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Introduction

ENERGY INTAKE AND EXPENDITURE IN HUMANS

In nutrition, energy balance is the difference between the amount of energy consumed through the diet and the amount required to sustain the body’s functions, such as respiration, circulation, metabolism, and physical activity. Overconsumption of dietary energy that is not matched by increased physical activity energy expenditure can result in weight gain. Similarly, underconsumption of dietary energy without a reduction in physical activity energy expenditure can result in weight loss. Although fluctuation in daily energy intake is common, individual responses to energy balance and body weight are variable

All energy supplied by foods derives from consumption of macronutrients: carbohydrates, fats, protein, alcohol, and to a lesser extent, polyols, organic acids, and novel compounds. A high-level view of how food energy flows through the body is shown in Figure 1-1. Energy remaining after accounting for the losses shown in the first two steps of the figure is referred to as metabolizable energy, which is defined as the “amount of energy available for total (whole body) heat production at nitrogen and energy balance” (Livesey, 2001, p. 283). After this point, additional energy is used for the metabolic processes of digestion, absorption, and intermediary metabolism, which is measured as heat production and commonly referred to as the thermic effect of food (TEF) or diet-induced thermogenesis (DIT). While there has been an evolution of the food energy conversion factors over the years, the Atwater general factor is typically used for its simplicity. It uses a single factor for each macro-

nutrient and alcohol as follows: 4 kcal/g for carbohydrate and protein, 9 kcal/g for fat, and 7 kcal/g for alcohol.

Over the past few decades, many individuals residing in the United States and Canada have experienced an imbalance in their energy input

and output such that weight status has trended toward increased overweight and obesity across demographic groups. Since 2016, the National Health and Nutrition Examination Survey (NHANES), the Canadian Community Health Survey (CCHS), and the Canadian Health Measures Survey (CHMS) have indicated that the combined prevalence of overweight and obesity among U.S. adults aged 20 years and over is 73.6 percent and 63.1 percent among Canadian adults aged 18 and over (Fryar et al., 2020; Statistics Canada, 2019). This trend of higher weight status is especially challenging for younger individuals, who are at risk of living with this condition over the long term. Results from the NHANES 2017–2020 estimate that prevalence of obesity among U.S. children aged 2 to 19 years is 19.7 percent (Stierman et al., 2021).

These trends in overweight and obesity have been identified using body mass index (BMI) as cut points. BMI is defined as body weight in kilograms divided by the square of height in meters. The National Institutes of Health (NIH) and the Centers for Disease Control and Prevention (CDC) guidelines use the original World Health Organization (WHO) cut points to define (for adults) underweight as BMI < 18.5, normal weight as BMI 18.5–24.9, overweight as BMI 25.0–29.9, and obese as BMI ≥ 30 (CDC, 2022; NIH, 2021; WHO, 2010).

An update of the WHO classifications added categories to identify severe underweight (BMI < 16.5) and to further define obesity risk in adults as class I (BMI 30.0–34.9), class II (BMI 35.0–39.9), and class III (BMI ≥ 40.0) (WHO Expert Consultation, 2004). In both clinical and community settings, calculation of BMI remains the easiest and most readily accessible tool for identifying individuals at risk of adverse health outcomes related to being overweight or underweight (Gonzalez et al., 2017). It is important to note that BMI is an insensitive measure because it assumes that there is an optimal weight range, regardless of body composition or association of BMI with morbidities and mortality. BMI does not account for interindividual variability by age, sex, ethnicity, or health status (Gonzalez et al., 2017; Pasco et al., 2012).

The need to reexamine the Dietary References Intakes (DRIs) for energy, last updated in 2005 (IOM, 2002/2005), arose primarily from two factors. First is the continued rise in BMI-defined prevalences of overweight and obesity. Second, new scientific evidence has advanced knowledge about the energy requirements of individuals to balance energy expenditure and promote a normal weight status and reduce risk of chronic disease. This report examines that evidence and provides updated estimated energy requirements for the United States and Canada by age, sex, and life-stage group.

BACKGROUND FOR THE STUDY

History and Changing Nature of the DRIs

The DRIs are a set of evidence-based nutrient reference intake values for a range of age, sex, and life-stage groups that are used in the United States and Canada for planning and assessing diets of individuals and groups. The DRIs also serve as reference values in design and evaluation of research studies; development of dietary guidelines, food guides, and product labeling; planning and monitoring of nutrition-related public health programs and initiatives including military nutrition standards; and nutrition counseling and education programs.

The current DRIs are an expansion of the original intake value, the Recommended Dietary Allowance (RDA), which served as the nutrient intake standard for the U.S. population from 1941 through 1989. For all nutrients other than energy, DRI values may include the Estimated Average Intake (EAR), the Recommended Dietary Allowance (RDA), the Average Intake (AI), and, when applicable, the Tolerable Upper Intake Level (UL). Acceptable Macronutrient Distribution Ranges (AMDR) were developed to provide guidance on the relative ranges of energy intakes from carbohydrates, proteins, and fats. The AMDR was intended to provide guidance on intake ranges of energy nutrients associated with reduced risk of chronic diseases while also ensuring that the intakes of essential amino acids and fatty acids and total protein could be met. More recently, the Chronic Disease Risk Reduction Intake (CDRR) was added for evaluating the relationship of nutrients to chronic disease risk. Table 1-1 provides definitions of the current DRI values.

For energy, the DRI value is expressed as the estimated energy requirement (EER). This value is unique among DRIs because energy intakes outside the EER would be expected to result in weight gain or loss rather than nutrient deficiency or toxicity. The EER as originally defined for DRIs is a level of energy intake from food that is predicted to balance energy expenditure relative to an individual’s body size and composition and level of physical activity that is consistent with long-term health, and that allows for the maintenance of normal physical activity. In children and pregnant or lactating females, the EER includes energy needs associated with tissue accretion or production of milk at rates consistent with maintaining health (IOM, 2002/2005).

Since publication of the first DRIs for macronutrients (IOM, 2002/2005), considerable attention has focused on developing more robust and transparent approaches to the DRI process, such as the use of systematic reviews rather than narrative reviews of the evidence. In 2007, the Institute of Medicine (IOM) held a workshop to identify issues important for enhancing the process of DRI development. This workshop

TABLE 1-1 Definitions of Dietary Reference Intake (DRI) Values

SOURCES: IOM, 2002/2005; NASEM, 2017.

highlighted the many challenges related to incorporating chronic disease endpoints into the DRI process. The workshop summary, The Development of DRIs 1994–2004: Lessons Learned and New Challenges, helped advance some of these issues, including a proposed overall organizing framework, but scientific challenges related to the use of chronic disease endpoints remained (IOM, 2008).

In 2015, the Joint U.S. and Canadian Federal DRI Working Group (federal working group) convened a public workshop and published its report, Options for Basing Dietary Reference Intakes (DRIs) on Chronic Disease Endpoints: Report from a Joint U.S./Canadian-Sponsored Working Group (Yetley et al., 2017). A committee convened by the National Academies of Sciences, Engineering, and Medicine (the National Academies) subsequently used the federal working group’s report as a resource for a consensus study. The committee’s report, Guiding Principles for Developing Dietary Reference Intakes Based on Chronic Disease, provided guidance for facilitat-

ing and standardizing the use of chronic disease endpoints in future DRI reviews (NASEM, 2017).

As noted above, prior to the publication of the CDRR, the AMDR was calculated as an intake amount needed to achieve energy balance to reduce risk of overweight or obesity. When intakes of macronutrients fall above or below the AMDR, the risk for the development of chronic disease (e.g., diabetes, cardiovascular disease, cancer) appears to increase. There is emerging evidence about the role of factors influencing energy balance in chronic disease risk that will influence future DRI reviews linked to the EER, the AMDR, and energy intake recommendations. Notably, digestible dietary fiber has been identified as the primary source of energy needed to support gut microbiota. Insufficient fiber intake can lead to the depletion of human gut microbiota diversity and beneficial metabolites. Inadequate intake of digestible fiber may increase risk of adverse health outcomes, particularly metabolic syndrome and obesity-related chronic disease (Hervik and Svihus, 2019).

In response to the challenges identified in these reports and at the request of the federal working group, the National Academies convened an expert panel to explore these issues in depth. In 2020, it held a series of internal expert meetings to develop a strategic approach to review the DRIs for all macronutrients and energy. The committee identified and invited subject-matter experts to participate in open session discussions with the committee on the DRI conceptual framework, the state of the science relevant to the review of macronutrients, and the process for approaching a new DRI review. The invited experts discussed using an umbrella literature review (a review of existing systematic reviews), with additional studies included when necessary for identifying new data related to DRIs for energy and macronutrients and chronic disease endpoints. The invited experts also discussed prioritization criteria, including significant new data, implications for public health, current controversies related to macronutrients, and the usability of current DRIs for macronutrients and energy. Lastly, they identified first steps in a prioritization process to ascertain user needs and consult with subject-matter experts to determine the status of DRI-relevant data.

Updating and Revising the 2005 DRIs for Energy

In the IOM (2002/2005) report, EER prediction equations for free-living individuals with normal weight were developed from data on total daily energy expenditure measured by the doubly labeled water (DLW) technique. As identified in the above-mentioned expert meeting discussions, an expanded DLW database has since been assembled by Speakman and colleagues (2019). The DLW database, hosted by the International Atomic Energy Agency (IAEA), consists of 6,621 DLW

measurements recorded since the early 1980s across 23 countries. These cumulative data provide an opportunity to expand and fill gaps in the original DLW database as well as reevaluate the equations used to derive the EER. A specific advantage of using the IAEA DLW database is that it provides an estimate of total energy expenditure over a period of several days, enabling adjustment for reducing the effect of day-to-day variability in energy intake and physical activity.

Integral to the calculation of EERs is the delineation of energy needs by physical activity level (PAL). Recent evidence indicates that the PAL coefficient is not constant but varies significantly across age groups, particularly during the first 20 years of life. The incorporation of newer DLW and outcome data in the IAEA database may better define physical activity levels across age/sex groups relative to body weight, fitness, and other aspects of age and sex groups.

The range of new evidence published since the first DRI review of energy and macronutrients (IOM, 2002/2005) provides a compelling reason to reevaluate the factors that affect derivation of the EER to determine human requirements for energy intake and expenditure and to assess the role of energy in reducing the risk of chronic disease. Further, evidence of changes in population health (i.e., increased prevalence of obesity and risk of chronic disease) necessitated reconsideration of the DRI population in order to be more inclusive. The committee hereinafter defines the DRI population as the general population, rather than the generally healthy population. This reevaluation is best done in advance of performing new DRI reviews of dietary macronutrients.

THE COMMITTEE’S TASK AND APPROACH

In response to new and emerging evidence related to factors affecting derivation of the EER, the role of energy in supporting metabolic functions, and energy’s relationship to the risk of chronic disease, the federal working group asked the National Academies to convene a consensus committee to undertake a review of the Dietary Reference Intakes for Energy (Box 1-1). Specifically, the committee was asked to assess the human requirements for energy intake and expenditure and to consider age, sex, body size, body composition, level of physical activity, race/ethnicity, and other factors that may be warranted based on available data. Other significant variables for consideration include energy for growth and maturation and to support pregnancy, energy needs postpartum, energy intake amounts to achieve and maintain weight loss or weight gain, energy requirements to support recovery from disease and treatments or interventions such as surgery, and the health consequences of chronic overnutrition or undernutrition across the life span.

In response to the sponsor’s request, the Health and Medicine Division of the National Academies established a committee with expertise in the following:

• Energy metabolism across the life span, including pregnancy, lactation, and menopause;
• Physical activity;
• Human nutrition across the life span;
• Clinical trials, including design and conduct of diet interventions;
• Methods in energy metabolism and body composition;
• Systematic review methodology, including quality and risk-of-bias assessment;
• Statistics, modeling, and analysis methods;
• Application of the DRI framework; and
• Use of doubly labeled water data.

Biographical sketches of the committee members are provided in Appendix B.

The committee began by gathering evidence from several sources, which involved conducting an umbrella review of systematic reviews and gathering information during open meetings that it convened with subject-matter experts (see Chapter 3 and Appendix C). The committee also engaged expert consultants and requested data analyses from CDC and Statistics Canada.

The committee also participated in open-session discussions held by the Standing Committee for the Review of the Dietary Reference Intakes Framework (the standing committee) to discuss questions about defining the DRI population. In a subsequent open session, the standing committee reported its guidance on this question to the federal working group and the Committee to Review the DRIs for Energy (i.e., the present

committee). In a letter report to the federal working group, the standing committee noted that the report, Guiding Principles for Developing Dietary Reference Intakes Based on Chronic Disease (NASEM, 2017) stated that the general U.S. and Canadian populations included individuals with obesity and other chronic conditions such as hypertension or diabetes, as well as individuals at risk of chronic disease who do not meet DRI exclusion criteria where they exist. Consistent with the findings in the National Academies’ report (NASEM, 2017), the standing committee concluded:

Based on the totality of evidence gathered, open-session discussions with subject-matter experts, guidance from the standing committee, and its deliberations, the committee formulated an approach to address its work and derive the findings, conclusions, and recommendations that are presented in this report.

ORGANIZATION OF THE REPORT

This report is organized into nine chapters. This first chapter describes the background for the study, the statement of task, and the study approach. Chapter 2 provides an overview of the DRI process. Chapter 3 describes the committee’s methodological approach to its task. Chapter 4 reviews metabolic factors that affect energy expenditure and requirements. Chapter 5 presents the prediction equations for estimated energy requirements that the committee developed. Chapter 6 describes dietary intake assessment and body composition from national surveys and compares them between U.S. and Canadian populations. Chapter 7 illustrates applications of the DRIs to assess and plan energy intakes for individuals and groups. Chapter 8 characterizes relative risk and discusses public health implications of inadequate and excessive energy intakes and expenditure. Chapter 9 presents research gaps and recommendations.

REFERENCES

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