GRADE GUIDANCE 38: Updated guidance for rating up certainty of evidence due to a dose-response gradient

Effects of Selenium Administration on Blood Lipids: A Systematic Review and Dose-Response Meta-Analysis of Experimental Human Studies

CONTEXT

Overexposure to the essential trace element selenium has been associated with adverse metabolic and cardiovascular outcomes, hypertension, and diabetes. However, dose-response meta-analyses analyzing the effects of selenium administration on the lipid profile in experimental human studies are lacking.

OBJECTIVE

Through a restricted cubic spline regression meta-analysis, the dose-response relation between the dose of selenium administered or blood selenium concentrations at the end of the trials and changes over time in blood lipids, ie, total, high-density lipoprotein (HDL) and low-density lipoprotein (LDL) cholesterol, and triglycerides was assessed.

DATA SOURCES

Searches were performed on PubMed, Web of Science, Embase, and the Cochrane Library from inception up to January 11, 2025 to identify randomized controlled trials (RCTs) investigating the impact of selenium supplementation on blood lipid profiles among adults.

DATA EXTRACTION

A total of 27 eligible RCTs that enrolled healthy individuals, pregnant individuals, and participants with specific health conditions were identified and the relevant data was extracted.

DATA ANALYSIS

Dose-response analysis indicated that selenium administration at and above 200 µg/day decreased HDL and LDL cholesterol and increased triglyceride levels. Blood selenium concentrations at the end of the trial above approximately 150 µg/L were positively associated with triglyceride and LDL cholesterol concentrations, and inversely associated with HDL cholesterol. Inorganic selenium supplementation showed stronger associations than organic selenium. At the lowest levels of baseline intake, selenium supplementation appeared instead to have beneficial effects on the lipid profile, with an overall indication of U-shaped curves, apart from HDL-cholesterol. The adverse effects of selenium were stronger in studies involving healthy participants as compared with unhealthy participants and pregnant females, in those having a longer duration of the intervention, particularly more than 3 months, and in European populations at selenium intake levels of above 300 µg/day.

CONCLUSIONS

In this dose-response meta-analysis of experimental human studies, an adverse effect of selenium administration on blood lipids at levels around or above the current upper level of intake was observed.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO registration No. CRD42022380432.

The waterpipe smoking and human health: a systematic review and meta-analysis of 191 observational studies

BACKGROUND

While growing evidence highlights the harmful effects of waterpipe smoking (WPS), detailed information about its association to chronic diseases remains limited. This systematic review and meta-analysis exploring the association between WPS and various health conditions.

METHODS

A systematic search of MEDLINE (via PubMed), Embase, Scopus, and Web of Science was conducted from inception to January 2025. Eligible observational studies on WPS and health outcomes were selected through a duplicate, independent process. Data extraction, including study details, participant characteristics, methods, and results, was performed independently by two reviewers using a standardized form. Methodological quality was assessed using the Newcastle-Ottawa scale (NOS), and studies were classified as high, moderate, or poor quality. The GRADE approach was applied to evaluate evidence certainty for each outcome, considering factors such as study design, risk of bias, consistency, precision, and publication bias.

RESULTS

A total of 191 studies with 807,174 participants were included, comprising 98 case-control, 77 cross-sectional, and 16 cohort studies from 24 countries. The median number of studies analyzed per outcome was 5, with a range of 3 to 30. Among the 62 outcomes evaluated, 31 (50%) demonstrated statistically significant effect sizes based on a random-effects model, with stroke, coronary artery disease (CAD), and cancer mortality exhibiting a significant prediction interval. Credibility evaluations identified low-quality evidence for birth weight, CAD, and cardiovascular and cancer mortality, whereas the evidence for the remaining outcomes was graded as very low quality. Significant associations were found between WPS and several health outcomes: gastric cancer, lung cancer, bladder cancer, esophageal cancer, CAD, stroke, diabetes, metabolic syndrome, overall mortality, cardiovascular mortality, cancer mortality, infertility, sperm normal form, sperm DNA fragmentation, chronic bronchitis, cough, sputum, low birth weight (LBW), spirometry parameters, and several dental health indicators.

CONCLUSION

This study reveals strong links between WPS and adverse health outcomes, but low evidence quality calls for rigorous research and public health interventions to mitigate its effects.

Dietary Sugar Intake and Incident Type 2 Diabetes Risk: A Systematic Review and Dose-Response Meta-Analysis of Prospective Cohort Studies

The dose-response relationship between dietary sugar and type 2 diabetes (T2D) risk is uncertain. MEDLINE, Embase, CINAHL, Web of Science and Cochrane databases were searched through July 9, 2024 for prospective cohort studies reporting relative measures of incident T2D risk by categories of dietary sugar (total, free, added, fructose, sucrose) or 2 beverage sources (non-diet sugar-sweetened beverages [SSBs], fruit juice) in healthy adults. Linear and restricted cubic spline dose-response models were fitted for each exposure, and study-specific slopes and confidence intervals (CIs) were calculated. Heterogeneity was evaluated using Q-statistics. Risk of bias was evaluated using the Risk of Bias in Non-randomized Studies of Exposures (ROBINS-E) tool. The Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach was applied to assess the certainty of evidence. Of 10,384 studies, 29 cohorts were included: SSB: 18 (n = 541,288); fruit juice: 14 (n = 490,413); sucrose: 7 (n = 223,238); total sugar: 4 (n = 109,858); fructose: 5 (n = 158,136); and added sugar: 2 (n = 31,004). Studies were conducted in Europe (13), United States (11), Asia (6), Australia (4), and Latin America (3). Each additional serving of SSB and fruit juice was associated with a higher risk of T2D (risk ratio [RR]: 1.25; 95% CI: 1.17, 1.35 and RR: 1.05; 95% CI: >1.00, 1.11, respectively; moderate certainty). In contrast, 20 g/d intakes of total sugar and sucrose were inversely associated with T2D (RR: 0.96; 95% CI: 0.94, 0.98; low certainty; and RR: 0.95; 95% CI: 0.91, <1.00; moderate certainty, respectively). No associations were found for added sugar (RR: 0.99; 95% CI: 0.96, 1.01; low certainty) or fructose (RR: 0.98; 95% CI: 0.83, 1.15; very low certainty). These findings suggest that dietary sugar consumed as a beverage (SSB and fruit juice) is associated with incident T2D risk. The results do not support the common assumption that dietary sugar (i.e., total sugar and sucrose), irrespective of type and amount, is consistently associated with increased T2D risk. This study was registered in PROSPERO as CRD42023401800.

Calling for a comprehensive risk assessment of selenium in drinking water

In the last two decades, research has elucidated that selenium, a trace element, has both nutritional and toxicological effects on human health, depending on its dose and chemical form. Recent animal, laboratory, and human studies have shown harmful effects of certain selenium species at specific exposure levels, prompting the need to reassess overall exposure to this element, including that occurring through drinking water, a primary source of inorganic selenium. Drinking water selenium standards worldwide are scarce and existing standards are inconsistent, likely because they have been informed by an incomplete and outdated assessment of the scientific evidence. Incorporating all the available human and laboratory evidence into a precautionary regulatory framework indicates that a drinking water limit of around 5 μg/L of selenium is needed to protect human health, i.e. with an uncertainty factor of 2 versus the lowest adverse effect level observed in human studies, and that higher values may pose unacceptable risks to humans. Despite the rarity of such high levels of selenium in underground and potable waters, coal mining and other sources of environmental pollution as well as geological factors may raise drinking water selenium content above a safe threshold, triggering the need to protect consumers, and to face challenging technological issues for selenium removal, currently under active investigation.

Cadmium exposure and risk of hypertension: A systematic review and dose-response meta-analysis

BACKGROUND

Exposure to environmental toxic metals represents a significant global public health concern. Many studies have reported that cadmium (Cd) exposure increases the risk of hypertension. Since the shape of such relation has not been well characterized, we assessed it by performing a systematic review and dose-response meta-analysis of human studies.

METHODS

We searched the literature through September 5, 2024 to identify papers related to Cd, hypertension, and blood pressure. Inclusion criteria were: observational design, adult population, assessment of exposure using Cd biomarkers, and availability of exposure category-specific risk estimates for hypertension. We performed a dose-response meta-analysis of the results from included studies.

RESULTS

Of the 18 studies published between 2006 and 2024, most had a cross-sectional design. Cd was measured in whole blood and/or urine in almost all studies, whereas only two studies measured Cd in serum. The dose-response meta-analysis indicated an almost linear relation between urinary Cd concentrations and hypertension risk with RR = 1.18, 95% CI 1.02-1.37 at 2.0 μg/g creatinine compared with no exposure. In contrast, the association between blood Cd concentrations and hypertension risk was non-linear: there was a steep monotonic increase in risk for Cd concentrations below 2 μg/L, reaching a RR of 1.48 (95% CI 1.17-1.86) at 2.0 μg/L, after which a plateau seemed reached. We found similar trends when restricting to studies of Asian population, while when considering North American studies, hypertension risk increased above 1.0 μg/g creatinine.

CONCLUSIONS

In this dose-response meta-analysis, risk of hypertension showed a non-linear positive association with blood Cd concentrations and a linear positive association with urinary Cd concentrations. Inconsistency in the shape of associations could relate to the different timing of exposure assessed by the biomarkers or the alteration Cd excretion at increasing exposure levels. Mitigation of Cd exposure is confirmed as a public health priority for chronic disease prevention.

Association between substituting macronutrients and all-cause mortality: a network meta-analysis of prospective observational studies

Background

Suboptimal diet quality is a key risk factor for premature death. Assuming relatively stable energy intake among individuals, changes in nutrient intakes occur by exchanging different nutrients. Therefore we aimed to examine the association of isocaloric substitution of dietary (macro)nutrients with all-cause mortality using network meta-analysis (NMA).

Methods

For this systematic review and NMA of prospective observational studies MEDLINE, Embase, and Scopus were searched from inception to February 13th, 2024. Eligible studies reported substitution analyses for quantity and/or quality of macronutrients, including carbohydrates, proteins, and fatty acids on all-cause mortality. Random-effects NMA were used in order to evaluate the pooled hazard ratios (HR) and 95% confidence intervals (CI) of substituting each included nutrient with another. We assessed risk of bias with the ROBINS-E tool, and the certainty of evidence (CoE) using the Grading of Recommendations Assessment, Development and Evaluations (GRADE) approach. This study is registered with PROSPERO (CRD42023450706).

Findings

Thirty-nine studies with 1,737,644 participants, 395,491 deaths, 297 direct comparisons, and seven nutrient-specific networks were included. Moderate CoE was found for an association with lower mortality risk when replacing 5% of energy intake from carbohydrates with polyunsaturated fatty acids (PUFA; HR: 0.90; 95%CI: 0.84, 0.95), n-6 PUFA (0.85; 0.77, 0.94), n-3 PUFA (0.72; 0.59, 0.86), and plant monounsaturated fatty acids (MUFA; 0.90; 0.85, 0.95), and when replacing 5% of energy from saturated fatty acids (SFA) and trans-fatty acids (TFA), with PUFA, MUFA, and plant-MUFA (HRrange: 0.75 to 0.91). A lower mortality risk was additionally found when 5% of animal-MUFA was replaced with plant-MUFA, and when replacing animal protein, and SFA with plant protein (HRrange: 0.81 to 0.87, moderate CoE).

Interpretation

Our results provide practical knowledge for public health professionals and can inform upcoming dietary guidelines. The beneficial association of increasing PUFA (both n-3 and n-6) and (plant-) MUFA intake while reducing carbohydrates, SFA and TFA, along with replacing animal protein and animal-MUFA with plant-based sources of protein and fat (MUFA) on the all-cause mortality risk, underscores the importance of plant-based dietary recommendations.

Funding

None.

Nutrition users' guides: systematic reviews part 1 -structured guide for methodological assessment, interpretation and application of systematic reviews and meta-analyses of non-randomised nutritional epidemiology studies

Due to the challenges of conducting randomised controlled trials (randomised trials) of dietary interventions, evidence in nutrition often comes from non-randomised (observational) studies of nutritional exposures-called nutritional epidemiology studies. When using systematic reviews of such studies to advise patients or populations on optimal dietary habits, users of the evidence (eg, healthcare professionals such as clinicians, health service and policy workers) should first evaluate the rigour (validity) and utility (applicability) of the systematic review. Issues in making this judgement include whether the review addressed a sensible question; included an exhaustive literature search; was scrupulous in the selection of studies and the collection of data; and presented results in a useful manner. For sufficiently rigorous and useful reviews, evidence users must subsequently evaluate the certainty of the findings, which depends on assessments of risk of bias, inconsistency, imprecision, indirectness, effect size, dose-response and the likelihood of publication bias. Given the challenges of nutritional epidemiology, evidence users need to be diligent in assessing whether studies provide evidence of sufficient certainty to allow confident recommendations for patients regarding nutrition and dietary interventions.

Alcohol Intake and Risk of Hypertension: A Systematic Review and Dose-Response Meta-Analysis of Nonexperimental Cohort Studies

BACKGROUND

Alcohol consumption has been associated with higher blood pressure and an increased risk of hypertension. However, the possible exposure thresholds and effect-modifiers are uncertain.

METHODS

We assessed the dose-response relationship between usual alcohol intake and hypertension incidence in nonexperimental cohort studies. After performing a systematic literature search through February 20, 2024, we retrieved 23 eligible studies. We computed risk ratios and 95% CI of hypertension incidence using a nonlinear meta-analytic model based on restricted cubic splines, to assess the dose-response association with alcohol consumption.

RESULTS

We observed a positive and almost linear association between alcohol intake and hypertension risk with risk ratios of 0.89 (0.84-0.94), 1.11 (1.07-1.15), 1.22 (1.14-1.30), and 1.33 (1.18-1.49) for 0, 24, 36 and 48 g/d, respectively, using 12 g alcohol/d as the reference value. In sex-specific analyses, the association was almost linear in men over the entire range of exposure but only observed above 12 g/d in women, although with a steeper association at high levels of consumption compared with men. The increased risk of hypertension above 12 to 24 g alcohol/d was similar in Western and Asian populations and considerably greater in White than in Black populations, mainly due to the positive association in women at moderate-to-high intake.

CONCLUSIONS

Overall, our results lend support to a causal association between alcohol consumption and risk of hypertension, especially above an alcohol intake of 12 g/d, and are consistent with recommendations to avoid or limit alcohol intake. Sex and ethnicity appear to be major effect-modifiers of such association.

Does fluoride exposure affect thyroid function? A systematic review and dose-response meta-analysis

INTRODUCTION

Fluoride exposure may have various adverse health effects, including affecting thyroid function and disease risk, but the pattern of such relation is still uncertain.

METHODS

We systematically searched human studies assessing the relation between fluoride exposure and thyroid function and disease. We compared the highest versus the lowest fluoride category across these studies, and we performed a one-stage dose-response meta-analysis for aggregated data to explore the shape of the association.

RESULTS

Most retrieved studies (27 of which with a cross-sectional design) were conducted in Asia and in children, assessing fluoride exposure through its concentrations in drinking water, urine, serum, or dietary intake. Twenty-four studies reported data on thyroid function by measuring thyroid-related hormones in blood (mainly thyroid-stimulating-hormone - TSH), 9 reported data on thyroid disease, and 4 on thyroid volume. By comparing the highest versus the lowest fluoride categories, overall mean TSH difference was 1.05 μIU/mL. Dose-response curve showed no change in TSH concentrations in the lowest water fluoride exposure range, while the hormone levels started to linearly increase around 2.5 mg/L, also dependending on the risk of bias of the included studies. The association between biomarkers of fluoride exposure and TSH was also positive, with little evidence of a threshold. Evidence for an association between fluoride exposure and blood concentrations of thyroid hormones was less evident, though there was an indication of inverse association with triiodothyronine. For thyroid disease, the few available studies suggested a positive association with goiter and with hypothyroidism in both children and adults.

CONCLUSIONS

Overall, exposure to high-fluoride drinking water appears to non-linearly affect thyroid function and increase TSH release in children, starting above a threshold of exposure, and to increase the risk of some thyroid diseases.

Dairy Intake and Risk of Cognitive Decline and Dementia: A Systematic Review and Dose-Response Meta-Analysis of Prospective Studies

Dairy intake may influence cognition through several molecular pathways. However, epidemiologic studies yield inconsistent results, and no dose-response meta-analysis has been conducted yet. Therefore, we performed a systematic review with a dose-response meta-analysis about the association between dairy intake and cognitive decline or incidence of dementia. We investigated prospective studies with a follow-up ≥6 mo on cognitive decline or dementia incidence in adults without known chronic conditions through a systematic search of Embase, Medline, Cochrane Library, Web of Science, and Google Scholar from inception to 11 July 2023. We evaluated the dose-response association using a random-effects model. We identified 15 eligible cohort studies with >300,000 participants and a median follow-up of 11.4 y. We observed a negative nonlinear association between cognitive decline/dementia incidence and dairy intake as assessed through the quantity of consumption, with the nadir at ∼150 g/d (risk ratio: 0.88; 95% confidence interval: 0.78, 0.99). Conversely, we found an almost linear negative association when we considered the frequency of consumption (risk ratio for linear trend: 0.84; 95% confidence interval: 0.77, 0.92 for 1 time/d increase of dairy products). Stratified analysis by dairy products showed different shapes of the association with linear inverse relationship for milk intake, whereas possibly nonlinear for cheese. The inverse association was limited to Asian populations characterized by generally lower intake of dairy products, compared with the null association reported by European studies. In conclusion, our study suggests a nonlinear inverse association between dairy intake and cognitive decline or dementia, also depending on dairy types and population characteristics, although the heterogeneity was still high in overall and several subgroup analyses. Additional studies should be performed on this topic, including a wider range of intake and types of dairy products, to confirm a potential preventing role of dairy intake on cognitive decline and identify ideal intake doses. This review was registered at PROSPERO as CRD42020192395.