Abstract
To examine the association of consumption of dairy foods with risk of total and cause specific mortality in women and men. DESIGN Three prospective cohort studies with repeated measures of diet and lifestyle factors. SETTING Nurses’ Health Study, Nurses’ Health Study II, and the Health Professionals Follow-up Study, in the United States. PARTICIPANTS 168 153 women and 49 602 men without cardiovascular disease or cancer at baseline. MAIN OUTCOME MEASURE Death confirmed by state vital records, the national death index, or reported by families and the postal system. During up to 32 years of follow-up, 51 438 deaths were documented, including 12 143 cardiovascular deaths and 15 120 cancer deaths. RESULTS Compared to the lowest category of total dairy consumption (average 0.8 servings/day), the multivariate pooled hazard ratio for total mortality was 0.98 (95% confidence interval 0.96 to 1.01) for the second category of dairy consumption (average 1.5 servings/day), 1.00 (0.97 to 1.03) for the third (average 2.0 servings/day), 1.02 (0.99 to 1.05) for the fourth (average 2.8 servings/day), and 1.07 (1.04 to 1.10) for highest category (average 4.2 servings/day; P for trend <0.001). For the highest compared to the lowest category of total dairy consumption, the hazard ratio was 1.02 (0.95 to 1.08) for cardiovascular mortality and 1.05 (0.99 to 1.11) for cancer mortality. CONCLUSION These data from large cohorts do not support an inverse association between high amount of total dairy consumption and risk of mortality. The health effects of dairy could depend on the comparison foods used to replace dairy. Slightly higher cancer mortality was non-significantly associated with dairy consumption, but warrants further investigation.
Generated Summary
This study, conducted through three prospective cohort studies, examined the relationship between dairy food consumption and the risk of total and cause-specific mortality in women and men. The research involved data from the Nurses’ Health Study (NHS), Nurses’ Health Study II (NHSII), and the Health Professionals Follow-up Study (HPFS) in the United States. The studies utilized repeated measures of diet and lifestyle factors to assess the association between dairy intake and mortality outcomes. A total of 168,153 women and 49,602 men, without cardiovascular disease or cancer at baseline, were included in the analysis. Data on deaths, including cardiovascular and cancer-related deaths, were documented over a follow-up period of up to 32 years. The primary outcome measure was death, confirmed by various sources. Multivariable analysis was performed to adjust for a range of factors, including family history of disease, physical activity, dietary patterns, smoking status, and other lifestyle variables. The core methodology involved Cox proportional hazards regression models to examine the associations between total dairy consumption and mortality risk, including dose-response analyses using restricted cubic splines to model non-linear associations. Additionally, substitution analyses were conducted to assess the impact of replacing dairy with other food groups on mortality risk. The findings were assessed for heterogeneity across cohorts.
Key Findings & Statistics
- The study documented 51,438 deaths during the follow-up period, which included 12,143 cardiovascular deaths and 15,120 cancer deaths.
- Compared to the lowest category of total dairy consumption (average 0.8 servings/day), the multivariate pooled hazard ratio for total mortality was 0.98 (95% confidence interval 0.96 to 1.01) for the second category (average 1.5 servings/day), 1.00 (0.97 to 1.03) for the third (average 2.0 servings/day), 1.02 (0.99 to 1.05) for the fourth (average 2.8 servings/day), and 1.07 (1.04 to 1.10) for the highest category (average 4.2 servings/day; P for trend <0.001).
- For the highest compared to the lowest category of total dairy consumption, the hazard ratio was 1.02 (0.95 to 1.08) for cardiovascular mortality and 1.05 (0.99 to 1.11) for cancer mortality.
- Whole milk intake was significantly associated with higher risks of total mortality (hazard ratio per 0.5 additional serving/day 1.11, 1.09 to 1.14), cardiovascular mortality (1.09, 1.03 to 1.15), and cancer mortality (1.11, 1.06 to 1.17).
- In food substitution analyses, consumption of nuts, legumes, or whole grains instead of dairy foods was associated with a lower mortality.
- Substitution for red and processed meat instead of dairy foods was associated with higher mortality.
- The study found a non-linear relation between total dairy intake and risk of total mortality (P for non-linearity <0.001; P for overall significance of the curve <0.001; P for linear association <0.001) and risk of cardiovascular mortality (P=0.001; P=0.004; P=0.60; fig 1), such that moderate intake was associated with a slightly lower risk, whereas high intake was associated with an increased risk.
- Total dairy intake was associated with higher risk of cancer mortality in a linear fashion (P=0.10; P=0.04; P=0.048).
- Intake of skimmed or low fat milk was associated with slightly higher risk of total mortality and cardiovascular mortality, and with a lower risk of colorectal cancer.
- Whole milk intake was significantly associated with higher risk of total mortality, cardiovascular mortality, and cancer mortality, including lung cancer, ovarian cancer, and prostate cancer.
- The associations of whole milk intake and risk of total mortality and cancer mortality remained significant after correcting for multiple testing (P for trend <0.001).
- Substituting one serving per day of total dairy for nuts and legumes was associated with a 14% lower risk of total mortality (hazard ratio 0.86, 95% confidence interval 0.78 to 0.95; fig 2).
- Substituting one dairy serving per day for whole grains was associated with an 11% lower risk of total mortality (0.89, 0.84 to 0.93).
- Substitution for red and processed meat was also associated with a 5% higher risk of total mortality (1.05, 1.01 to 1.09).
- During 29-32 years of follow-up, 51438 deaths were documented in the three cohorts, including 12143 cardiovascular deaths and 15120 cancer deaths. In an age adjusted model, dairy intake was positively associated with risk of total mortality in the NHSII and HPFS, but not NHS (table 2), which might be due to the different distribution of covariates according to dairy intake across the three cohorts as shown in table 1. However, after adjusting for covariates, dairy intake was positively associated with risk of mortality across the three cohorts (P for heterogeneity >0.1). Compared to the lowest category of total dairy consumption, the multivariable pooled hazard ratio for death was 1.07 (95% confidence interval 1.04 to 1.10) for the highest category of dairy consumption (P for linear trend <0.001; table 2).
Other Important Findings
- The study found that total dairy intake was not associated with lower risk of total mortality.
- The health effects of dairy could depend on the comparison foods used to replace dairy.
- Slightly higher cancer mortality was non-significantly associated with dairy consumption, but warrants further investigation.
- The associations of dairy intake with risk of mortality using a dose-response analysis revealed a non-linear relation between total dairy intake and risk of total mortality.
- Substituting dairy with nuts or legumes, poultry, and whole grains was associated with lower mortality.
- Substituting total dairy with red and processed meat was associated with increased mortality.
Limitations Noted in the Document
- The study’s findings may not be generalizable to other populations due to the predominantly white healthcare professional participants.
- Given the observational design, a direct causal relationship between dairy intake and mortality could not be definitively established.
- Residual confounding remains a possibility, despite adjusting for various diet and lifestyle factors.
- Reverse causation could exist, particularly as participants with health issues might alter their dairy intake.
- The study acknowledges that the health effects of dairy consumption could depend on the background diet of the population.
Conclusion
The research, conducted across three large cohort studies, examined the association between dairy consumption and mortality risks in women and men. The primary finding indicates that high amounts of total dairy intake were not associated with a reduced risk of overall mortality, contradicting the hypothesis of an inverse relationship. Moreover, the study revealed a complex relationship, showing a non-linear association between total dairy intake and the risk of mortality, with higher intakes potentially increasing risk. Whole milk intake, in particular, was significantly associated with increased risks of total, cardiovascular, and cancer-related mortality, including lung, ovarian, and prostate cancers. This is a crucial point, as it underscores the importance of distinguishing between different types of dairy products. In contrast, skimmed or low-fat milk consumption showed a different profile, linked with a slightly higher risk of total mortality and cardiovascular mortality, yet a lower risk of colorectal cancer. These findings suggest that the health effects of dairy consumption are not uniform and depend heavily on the specific type of dairy consumed. The study’s substitution analyses further add to the complexity, indicating that replacing dairy with healthier food options such as nuts, legumes, or whole grains was associated with lower mortality risks, whereas substituting with red and processed meats was associated with higher risks. The study also found that substituting dairy for red and processed meat was associated with an increase in mortality, emphasizing the importance of the context of the overall diet in determining health outcomes. While the study provides valuable insights, it also acknowledges certain limitations. The observational nature of the study design prevents direct causal inferences. The population studied, composed mainly of white healthcare professionals, may limit the generalizability of the findings to other demographics. In addition, the study recognizes the potential for residual confounding, despite adjusting for numerous lifestyle factors. The results suggest that dietary recommendations on dairy consumption should be nuanced, considering the type of dairy products consumed and the dietary context in which they are consumed. The findings emphasize that simply increasing dairy intake may not necessarily translate into improved health outcomes, and that healthier alternatives should be considered in dietary strategies. The implication of the study’s findings is that dietary advice should move away from blanket recommendations about dairy intake and consider the type of dairy products consumed, and the overall dietary pattern.
IFFS Team Summary
- Nurses’ Health Study, Nurses’ Health Study II, and the Health Professionals Follow-up Study, in the United States.
- 168 153 women and 49 602 men, avg 32 years follow up, appropriately adjusted
- Highest to lowest quintiles of dairy consumption shows HR of 1.07
- No difference shown for CVD or Cancer mortality individually
- However the data is markedly positive depending on what dairy is substituted for:
- Each serving of dairy substituted by whole grains showed 11% decrease in mortality
- Each serving of diary substituted by nuts and legumes showed 14% decrease in mortality
- Each serving of dairy substituted by red meat showed 5% increased mortality
- Each serving of dairy substituted by whole grains showed 11% decrease in mortality
- Each serving of diary substituted by nuts and legumes showed 14% decrease in mortality
- Each serving of dairy substituted by red meat showed 5% increased mortality
- Whole milk: for each 0.5 additional serving per day (vs other dairy products)
- HR = 1.11 for total mortality
- HR = 1.09 for CVD mortality
- HR = 1.11 for Cancer mortality
- HR = 1.11 for total mortality
- HR = 1.09 for CVD mortality
- HR = 1.11 for Cancer mortality
- Looking at quantity of dairy alone show only a slight increased mortality, however
- Data strongly suggests that this depends on what people are eating instead of dairy
- Whole milk has the most marked detrimental effect
- Data strongly suggests that this depends on what people are eating instead of dairy
- Whole milk has the most marked detrimental effect
- Referenced in Total Mortality, Total Cancer and Cardiovascular Disease sections