profile than our women who were premenopausal. A postmenopausal endocrine profile may be more responsive to the influence of high-calcium and vitamin D intakes than those in premenopausal women and adult men that were research volunteers in the present study.
The short duration and small sample size of the present study did not allow us to detect changes in body weight or body fat as small as 1
kg as observed in the WHI data. Additionally, a statistically significant finding for an outcome variable of weight gain over time is not the same as testing the outcome variable of weight loss and, therefore, cannot be directly compared.
Adipose tissue is an active endocrine organ that releases a number of metabolism-related hormones and cytokines into the circulation, and obesity is often marked by sub-clinical inflammation and WAT macrophage infiltration. However, research findings in this area [34
] have reported conflicting results. We observed no changes in gluteal subcutaneous WAT inflammatory markers in our overweight and obese adults even with weight loss. Our subjects had low circulating levels of inflammatory markers at the start of the study, suggesting there was no underlying inflammation at the time of enrollment, and; therefore, no improvement could be detected with treatment. Furthermore, the hs-CRP values in our study subjects at the outset were on average within the AHA/CDC recommended normal range (1.0–3.1
]. However, we did see small, but statistically significant improvements in leptin, CRP, and PAI-1 associated with weight loss.
Weight loss studies that focus on dietary interventions generally instruct the volunteers to maintain current physical activity levels; however, supporting data to confirm compliance with this request are generally lacking. In our study we demonstrated no change in daily physical activity level in the LD group over time and a small (~13–15%) decline in the AD group. However, due to the large variation in total activity counts (standard deviations of ~90,000 and 120,000 counts), there was no significant difference in activity between groups. Nonetheless, the ~13–15% reduction in activity counting in the AD group during the latter half of the intervention may have resulted in somewhat less weight loss and fat loss as a result of a lower energy expenditure. To more thoroughly examine this possibility, we corrected energy expenditure for body weight and the change in body weight over the course of the intervention. When energy expenditure associated with daily physical activity was corrected for body weight, no differences were observed between the treatment groups. We also saw no significant change in lean body mass, and; thus, no change in the resting metabolic rate. Respiratory exchange ratio also did not change suggesting no change in substrate utilization between the groups. In summary, the fact that we observed no changes in lean mass, RMR, RER, and physical activity energy expenditure provide additional support for the concept that the change in body weight, body fat, and IAAT was due to caloric restriction and not other factors.
A limitation of our study was the power calculation and sample size determination. Although our calculations were based on previous research results, we may have been too ambitious in our expectation that we would observe more than a 4
kg difference in weight loss between the groups. In retrospect, a poststudy power analysis revealed a sample size of 112 subjects per group in order to detect a 1
kg difference in body weight ±3
kg standard deviation with 80 percent power and an alpha level of 0.05. Our inability to see treatment-related changes in endocrine and inflammatory parameters may also have been due to our strict inclusion criteria, particularly since healthy subjects without metabolic abnormalities may have had little room for improvement in their endocrine or inflammatory parameters. Additionally, the BMI range was limited to overweight and moderately obese individuals, thereby limiting comorbidities commonly seen with heavier individuals. Thus, inflammatory markers after weight loss may not similarly reflect those markers in body-weight- and adiposity-matched subjects that have not lost weight. Whether the inclusion of an adequate number of dairy servings in weight reduction diets of more obese individuals with metabolic disturbances improves metabolic and inflammatory parameters deserves further investigation.
In conclusion, the addition of 3-4 servings of dairy foods in a moderate energy-restricted diet of overweight and obese adult women and men did not result in greater weight and fat loss compared to the low-dairy diet. However, no adverse effects were seen on serum lipids with the inclusion of 3-4 servings of dairy foods, and increasing dairy and calcium intakes in the course of a calorie-restricted diet did not compromise fat or weight loss. Thus, given the importance of calcium and vitamin D to bone health, for weight loss diets that meet the US Dietary Guidelines for dairy foods, calcium and vitamin D are recommended. Whether increased dairy intake during weight loss results in greater weight and fat loss for individuals with metabolic syndrome deserves further investigation, but attention to study duration, controlled or free-living environment, and sufficient sample size will be critical for the detection of significant changes due to treatment. Consideration should also be given to assessment of appetite, hunger, and satiety with postintervention followup on weight regain.