4.1. Diet
A large body of peer-reviewed journal literature reports that diet is a major risk-modifying factor for lung cancer. A case–control study in Texas involving 2139 non–small-cell lung cancer (NSCLC) cases who completed food frequency questionnaires for the year before cancer diagnosis were compared with 2163 matched controls [
34]. Participants were from many races/ethnicities, which the analysis did not consider. Three dietary patterns were evaluated: fruits and vegetables, American/Western, and Tex-Mex. The multivariable adjusted odds ratio (aOR) for NSCLC for quantile 5 versus quantile 1 of fruits and vegetables was 0.68 (95% CI, 055–0.85); for American/Western, 1.45 (95% CI, 1.18–1.78); and Tex-Mex, 0.45 (95% CI, 0.37–0.56). For never smokers, the aOR for fruits and vegetables was 0.99 (95% CI, 062–1.58); for American/Western, 2.01 (95% CI, 1.25–3.24); and Tex-Mex, 0.50 (95% CI, 0.32–0.78). The aORs for former smokers and current smokers were similar to the results for all participants.
In this ecological study, the association with lung cancer for diabetes was stronger than for obesity. Obesity is not considered as strong a risk factor for lung cancer as is waist circumference [
35]. The same holds true for diabetes [
36,
37].
Obesity has been identified as a risk factor for several cancers. A 2013 review listed six cancers caused by obesity: breast, colorectal, endometrial, pancreatic, prostate, and renal cell carcinoma [
38]. The mechanisms for the three cancers which this study supports are, for breast cancer, decrease in sex hormone–binding globulin and hormonal factors; for colorectal cancer, steroid hormones and chronic inflammation; and for renal cell carcinoma, increased level of estrogen. A 2016 review also listed high BMI as a modifiable risk factor for breast cancer among white women in the US [
39]. A 2019 review listed obesity, insulin resistance and adipokine aberrations as being jointly linked to cancer risk [
40]. Adipose tissue increases in obesity and results in production of adipokines, which trigger low-grade inflammation and insulin resistance [
41]. Also, the altered gut microbiome contributes to inflammation and carcinogenic products [
40].
Obesity rates have risen in the US recently. Obesity rates for NHW adult men aged 20 years or older rose from a mean of 26.6% in 1999–2000 to 38.0% in 2015–2016 according to National Health and Nutrition Examination Survey (NHANES) data from 1999–2016 [
42]. For NHW adult women, the corresponding values were 33.5% and 41.5%.
A recent article [
43] suggested following the Mediterranean diet [
44] to manage obesity. The main guidelines are low intake of red and processed meat and refined sugar; moderate intake of low-fat dairy products, poultry, fish, and red wine; and high intake of virgin olive oil, nuts, fruit and vegetables, legumes, and unrefined whole grains. Those recommendations are in general agreement with finding in a 2023 Harvard cohort study [
45]
Good evidence exists that diet affects risk of colorectal cancer (CRC). A 2015 article from the Adventist Health Study 2 reported that in a prospective observational study of vegetarians and nonvegetarians, the adjusted hazard ratio for CRC was 0.78 (95% CI, 0.64–0.95) [
46]. In an analysis of food intake based on data from NHANES, 2007–2010, and the USDA Food Patterns Equivalents Database, 2007–2010, vegetarians consumed 1862 kcal, whereas nonvegetarians consumed 2058 kcal [
47]. A 2019 review listed the driving forces behind the increase in CRC as obesity, sedentary lifestyle, red meat consumption, alcohol, and tobacco
Studies of changes in cancer rates in countries that experienced the nutrition transition to the Western dietary pattern in the past half-century offer more support for diet’s role in cancer risk. For example, an analysis of data from China, Hong Kong, Japan, Korea, and Singapore showed remarkable increases in mortality rates of breast, colon, and prostate cancers and precipitous decreases in mortality of esophageal and gastric cancers [
48]. Those results are consistent with findings in the present ecological study for breast and colorectal cancer (with obesity as a risk factor). They also are probably consistent for the findings for esophageal and gastric cancers in that neither diabetes nor obesity was found to be a risk factor. In an ecological study involving eight countries—Brazil, China, Cuba, Egypt, India, Nigeria, Republic of Korea, and Sri Lanka—20-year increases of dietary supply of energy and animal fat were significantly associated with increases in Alzheimer’s disease and dementia rate [
49].
Diet is an important risk factor for type 2 diabetes mellitus (T2DM). A 2023 article reported findings from a cohort study involving 205,852 health professionals monitored for up to 32 years [
45]. The participants completed food frequency questionnaires every 4 years and described changes in health status. The study included 37 food groups. The data were then correlated with various dietary patterns such as DASH and an American version of the Mediterranean diet. In addition, two empirical dietary patterns were developed: the reversed empirical dietary index for hyperinsulinemia (rEDIH) and reversed empirical dietary inflammatory pattern (rEDIP). Both insulin resistance and systemic inflammation, often associated with obesity, are significant risk factors for many diseases, including T2DM [
50,
51] and cancer [
52]. The rEDIH and rEDIP dietary patterns had the strongest inverse correlations with T2DM. For the highest decile compared with the lowest decile, the multivariate adjusted risk for T2DM was 0.36 (95% CI, 0.35–0.37) for rEDIH and 0.38 (95% CI, 0.37–0.40) for rEDIP. When BMI was added, the values changed to 0.57 (95% CI, 0.54–0.59) and 0.57 (95% CI, 0.55–0.59), respectively. The food groups most strongly associated with high risk of disease were red meats, processed meats, energy drinks, french fries, and refined grains, whereas the food groups most strongly associated with reduced risk included coffee, leafy green vegetables, whole grains, fruit, dark-yellow vegetables, and salad dressing.
Further evidence shows that red meat and processed meat are important risk factors for cancer. A case–control study in Uruguay reported that both types of meat significantly correlated with incidence of NHL [
53]. A 2015 review showed that nine of 10 meta-analyses reported red and/or processed meat to be significantly correlated with risk of CRC [
54]. A 2021 meta-analysis of prospective studies showed red and/or processed meat to be significantly directly correlated with incidence of breast, colon, colorectal, lung, rectal and renal cancers [
55]. It has been proposed that intestinal microbiota helps mediate the link between red/processed meat consumption and risk of colon cancer [
56].
A study conducted from 2003 to 2007 reported that participants consuming the highest quartile of the Southern dietary pattern (characterized by added fats, fried food, eggs, organ and processed meats, and sugar-sweetened beverages) experienced an adjusted 37 (95% CI, 1–85)% higher risk of coronary heart disease than those in the lowest quartile [
57].
T2DM was treated with a high-fiber, low-fat, plant-predominant diet in Virginia, USA [
58], consisting of 40% vegetables, 20% beans, 15% whole grains, 10% fruits, 10% seeds/nuts, and 5% egg whites and nonfat milk. Mean BMI immediately before the lifestyle change was 33 (SD = 6), dropping to 30 (SD = 6) after 6 months. Fasting glucose decreased from 140 mg/dL (SD = 40 mg/dL) to 110 mg/dL (SD = 20 mg/dL). Twenty-two of 59 patients achieved T2DM remission.
An important but relatively little-known fact about the US food supply is that concentrations of essential minerals have been decreasing. A 2002 review outlined the evidence that mineral deficiencies are a major cancer risk [
59]. A 2007 article reported the weighted average depletions of essential minerals in the US food supply [
60]. It was based on data for cheeses, dairy, and meat from 1940 to 2002 and on fruits and vegetables from 1940 to 1991. Depletions were 29% for calcium, 62% for copper, 37% for iron, 19% for magnesium, 15% for potassium, and 34% for sodium. The reasons for the decreases include acid deposition [
61], extraction by harvested agriculture products, and widespread use of glyphosate fertilizer. Glyphosate fertilizer reduces seed and leaf concentrations of important minerals [
62]. It decreases mycorrhizal colonization and adversely affects plant–soil feedback [
63]. The fertilizer adversely affected soil bacteria, soil chemistry, and mycorrhizal fungi during restoration of a Colorado grassland [
64].
A quick search of publications regarding mineral intake and risk of cancer found that higher iron and zinc intake was associated with reduced risk of lung cancer in a 22-year study [
65]. Higher combined mineral intakes of 11 minerals were inversely correlated with risk of CRC in postmenopausal women in a prospective study conducted in Iowa [
66]. A 2022 review provides a recent overview of the importance of minerals in cancer risk [
67].
Minerals are also important for reducing risk of T2DM. A 2020 review outlines the role of minerals and trace elements in reducing risk of insulin resistance and T2DM [
68]. Studies in China found that copper and zinc concentrations were inversely correlated with T2DM [
69], and that while iron was directly correlated with T2DM but that this association was reduced to a non-significant correlation with higher concentrations of antioxidant minerals including chromium, copper, magnesium, selenium, and zinc [
70]
A 2022 review of spatial-temporal patterns of incidence, mortality, and attributable risk factors for T2DM from 1990 to 2019 among 21 world regions showed high BMI (52%), ambient particulate matter (14%), smoking (10%), and secondhand smoke (9%) to be the major contributing factors to T2DM disability-adjusted life-years [
71].
4.4. Solar UVB and Vitamin D
The role of solar UVB and vitamin D in reducing risk of cancer incidence and mortality rates was reviewed in 2022 [
12]. Supporting evidence comes from various studies stretching back to 1936, when researchers recognized that sun exposure can cause skin cancer but reduce risk of internal cancers [
78]. As discussed, ecological studies in the US have yielded good evidence that solar UVB reduces risk of incidence and mortality rates for many cancers [
7,
8]. Similar results have been reported from China [
9], Russia [
10], and Nordic countries [
11]. No factor other than vitamin D production has been proposed to explain the inverse correlation of solar UVB doses with cancer risk.
Solar UVB doses might have had lower correlations with cancer incidence rates in the 2016–2020 period than in earlier periods in the US for several reasons:
Reduced time spent in the sun when vitamin D can be produced. Because solar UVB reaching Earth’s surface increases as the solar elevation angle increases [
79], it is generally recognized that vitamin D can be made effectively when the angle is greater than about 45°.
Wearing sunscreen or sunblock. Many cosmetics now contain sunscreen [
80].
Increased prevalence of overweight and obesity. An inverse correlation generally exists between serum 25(OH)D concentration and weight or BMI. A meta-analysis reported: “The prevalence of vitamin D deficiency was 35% higher in obese subjects and 24% higher than in the overweight group [
81]. Also, obesity is associated with increased systemic inflammation, thereby increasing risk of cancer [
52]. This mechanism could possibly explain why solar UVB and vitamin D are less effective in reducing risk of cancer as found in the VITAL trial [
82]. In that trial, participants in three BMI categories, <25 kg/m
2, 25 to <30 kg/m
2, and
>30 kg/m
2 had the same increased in 25(OH)D concentration, ~12 ng/mL, but only those with BMI <25 kg/m
2 had a significantly reduced risk of cancer incidence.
Prospective cohort studies of cancer incidence with respect to serum 25(OH)D at time of enrollment have shown inverse correlations for bladder, breast, colorectal, liver, lung, and renal cancers [Table 5 in [
12]]. An important problem in conducting meta-analyses of such studies is to properly account for changes in serum 25(OH)D since enrollment [
83]. As shown in Figure 1 in [
12], a nearly linear change occurs in the odds ratio with follow-up time for CRC. When properly accounted for, the relative risk (RR) drops to 0.74 for men and 0.77 for women. That finding differs from what was reported in the 2019 article by McCullough and colleagues in which it was reported that men had considerably lower reduction of CRC than did women [
84].
Randomized controlled trials (RCTs) offer less support for vitamin D’s role in reducing risk of cancer incidence and death. The main reason is that most RCTs are based on guidelines for pharmaceutical drugs, not for nutrients. In drug trials, the only source of the drug is the trial itself, participants in the control arm are given a placebo, and results are analyzed on an intention-to-treat basis. That approach is not appropriate for vitamin D because vitamin D is available from other sources besides the trial, and cancer outcomes are related to serum 25(OH)D concentrations, not vitamin D dose. Heaney outlined guidelines for nutrients in 2014 [
85]. The important guidelines include that serum 25(OH)D concentrations should be measured before enrollment and that people with low values should be included in the trial; that the vitamin D dose should be large enough to raise serum 25(OH)D concentrations enough to significantly reduce the risk of the health outcome of interest; and that achieved serum 25(OH)D concentration should be measured and used in analyzing the results. A 2022 review further discusses the topic [
86].
An example of the importance of following such guidelines is found in the prediabetes-to-diabetes trial conducted by Tufts University gave people in the treatment group 4000 IU/d of vitamin D
3 [
87]. When results were analyzed by intention to treat, no significant difference in progression to diabetes was apparent between the treatment and placebo arms. However, when results were analyzed by achieved 25(OH)D concentration in the treatment group, researchers found that participants in the vitamin D treatment arm who had 25(OH)D concentrations above 50 ng/mL during the trial had a hazard ratio for progression to diabetes of 0.29 [95% CI, 0.17–0.50]compared with those who maintained a level of 20–30 ng/mL [
88].
The largest vitamin D–cancer RCT conducted was Harvard Medical School’s
VITamin D and
OmegA-
3 TriaL (
VITAL) [
82]. More than 25,000 participants were enrolled, including more than 5000 African Americans. Participants in the treatment arm were given 2000 IU/d of vitamin D
3, but participants in both the treatment and placebo arm were permitted to take up to 600 or, if older than 70 years, 800 IU/d of vitamin D
3. Nearly 17,000 participants submitted serum 25(OH)D concentrations near time of enrollment. The mean 25(OH)D concentration of those in the treatment arm was near 31 ng/mL. The median follow-up time was 5.3 years. The abstract reported that vitamin D did not significantly reduce risk of cancer incidence but seemed to modestly reduce risk of cancer mortality rates. However, the article reported that the HR for cancer incidence for those with BMI <25 kg/m
2 was 0.76 (95% CI, 0.63–0.90). In addition, the HR for African Americans was 0.77 (95% CI, 0.59–101), which barely failed the
p = 0.05 test of significance. Those results were not discussed in press conferences regarding the findings, and so busy physicians that read only the abstract were unaware of those results.
The mechanisms whereby vitamin D reduces risk of cancer incidence and mortality rates are well known [
12]. Vitamin D reduces cancer risk by surveilling cells and regulating apoptosis, differentiation, and progression. Vitamin D reduces progression by reducing angiogenesis around tumors and reduces metastasis by regulating concentrations of MMP-9. Matrix metalloproteinases (MMPs) are zinc-dependent proteolytic metalloenzymes, of which MMP-9 is one of the most complex. MMP-9 can degrade the components of the extracellular matrix [
89]. Many more mechanisms also exist.
Researchers recently determined that patients with digestive tract cancers who are p53-immunoreactive have a much better survival rate with vitamin D supplementation [
90]. Holick wrote the accompanying editorial pointing out its importance in treating cancer [
91]. That finding seems likely to apply to all types of cancer.
Several reviews make recommendations regarding vitamin D supplementation. A 2024 review outlined the rationale for supplementing with 2000 IU/d (50 μg/d) of vitamin D
3 for most adults [
92].
4.6. Public Health Implications
The findings in this ecological study could play an important role in public health policy and cancer prevention. The results indicate that dietary risk factors for lung cancer, obesity, and T2DM and solar UVB exposure/vitamin D are imporant risk-modifying factors for cancer incidence. Government agencies and disease organizations are important determants of public health policies for diet and vitamin D. This section will compare public health policies from those sources with current scientific evidence.
Solar UV and vitamin D policies are considered first. The National Institutes of Health, Office of Dietary Supplements, states the following in their report on vitamin D for health professionals: “
Although 25(OH)D functions as a biomarker of exposure, the extent to which 25(OH)D levels also serve as a biomarker of effect on the body (i.e., relating to health status or outcomes) is not clear. Researchers have not definitively identified serum concentrations of 25(OH)D associated with deficiency (e.g., rickets), adequacy for bone health, and overall health.” That statement is based on a report from 2011 [
99]
, prepared before 14 years of additional research. It is in need of an update. While the American Cancer society has good information regarding several ways to reduce risk of cancer, it recommends staying safe in the sun through use of sunscreen, etc., without mentioning that UVB, through production of vitamin D, can reduce risk of cancer so that regular sunscreen use should be accomapied by regular vitamin D supplementation [
100].
The US Preventative Services Task Force recommended in 2021 that adults should not be screened for vitamin D deficiency [
101]
. It concluded “Among asymptomatic, community-dwelling populations with low vitamin D levels, the evidence suggests that treatment with vitamin D has no effect on mortality or the incidence of fractures, falls, depression, diabetes, cardiovascular disease, cancer, or adverse events.” - a statement that clearly needs to be updated.
Red meat and processed meat are among the most important risk factors for T2DM [
45,
102,
103]. There is also good evidence that T2DM can be reversed through a plant-based diet [
104]. A 2020 consensus report of the American Diabetes Association and several other organizations recommended that patients work with dieticians but did not make any dietary recommendations [
105]. Disease organizations, such as the American Diabetes Association, could do more to inform people how to prevent and reverse DM.
Meat and ultraprocessed food (UPF) are important risk factors for obesity. A 2009 study found that people in the upper quintile of meat consumption consumed 700 more kCal/day than those in the lowest quintile [
106]. A 2022 paper opined that the major cause of the obesity epidemic in the US is UPF [
107]. The author noted that healthy foods cost more than unhealthy foods, due perhaps to increased subsidies to farmers beginning in the 1970s, leading to more production of foods that could be made into UPF and sold cheaply. A 2023 meta-analysis found that a 10% increase in UPF consumption was associated with a 6% increased risk of obesity [
108]. UPF consumption is also an important risk factor for T2DM [
109].
The Dietary Guidelines for Americans, 2020—2025 recommended keeping added sugars to less than 10% of total calories/day, saturated fat to less than 10% of calories/day, and sodium to less than 2300 mg/day [
110]. There was a recommendation to minimize foods with added sugars, UPF, processed meats, foods high in salt, alcoholic beverages, and topic oils. However, there was no recommendation to limit red meat consumption. A 2022 article noted that the vast majority of dieary guideline committee members had at least several conflicts of interest directly relevant to their work on the scientific report such as working for, or having research funded by, food and/or pharmaceutical companies [
111].
Thus, at present, the effort to inform the general public on how to improve their health though dietary choices, vitamin D supplements, and careful sun exposure seems to fall mainly on non-profit organizations that recommend support such measures.