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N E W S A N D V I E W S

Challenges in dietary guidance:a US perspective

J. SlavinDepartment of Food Science and Nutrition, University of Minnesota, USA

Summary Good nutrition is essential to human health. But how do we define good nutrition?

Nutritional needs vary greatly through the life cycle so a diet that promotes healthy

growth and development for a child may not be optimal for the elderly. Also,

humans are adaptable to a wide range of dietary patterns and food intakes and their

varying cultural preferences must be considered when making dietary recommen-

dations. This paper describes the history of the development of dietary guidelines in

the US and discusses some of the complexities around the development of advice for

an optimal diet at a population level.The Dietary Guidelines for Americans have been published every five years since

1980. The development of the most recent Dietary Guidelines for Americans (2010)

involved an evidence-based review process to evaluate the strength of the relation-

ships between food intake and disease outcomes. Unfortunately, many accepted

relationships between food patterns and disease outcomes are not strongly based on

evidence-based reviews. This creates a challenge for government agencies when

developing population guidelines for dietary intake, as well as for future committees

charged with agreeing on the basis of an optimal diet for good nutrition.

Keywords: chronic disease, diet, dietary guidance, evidence-based review, nutrition, United

States

The search for the holy grail of diets, what we should

and should not eat and in what quantities continues tocreate debate. Diet wars are typically waged by the

low-carbohydrate and low-fat camps. Yet, one mustremember that humans are omnivores, with gastrointes-

tinal tracts most similar to pigs. Thus, humans canadapt to a wide range of diets and food intakes.

Through the ages, traditional diets mostly reflect human

access to food supply. Although protein requirementsare set based on ideal bodyweight (i.e. 0.8 g/protein/kg

bodyweight for adults), the amount of carbohydrateand fat in nutritionally adequate diets varies greatly. For

example, traditional Arctic diets contain 80% of energy

from fat, whereas traditional African diets provide 80%

of energy from carbohydrate (Harper 1988). Therefore,the trick perhaps for good nutrition is to consume diets

that contain the appropriate amount of energy, along-side an adequate amount of protein and sufficient quan-

tities of essential vitamins, minerals and fluids; however,such diets may vary in their carbohydrate and fat

content.

Accepted nutrition facts

The relationships between vitamin intake and deficiency

diseases were first studied in the early 20th century andour understanding of the essential role that nutrients

play in health led to the development of nutrient recom-mendations to prevent such deficiency diseases. The US

National Academy of Sciences began issuing Recom-mended Dietary Allowances (RDAs) in 1941. RDAs are

the quantity of nutrients a person needs to consume

Correspondence: Dr. Joanne Slavin, Professor, Department of Food

Science and Nutrition, University of Minnesota, 1334 Eckles Ave.,

St. Paul, MN 55108, USA.

E-mail: jslavin@umn.edu

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DOI: 10.1111/j.1467-3010.2012.01996.x

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carbohydrate) as defined by the Dietary Goals (1977)improve our health? Unfortunately, long-term human

intervention studies investigating the effect of dietsadhering to the DGA guidelines simply do not exist.

Generally, adherence to DGAs is measured in epidemio-logical studies by means of a scoring system the

healthy eating index (HEI), which quantifies adherenceto the various dietary guidelines. One such study by

McCullough et al. (2000) in a population of over100 000 US adult men found that the HEI was only

weakly negatively associated with risk of major chronicdisease. In a subsequent analysis of both the HEI and an

alternative healthy eating index (that considered otherfactors such as type of fat and quality of carbohydrate)

among more than 70 000 women from the NursesHealth study and over 40 000 men from the Health

Professional Follow-up Study over two years of follow-up, adherence to both indices was found to be protective

against cardiovascular disease and diabetes (Chiuveet al. 2012). The authors therefore suggested that adher-

ence to the 2005 Dietary Guidelines may lower risk ofmajor chronic diseases. However, using a 100-point Diet

Quality Index to rate participants diets based onmeeting the 2005 DGA key recommendations Zemora

et al. (2010) did not find a relationship between weightgain among young adults from different ethnic groups

who participated in the Coronary Artery Risk Develop-

ment in Young Adults study (19852005). Their find-ings did not, therefore, support the hypothesis that a

diet consistent with the DGAs benefits long-term weight

maintenance in young American adults.

How were the most recent US DGAsdefined?

Responsibility for the development of DGAs rests withan expert scientific group, the DGAC. In the last DGA

review (in 2010), the DGAC consisted of 13 scientists(including the present author) with a broad range of

expertise in nutrition, physical activity, food behaviourand nutrition through the life cycle. The DGAC is

divided into subcommittees to address specific questions

relating to diet and disease risk. In 2010, there wereeight subcommittees focusing on different dietary issues:(1) alcohol, (2) carbohydrate, (3) energy balance and

weight maintenance, (4) fatty acids and cholesterol, (5)food safety and technology, (6) nutrient adequacy, (7)

protein and (8) sodium, potassium and water. Being amember of the 2010 DGAC, the author of this paper

served as chair of the carbohydrate and protein subcom-mittees and also as a member of the energy balance and

the nutrient adequacy subcommittees.

In their review, the DGAC and related subcommitteesaddressed questions regarding the relationship between

diet and health outcomes by following an evidence-basedreview process with a strict hierarchy of evidence, with

the strongest evidence being found in randomised-controlled trials (RCTs), preferably double blinded. Of

course, dietary food studies typically suffer in this arenaas it is difficult to carry out blind food treatments (e.g.subjects know they are consuming an apple or applejuice). Such trials can work with nutrients however as

nutrients can be added to food or drinks without theknowledge of the participants or investigators (i.e.double blind). Following RCTs, the next strongeststudies are prospective cohort studies (PCs), where a

group or cohort of subjects is studied over time. Foodfrequency instruments are often used to collect dietary

information before any diagnosis of disease, makingthese studies more reliable than cross-sectional studies

where diet and outcome measures are assessed simulta-neously. No case-control studies, animal research or in

vitro studies are considered in any of the DGAC reviews.Typically, cross-sectional studies are only included if no

stronger prospective studies are available for review.Having gathered all of the scientific literature together

(i.e. from RCTs and PCs) the body of evidence foreach question (e.g. what is the relationship between

dietary fibre intake and health outcomes) is then exam-ined in great detail (www.nutritionevidencelibrary.com).

Within a systematic, evidence-based review, the conclu-sions drawn can be deemed as strong, moderate, limited

or lacking data to support them. There may also bestrong evidence of no relationship. For example, nostrong evidence was found to support a relationship

between glycaemic index and disease outcomes in the2010 DGAC review (www.dietaryguidelines.gov).

Agreeing on the strength of the relationship isalways difficult, as, for each question, different types

of studies with a variety of outcomes have been pub-lished. For each question addressed in the evidence-

based report, the precise search criteria, inclusion andexclusion criteria for all of the studies referred to,

including the range of dates searched, is available

on the US Department of Agriculture portal (www.nutritionevidencelibary.com). Such transparency in anevidence-based approach minimises bias, therefore

adding credibility to the findings.

Challenges in evaluating diet anddisease relationships

As is normal in reviews of this kind, there can be issues

relating to contradictory evidence and the DGAC 2010

US dietary guidance 361

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report was no exception. In a critique of the DGAC2010, Hite et al. (2010) suggested that the DGAC

Report does not provide sufficient evidence to concludethat increases in intake of wholegrain foods and fibre,

and decreases in dietary saturated fat, salt and animalprotein can lead to positive health outcomes. Moreover,

they stated that the lack of supporting evidencelimits the value of the proposed recommendations as

guidance for consumers or as the basis for public healthpolicy.

Accepted carbohydrate policy

Although the amount of dietary carbohydrate that

confers optimal health in humans is unknown, the DRIsrecommend adults consume 4565% of their total

energy from carbohydrates (DRIs 2005). The acceptablemacronutrient distribution range for fat is 2035% and

for protein is 1035%. These wide ranges exist becauseprotein recommendations when following a low-calorie

diet require up to 35% of calories to come from protein.Also, achievement of the recommended intake of dietary

fibre requires a certain amount of carbohydrate to beingested.

Carbohydrate is found in a variety of foods anddrinks [i.e. present as starch, sugar (intrinsic/extrinsic)

or fibre]. Fruit and vegetables, wholegrains and milkand milk products are major food sources of carbohy-

drates. Grains and certain vegetables including corn and

potatoes are rich in starch. Fruits contain little or no

starch but sugar in the form of fructose. Regular softdrinks, sugar/sweets, sweetened grains and regularfruitades/drinks (i.e. sugar sweetened) comprise 72% of

the intake of added sugar in the US (Marriott et al.2010).

Limited data exist to suggest that added sugarintake is linked to any adverse health outcome. Gen-

erally, intake of all types of carbohydrates is linked tolower bodyweight in PCs (Gaesser 2007). It is difficult

to measure added sugar in epidemiological studies, sointake of sugar-sweetened beverages (SSBs) is the

proxy for added sugar intake. In PCs there are few

data that support a link between intake of SSBs andhigher energy intake or bodyweight in adults. Evencarbohydrate foods that are generally accepted as

healthy, such as fruits and vegetables, do not havestrong scientific support for a positive effect on health

outcomes (Slavin & Lloyd 2012). For example, arecent systematic review and meta-analysis of the

effects of fruit and vegetable intake on incidence oftype 2 diabetes included six studies, four of which pro-

vided separate information on the consumption of

green leafy vegetables (Carter et al. 2010). Findingsshowed no significant benefits on the incidence of type

2 diabetes with increased consumption of vegetables,fruit or fruit and vegetables combined.

ConclusionsThe DGAC report is an evidence-based, systematic

review written by the DGAC. The DGAs are written bythe US government based on the DGAC report. Food

guides, such as myplate.gov are also produced by USgovernment staff. The translational process taking the

scientific report and turning it into the DGAs has beencriticised for not being transparent to the public or even

to the members of the DGAC (Hite et al. 2010). Asystematic evidence-based review of diet and health out-

comes does not, however, yield a blueprint for theperfect diet. Nutrient needs vary greatly over the life

cycle, so a relatively high-fat diet, as recommended forinfants and toddlers, may not be appropriate for an

overweight adult. Generally, a balanced diet containsadequate protein (both in terms of quantity and quality)

alongside sufficient amounts of essential vitamins andminerals. However, such diets can be either high or low

in carbohydrates and fats, and the choice will reflect thecultural norms and traditions of consumers.

Conflict of interest

The author was a member of the 2010 Dietary Guide-

lines Advisory Committee. The views expressed in thispaper are those of the author alone. The author reportsno conflict of interest.

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