Review of Evidence on Alcohol and Health (2025)
Chapter: 1 Introduction
1
Introduction
BACKGROUND
The Dietary Guidelines for Americans (DGA) serve as the primary source of dietary guidance from the federal government and are used to inform food and nutrition programs and as a resource for recommendations for dietary intake and healthful dietary patterns for the U.S. population (Box 1-1). The original systematic reviews (SRs) informing DGA guidance are conducted by the Dietary Guidelines Advisory Committee (DGAC) with support from the Nutrition Evidence Systematic Review (NESR) team, which operates under the auspices of the U.S. Department of Agriculture’s (USDA) Center for Nutrition Policy and Promotion. NESR, working with its collaborators, supports the conduct of systematic reviews that serve as a central resource for the federal government in making evidence-informed decisions, including development of the DGA.
Since its inception in 1980, the DGA has provided guidance that includes recommendations regarding alcoholic beverages. Previous editions, particularly the 2010–2015 DGA, have also provided guidance about the consumption of alcoholic beverages for population groups, including those who are breastfeeding (USDA and HHS, 2010). The current edition of the DGA recommends that individuals should not start drinking alcohol for any reason and further states that drinking less is better for health than drinking more (USDA and HHS, 2020). The DGA 2020–2025 also advise that some individuals should not drink alcohol at all, for example, those who are pregnant or might be pregnant, individuals under the legal age for drinking, individuals with certain medical conditions or who are taking
certain medications that can interact with alcohol, individuals recovering from an alcohol use disorder (AUD), and individuals unable to control the amount they drink (USDA and HHS, 2020).
The DGA also carried forward a recommended limit on alcoholic beverage consumption from guidance in previous editions. Specifically, for adults of legal drinking age who choose to drink, it should be done in moderation by limiting “alcohol intake to two drinks or fewer in a day for men and one drink or fewer in a day for women” (USDA and HHS, 2020). The DGA 2020–2025 further recognized that the decision to engage in alcohol consumption at low or moderate levels reflect personal considerations that balance the potential harms against the potential benefits of alcohol (USDA and HHS, 2020).
Context of Evidence for the Development of the Next Dietary Guidelines for Americans
The DGAC is a federal advisory committee convened by USDA and HHS prior to developing updated editions of the DGA. The DGAC conducts SRs, data analyses, and food pattern modeling with support from federal staff, including NESR. Collectively, this body of work is integrated into the findings of the Scientific Report of the Dietary Guidelines Advisory Committee and provided to the secretaries of USDA and HHS that, along with additional input from federal agencies and comments received from the public, contribute to the development of the DGA every 5 years.
For each 5-year cycle in the DGA process, proposed scientific questions, including systematic review questions, are identified by USDA and HHS based on input from previous DGAC, federal experts, and the public. The proposed questions are prioritized based on the following criteria: relevance, importance, potential effect on federal programs, and avoiding duplication. Research availability, whether sufficient evidence exists to conduct a new review or update an existing review, is also considered. NESR estimates research availability through continuous evidence monitoring or evidence scans. The proposed questions are provided to the DGAC, who further refine and prioritize the questions based on the same criteria.
During the open session for the public on January 25, 2024 (Appendix B), USDA provided background and the most recent systematic reviews for the DGAC (Box 1-2). The 2020 DGAC conducted one systematic review on alcohol and all-cause mortality. The remaining seven questions were last examined by the 2010 DGAC.
Overview of Alcohol and Health
The health effects of heavy drinking have been documented by a number of authoritative bodies, such as the Centers for Disease Control and Prevention (CDC) and the National Institute on Alcohol Abuse and Alcoholism (NIAAA).1,2 An understanding of the potential health effects of alcohol is shaped by its complex and diverse actions on physiological structures and processes, and these in turn are modified by the quantity, frequency, and pattern of intake. Alcohol exerts its pharmacologic actions through direct interactions with multiple proteins present throughout the body; ethanol’s displacement of water from hydrophilic pockets alters some proteins’ structure and activity (Mihic et al., 1997). This action is best understood for, but is not limited to, proteins that mediate neurotransmission, and the consequence depends on the individual protein target; for some, activity is increased (i.e., γ-aminobutryic acid A [GABAA] receptors) and for others it is decreased (N-Methyl-D-aspartic acid [NMDA] receptors).
Ethanol’s toxicity originates, in part, through its metabolic conversion to the chemical acetaldehyde, which can derivatize deoxyribonucleic acid (DNA) nucleotides to introduce mutations (Mizumoto et al., 2017). This metabolic conversion can also generate free radicals, which have the potential to damage cellular components including DNA, lipids, and mitochondria (Wu and Cederbaum, 2003). Alcohol also increases intestinal permeability, admitting microbial components that have proinflammatory effects in the circulation (Maccioni et al., 2023).
Alcoholic beverages contain myriad nonalcohol compounds, also known as congeners, that can have further physiological influences. Congeners range from phytochemicals present in grapes to contaminants that enter during processing, and their content varies with the type of alcoholic beverage consumed (wine versus beer versus spirits) (IARC, 1988). The complex composition of alcoholic beverages, combined with alcohol’s diverse actions, have made it challenging to reach a consensus regarding the health effects of moderate drinking as defined by the DGA for low-risk drinking.
As with other pharmacologic agents, lower alcohol consumption tends to have smaller and even different effects, and intermittent consumption may not have the same overall effect as daily or near-daily intake, which can promote tolerance such that a higher amount is necessary to produce the same effect as was previously produced by a lower amount of alcohol (Elvig et al., 2021). Likewise, while it is tempting to infer a linear relationship between level of alcohol intake and risk of an outcome, alcohol may
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1 https://www.cdc.gov/alcohol/about-alcohol-use/index.html (accessed September 23, 2024).
2 https://www.niaaa.nih.gov/alcohols-effects-health/alcohols-effects-body (accessed September 23, 2024).
have different effects at different doses, creating a J-shaped response curve that reflects greater or lesser effect on different health outcomes. This can reflect differences in the physiochemical properties of the proteins that alcohol interacts with, in which smaller exposures may activate protective or defensive mechanisms that repair cellular damage, enhance toxin disposal, or activate the immune system (Calabrese and Baldwin, 2001). Moreover, there is abundant evidence that individuals respond to the same alcohol dose differently with some of the differences attributable to variation in alcohol metabolism related to such factors as age, sex, and genetics, and this source of variance is discussed further in Chapter 2.
Current Drinking Patterns
Alcohol consumption is highly prevalent in the United States. According to the 2023 National Survey on Drug Use and Health, 62.5 percent of people 12 years and older reported drinking in the past year (NIAAA, 2024a). Even when excluding 11 percent of the U.S. population with a Diagnostic and Statistical Manual (DSM)-5 AUD diagnosis, about half the adult population engages in alcohol consumption. Nearly half a million visits to emergency departments annually were related to alcohol consumption. Further, CDC noted that death certificates listed chronic or acute alcohol as a factor in 178,000 deaths in 2020 and 2021, and this reflects a steady increase and tripling of numbers between 2000 and 2021, with a notable jump in prevalence in the first year of the COVID-19 pandemic (CDC, 2024). The percentage of women who consume alcohol has now caught up with the percentage of men. This convergence of proportion by sex may herald increasing alcohol-related problems in women (White, 2020). The recent increases in alcohol consumption in people aged 65 years or older similarly introduce additional health-related complications in that population (White et al., 2023). Thus, changing demographics in the populations that consume alcohol inform the importance of assessing the association between those behaviors and health outcomes.
Defining Alcohol Use, Misuse, and Abuse
There are personal, familial, and societal impacts associated with alcohol misuse, and there are important differences between moderate drinking, which may be considered as a term defined by exclusion, and problem drinking. Moderate drinking may not meet the American Psychiatric Association DSM-5 criteria for a diagnosis of AUD, which is based on presentation of symptoms rather than number of drinks consumed and is characterized on a spectrum.
In addition to considering symptoms to establish diagnosis, defined drinking patterns, notably binge drinking and heavy alcohol consumption, can be considered alcohol misuse and fall under the aegis of an AUD diagnosis. Heavy drinking is defined for men as consuming five or more drinks on any day or 15 or more per week. For women, heavy drinking means drinking four or more on any day or eight or more drinks per week.3 According to the Substance Abuse and Mental Health Services Administration, heavy alcohol consumption can include binge drinking on five or more days in the past month.
Moderate drinking (i.e., nonproblem drinking and not to be confused with the International Classification of Diseases, 10th edition (ICD-10) nomenclature of moderate use disorder) can be defined as consumption within or below the NIAAA/DGA limits for low-risk drinking and drinking in moderation (NIAAA, 2024b).4 The low-risk classification defines these limits as two drinks for men or one drink for women per day, with no more than 14 drinks for men and 7 drinks for women per week. Women who are pregnant should refrain from drinking alcohol.
According to NIAAA and CDC, one standard drink contains 14 grams (0.6 U.S. ounces) of alcohol, which is about 12 ounces of beer, 5 ounces of wine, or 1.5 ounces of brandy or distilled spirits (NIAAA, n.d.).5 These equivalents depend on the amount of alcohol contained in a beverage. For example, the alcohol content of table wine typically varies between 12 percent and 15 percent.
In making comparisons of health outcomes for moderate drinking compared with people who do not drink, it is important to note that categories of nondrinkers may well include former drinkers who may be persons with a former AUD or others who may have quit drinking because of health problems. Their inclusion with control or nondrinker groups may bias the health status of former regular drinker or current abstainer cohorts toward a compromised health status despite their current no-to-low level of drinking (often called abstainer bias). A further consideration is the possibility that heavy alcohol consumption during youth carries a liability for accelerated aging in older adults who are current no-to-low drinkers (Nannini et al., 2023).
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3 https://www.niaaa.nih.gov/alcohol-health/overview-alcohol-consumption/moderate-binge-drinking (accessed September 23, 2024).
4 https://www.niaaa.nih.gov/alcohol-health/overview-alcohol-consumption/moderate-binge-drinking (accessed September 23, 2024).
5 https://www.niaaa.nih.gov/alcohols-effects-health/overview-alcohol-consumption/what-standard-drink (accessed September 23, 2024).
COMMITTEE’S TASK AND APPROACH
The Consolidated Appropriations Act, 2023, section 772, requires that USDA, in consultation with the secretary of HHS, contract with the National Academies of Sciences, Engineering, and Medicine (the National Academies) to convene an ad hoc committee to undertake a review of the current scientific evidence on the relationship between alcohol consumption and health outcomes. The Statement of Task (Box 1-3) requests a review of evidence regarding eight questions related to alcohol consumption and health outcomes that were previously published by USDA and HHS and reviewed by NESR. The committee was asked to prioritize the evidence and determine whether it was sufficient to support a systematic review that could be used to answer each question. If enough research was available to conduct a systematic review, the committee was to determine if it should conduct an original systematic review or if a high-quality existing systematic review can be used to answer the question. The committee was then asked to produce a report summarizing the evidence in conclusion statements (graded to indicate the strength of evidence) but to not include dietary guidance statements, recommendations, or advice. In response to this congressional request, the National Academies empaneled a committee of 14 experts in the eight areas of health specified in the Statement of Task as well as experts in systematic reviews and those with previous experience with the DGA and in public.
Approach to the Task
To approach its task, the committee first convened public information gathering sessions, which included public comment sessions (see Appendix B). Based on the large body of evidence linking heavy alcohol consumption to health problems and the exclusion of binge drinking by the sponsor (Stoody, 2024), along with current dietary guidance that people should not initiate alcohol consumption to improve their health, the committee interpreted its task to focus on evidence related to moderate alcohol consumption. Although individual studies used terminology variations such as light-to-moderate, the committee adopted the term moderate as defined in Box 1-4. Evidence that met this definition of moderate served as the upper threshold of alcohol consumption that the committee considered when developing its findings and conclusions for this consensus study.
Based on the eight questions from the Statement of Task, the committee developed search strategies to support evidence scans of the published literature from multiple databases to support its assessment and prioritization process for reviewing the current literature to determine whether the quality and availability of peer-reviewed published evidence were sufficient to conduct a systematic review. For each of the eight questions in
the Statement of Task, the committee reviewed and assessed the results of the evidence scan and decided if a systematic review of more recent literature (i.e., articles published since the last systematic review used by the DGAC in developing the DGA—see Chapter 2) was needed. With a goal of completing this report in time to inform the DGA 2025–2030, the committee decided to undertake de novo systematic reviews rather than perform updates and reanalysis of past reviews. Protocols for these systematic reviews were registered in the PROSPERO international database for systematic reviews to avoid duplication of effort, reduce reporting bias, and promote transparency (Schiavo, 2019),6 and the protocols were carried out by the Academy of Nutrition and Dietetics Evidence Practice Center at the request of the committee.
The approaches to the eight questions limited the population studied to nonpregnant adults of legal “drinking age” (21 years of age and older). In identifying the literature, an important requirement was to have a comparison group that did not combine never drinkers with former drinkers because of the resulting abstainer bias that would occur; therefore, results in this report are not directly comparable to past evidence that does include such abstainer bias.
Each systematic review required tailoring to its topic. For example, the committee identified many recent peer-reviewed publications (and systematic reviews) on the relationship of alcohol consumption to the health outcomes for the first five questions specified in the Statement of Task. In contrast, the committee identified little evidence for the three lactation-related questions and noted that these topics were not systematically or consistently addressed in prior versions of the DGA (Box 1-2); accordingly, the committee used a different search strategy, as described in Chapter 8, for the lactation questions. Where evidence on any topic was determined to be insufficient for a quantitative synthesis (i.e., meta-analysis), the committee performed a systematic review with a narrative synthesis of the evidence.
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6 CRD42024563137, CRD42024566062, CRD42024564414, CRD42024563189, and CRD42024545562.
In interpreting the Statement of Task term “certain cancers” (Box 1-3), the committee chose to be inclusive of all cancers for which there were studies of risk associated with moderate alcohol consumption within the search time frames described in Chapter 2. The committee asked for systematic reviews addressing the seven types of cancer for which the National Cancer Institute reports increased risks associated with moderate alcohol use: breast, oral, pharyngeal, laryngeal, esophageal, colon, and rectal (ACS, 2020; NCI, 2021). All other cancers with articles published within the search time frame were determined to have insufficient numbers of articles to warrant systematic reviews (see Chapter 4).
In reviewing the evidence for the question in the Statement of Task, “What is the relationship between alcohol consumption and growth, size, body composition, and risk of overweight and obesity?” the committee chose to exclude body composition from its review, findings, and conclusions, owing to limitations of reporting this outcome even in clinical settings (see Chapter 5).
Similarly, for the question, “What is the relationship between alcohol consumption and risk of cardiovascular disease?” the committee restricted its review, findings, and conclusions to the composite three-point major adverse cardiovascular events (3P-MACE) outcome, which consists of nonfatal myocardial infarction, nonfatal stroke, and cardiovascular death. The committee restricted neurocognition outcomes to dementia, Alzheimer’s disease, and cognitive decline for the question, “What is the relationship between alcohol consumption and neurocognitive health?”
ORGANIZATION OF THE REPORT
This report is organized into nine chapters. Chapter 2 delineates the committee’s approach to the task including search strategies, methodological considerations with alcohol consumption research generally, and the methods used in this report. The next five chapters present discussions and de novo systematic reviews for the association between moderate alcohol consumption and health for five of the questions in the Statement of Task: all-cause mortality (Chapter 3), weight changes, (Chapter 4), cancer (Chapter 5), cardiovascular disease (Chapter 6), and neurocognition (Chapter 7). Chapter 8 presents the committee’s review of the three questions about lactation. Future research, including methodological considerations and research gaps regarding research on alcohol and health, is discussed in Chapter 9. Biographical sketches of the committee members are provided in Appendix A. Open session agendas are presented in Appendix B. The timeline of screening for eligibility and committee decisions are provided in Appendix C, and A Measurement Tool to Assess Systematic Reviews (AMSTAR-2) tables are in Appendix D. The search terms and results for literature searches conducted and all results for the commissioned systematic reviews conducted
by the Academy of Nutrition and Dietetics are presented in Appendixes E–I.7 Although the three questions in the Statement of Task about maternal alcohol consumption during lactation did not result in a systematic review, the search terms are provided in Appendix J.
CONTEXTUAL ISSUES FOR THIS REPORT
As described above, every edition of the DGA is informed by different sources of information. This study was congressionally mandated to address the effect of alcohol on health for consideration by the DGAC as a part of the systematic reviews informing the DGA. Within the Statement of Task and with consideration of previous findings and conclusions of the various DGAC scientific reports, the committee adhered to the standard scientific protocols for conducting systematic reviews while recognizing the urgency of delivering this report to inform recommendations of the DGA.
Within this context, the committee sought to apply the most comprehensive and rigorous methods available in the specified time frame to inform the DGA process. This report evaluates primary research published since 2010 (and 2019 for all-cause mortality, given that it was last reviewed for the 2020–2025 DGA) and is but one piece of a multifaceted process to develop the DGA. The committee notes that there is an additional body of research published before and subsequent to (e.g., Ortolá et al., 2024) the publication search dates used for identifying evidence examined in this report. As described in the methodology in Chapter 2 and consistent with the NESR process, this report relied on evidence synthesis of primary studies and did not include results from existing systematic reviews and meta-analyses. Lastly, the committee’s decision to address the effect of bias caused by including former drinkers with never drinkers in the comparison group for evaluating the health effects of moderate drinking resulted in the exclusion of several otherwise relevant studies. The committee has determined that the evidence presented herein may be useful for policy making and informing future research.
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7 Appendixes E through J are available online at https://nap.nationalacademies.org/catalog/28582.
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