The literature search yielded 1630 results: 379 from MEDLINE, 716 from EMBASE, and 535 from Web of Science. In addition, ten WCRF evidence synthesis reports, which can be categorized as systematic reviews, were included [22]. Two hundred and fifteen records were assessed as full texts, and a total of 95 reports were included in this scoping review, of which 92 contained meta-analyses. The title, abstract, and full-text phases, along with reasons for exclusion, are presented in a PRISMA flowchart (Fig. 1) [23] A list of excluded reports with reasons for exclusion can be found in Supplemental Table C.

Fig. 1 Flow diagram of the study selection. Full size image

Characteristics of included reports

The largest portion of included reports incorporated more than one study design (n = 43), followed by reports that included solely prospective cohort studies (n = 35). Seven reports examined follow-up data from RCTs, two were based exclusively on RCTs and four only included case–control studies (Supplementary Table A). Ten SRs were from the WCRF’s Continuous Update Project and include cohort studies, case–control studies and RCTs. The number of studies analyzed in the included reports ranged from five [24] to 152 [25], whereas participant numbers varied from 1427 [26] to 7,606,009 [27]. WCRF reports did not consistently provide the number of participants per included study in the report body, particularly for individual exposures/interventions such as dairy.

Health outcomes

Twenty-nine diet-related health outcomes were identified in the included reports. They fell into five groups:

I. cardiovascular outcomes including composite cardiovascular disease [27,28,29,30,31,32,33,34], coronary heart disease [9, 29,30,31,32,33, 35,36,37,38,39], all types of stroke [9, 29,30,31, 33,34,35,36,37, 39, 40], ischemic stroke [38], and hypertension [9, 17, 41,42,43], II. cancer types including bladder [28, 44,45,46,47,48], breast [49,50,51,52,53,54,55,56], colorectal [28, 52, 57,58,59,60,61,62,63,64], corpus uteri [65, 66], esophagus [28, 67, 68], kidney [69], leukemia [70], liver [71,72,73], lung [74, 75], non-Hodgkin lymphoma [76, 77], oral [78,79,80], ovarian [81,82,83], pancreatic [24], prostate [52, 84, 85], stomach/gastric [86,87,88,89,90], and “all types” [28], III. body composition including overweight and obesity [41, 91], and weight gain [25, 91,92,93], IV. mortality including all-cause mortality [32, 35, 94,95,96,97,98,99,100] and cardiovascular mortality [94,95,96, 100, 101], V. other outcomes including T2DM [34, 41, 102,103,104,105,106,107], bone health [108,109,110,111], joint health including rheumatoid arthritis and knee osteoarthritis [112, 113], and cognitive outcomes [114,115,116].

Health outcomes were represented with varied frequency across the reports as illustrated in Fig. 2. The total number of associations exceeds the number of included reports, as several reports addressed multiple health outcomes. Supplementary Table D provides a tabular summary.

Fig. 2 Treemap of included health outcomes. Full size image

Exposures and interventions

Exposures addressed in the included reports encompassed a range of dairy foods. In our analysis, we focused on total dairy, milk, yogurt, cheese, and fermented dairy products. These categories were determined by the authors of the included reports and may represent varying definitions. Broadly, “milk” refers to all milk consumption (not by sub-type), “total dairy” refers to all dairy products included in the individual reports, and “fermented dairy” refers to any fermented dairy foods categorized in individual reports as “fermented dairy”. Sixty-six reports assessed the association of total dairy on health outcomes, 52 examined associations with milk, 43 reports studied cheese and 40 investigated yogurt consumption. Thirteen reports presented findings about all fermented dairy products on health outcomes. A detailed overview can be found in Supplementary Table E.

Synthesis of results

Table 1 provides a comprehensive overview of the health outcomes associated with the consumption of five dairy product categories: total dairy, milk, cheese, yogurt, and fermented dairy products. Detailed information on how each report was charted is provided in Supplemental Table E.

Out of the total 281 identified associations, 106 (37.7%) suggested that dairy consumption is associated with a reduced risk of certain negative health outcomes. Meanwhile, 135 (48.0%) indicated a neutral effect of dairy on the investigated outcomes. Twelve associations (4.3%) found that dairy consumption may increase the risk of certain negative health outcomes, and 28 (10.0%) yielded inconclusive results.

In the following sections, we will examine the associations of different dairy exposures—total dairy consumption, milk, cheese, yogurt, and overall fermented dairy—on the five outcome categories listed in Table 1: cardiovascular outcomes, individual cancer types, body composition, mortality, and other outcomes. We highlight the most significant findings for each exposure category.

Overall dairy intake

Sixty-six reviews examined associations between overall dairy consumption and health outcomes. This is not an aggregate measure; we are reporting findings of included reports that presented associations with overall dairy consumption. Our findings show that dairy intake is either associated with reduced risk (n = 35) or has no association (n = 40) with most of the health outcomes considered. Six associations yielded inconclusive findings, and five showed that overall dairy consumption was linked to an increased risk of negative health outcomes (Table 1). Overall dairy intake was associated with a reduced or neutral risk of all of the cardiovascular outcomes listed in Table 1 [9, 26, 29, 31,32,33,34, 36,37,38, 40,41,42,43, 86,101] except cardiovascular mortality. A lower risk of bladder [44], breast [53,54,55], colorectal [57, 61,62,63,64], and oral cancers [79], as well as overweight/obesity [41, 93], T2DM [34, 41, 102, 103, 106, 107], and joint health [113] was associated with overall dairy consumption. Some reports found that increased risk of liver cancer [73], non-Hodgkin lymphoma [77], ovarian [81], prostate [85], and gastric [89] cancers were associated with overall dairy consumption.

Milk intake

Fifty-one of the included reports investigated the association of milk intake with health outcomes. Of these, thirteen associations indicated a decreased risk, 43 were neutral, four suggested an increased risk, and ten yielded inconclusive findings on the identified health outcomes (Table 1).

The results indicated that milk consumption was associated with either a decreased risk (n = 4) or neutral associations (n = 11) on cardiovascular outcomes, whereas two inconclusive findings were reported [9, 29, 32, 35, 36, 38,39,40,41,42]. Overall, milk consumption was associated with either a reduced risk or no risk of developing most cancers. Specifically, reports suggest that milk consumption is associated with a lower risk of the development of oral [78, 79], bladder [44, 47], and colorectal cancers [57, 62]. Four reports suggested that milk was linked to an increased risk of certain types of cancer, including one report each for breast [50], prostate [85], liver cancer [71], and non-Hodgkin lymphoma [76]. Only one report explored the relationship between body composition and milk intake [41], and it found a reduced risk of overweight and obesity.

Associations between milk intake and cardiovascular- and all-cause mortality were heterogeneous: five neutral and two inconclusive associations were reported [32, 35, 94, 97, 100]. One report showed that consuming milk was linked to a reduced risk of developing T2DM [107], while three reports identified neutral associations [41, 103, 106]. Neutral (n = 1) or inconclusive (n = 8) associations were found between milk consumption and bone health, while four inconclusive and two neutral associations were found for cognitive outcomes [114,115,116]. No reports examined the relationship between milk consumption and leukemia, “all cancers,” joint health, or weight gain.

Cheese intake

Forty-two reports investigated associations between cheese intake and various health outcomes (Table 1). Overall, 20 associations showed a decreased risk, 25 had neutral findings, two showed an increased risk, and seven had inconclusive results across the health outcomes studied. Eight reports examined associations between cheese consumption and stroke [9,29,30,31,36,38,39,40] and seven investigated its relationship with CHD [29,30,31,38]. Some reports found that that cheese intake was associated with a lower incidence of cardiovascular diseases [29,30,31,32,36,38,39,40,117] while others reported neutral results [29, 32, 38, 39, 41, 42]. One report did not draw definitive conclusions [9]. Cheese was linked to a lower risk of developing cancer (breast, colorectal, oral, ovarian) in some reports [28, 50, 57, 60, 78, 81], but no associations were found in others (bladder, breast, colorectal, uterine, esophageal, liver, gastric, non-Hodgkin lymphoma) [24, 45, 46, 54, 58, 62, 63, 66, 67, 71, 72, 76, 86, 118]. An increased risk of prostate cancer was observed in one report [85]. Three reports found no association between cheese consumption and mortality [32, 97, 99], whereas relationships with bone health were inconsistent: one report showed positive associations and two were inconclusive [108, 110]. Similarly, two reports showed no link between cheese intake and T2DM [41, 103] while one found an increased risk of T2DM [105]. Only one report examined the association between cheese and cognitive outcomes [116], noting inconclusive findings. None of the reports investigated possible associations between cheese consumption and lung cancer, kidney cancer, leukemia, body composition, overweight, obesity, joint health, or cognitive disorders.

Yogurt intake

Forty reports investigated yogurt consumption. Overall, yogurt intake was found to have 25 associations with reduced risk, 25 neutral findings, no increased associations, and five inconclusive results with the identified health outcomes. One report found that yogurt intake was linked to a decreased risk of adverse cardiovascular outcomes [117], while seven found no associations with these outcomes [27, 29, 32, 36, 38, 41, 42]. Yogurt intake was associated with either a reduced risk [45, 49, 54, 57, 59, 67, 71, 72, 78, 117] or no association with cancer development in all reports [50, 51, 60, 73, 76, 82, 85, 118, 119] except one, which reported inconclusive evidence of the association between yogurt consumption and colorectal cancer [63]. There was also evidence that yogurt consumption is associated with a reduced risk of developing bladder [28, 45], breast [49, 54], colorectal [28, 57, 59], esophageal [67], liver [71, 72], and oral cancers [78]. Two reports investigated body composition outcomes. One found that yogurt intake reduced the risk of weight gain [92] while the other found no association with overweight or obesity [41]. Two reports found that yogurt consumption was associated with a decreased risk of cardiovascular- and all-cause mortality [95, 96] while another reported neutral findings on all-cause mortality [32]. Five reports indicated that yogurt intake was linked to a reduced risk of developing T2DM [41, 103, 106, 107, 120]. None of the included reports found that yogurt consumption was linked to an increased risk of any of the included 29 adverse health outcomes, though two reports were unable to draw definitive conclusions [9, 63].

Fermented dairy intake

Thirteen included reports investigated associations of overall fermented dairy intake, such as cheese, yogurt, and cultured/sour milk, on health outcomes. Overall, 13 associations showed a reduced risk of poor health outcomes, four found neutral results, and one indicated an increased risk. Fermented dairy was associated with a decreased risk of cardiovascular disease [32, 117] and stroke [36], whereas both a reduced risk [42], and no association [41] were seen with hypertension. Fermented dairy intake was also associated with a lower risk of developing bladder [28, 46], breast [55], and esophageal cancer [28], as well as T2DM [103, 120], and all-cause mortality [32]. No association between fermented dairy consumption and colorectal cancer [57, 58] were found in two reports, while another report demonstrated a decreased risk [28]. A single report found an association between the consumption of fermented dairy and risk of all-cause mortality [97]. No reports investigated the association of fermented dairy consumption with body composition.

High- versus low-fat dairy and milk

Twenty-seven of the included papers reported subanalyses of high- versus low-fat milk and dairy consumption (Table 2). Most of the included studies focused primarily on dairy products in aggregate rather than milk alone, and predominantly investigated cardiovascular outcomes [9, 26, 29, 31,32,33,34,36,37,38,40,41,42,43]. In addition to cardiovascular health, associations between dairy fat content and various other outcomes were reported. These included four cancer types—breast, colorectal, ovarian, and prostate cancer [51, 52, 54, 55, 57, 61, 63, 81, 82, 85]—as well as body composition [41], mortality [32, 97, 98, 100], and T2DM [41, 102, 103, 105, 106]. Across outcomes, most reviews reported no association between fat content of milk or dairy foods and adverse health outcomes. Inverse associations with both full-fat and reduced-fat dairy were reported for many cardiovascular outcomes [9, 26, 29, 31, 33, 36, 40,41,42], though two studies reported increased associations for full-fat milk [36, 38]. With reference to cancers, most reports found either no association or inverse associations between full-fat dairy consumption and adverse health outcomes. A notable exception was found in one report showing an increased association between full-fat milk and ovarian cancer, contrasting with an inverse association for reduced-fat milk [81]. While most reports found no association between low- or high-fat milk or dairy and mortality, one reported an increased association of full-fat milk with both all-cause mortality and cardiovascular mortality [100].