chapter 10 Introduction to clinical nutrition: food and supplements

INTRODUCTION AND OVERVIEW

A healthy diet and lifestyle are prerequisites for wellness and the prevention of many chronic diseases. Identifying specific components in the diet that promote health remains a challenge, as the relationships are complex and often multifactorial. Nutritional epidemiology and interventional research with specific food components and nutritional supplements provide evidence that helps us to better understand the role of nutrition in health promotion and disease and provide patients with better advice.

Early studies in nutrition originated from observations of deficiency and its signs and symptoms. Probably the best known of these is the identification of scurvy as a diet-related disease with a potentially fatal outcome, sometime in the fifteenth century. Approximately 200 years later, James Lind identified citrus fruits as both a preventative treatment and a cure; however, it wasn’t until the 1920s that vitamin C was isolated and identified as the true source of the treatment’s success.

Contemporary research using more sophisticated methodologies has allowed new approaches to be used in nutritional studies. These include large prevalence and community studies, cohort and case-control studies and randomised trials. Generally, food intake diaries and surveys, biomarkers (e.g. vitamin C in urine), clinical indicators (e.g. cholesterol) and anthropometry are used as key measurements. Although these measures are useful, they are not without limitations, and research is still being conducted to identify new and more accurate measures and methods of assessment.

Clinical nutrition is the use of this information in diagnosis, disease prevention and treatment, and health promotion. Treatment consists of dietary manipulation and using food and nutritional supplements as medicine, based on individual assessment.

INADEQUATE NUTRITIONAL INTAKE

Many factors can affect an individual’s nutritional status. The most obvious factors are human biological factors and physiological phases, such as age, gender and the stages of growth, pregnancy, lactation and older age. Less obvious but just as influential can be system factors such as the healthcare system, the education system and the food supply system (industry, agriculture and institutions).

Factors affecting nutritional status can be divided into three broad categories: external, internal and food factors. These are listed in Table 10.1.

TABLE 10.1 Factors affecting food choices and nutritional status

External factors	Internal factors	Food factors
Educational system: nutrition and health knowledge	Age, gender and genetics affecting nutritional requirements	Food supply chain: • availability • nutritional quality • affordability • clean and uncontaminated
Peer beliefs and practices	Physiological phases affecting nutritional requirements, e.g. growth, pregnancy, lactation, older age	Quantity of food consumed
Family beliefs and practices	Altered organ function or metabolism, e.g. malabsorption syndromes, intolerances	Flavour and palatability of food
Ethnicity and cultural influences	Personal attitudes, beliefs and behaviours, e.g. following a fad diet, vegetarianism	Texture of food
Religious beliefs and practices	Appetite, e.g. poor appetite due to disease or disease treatment	Appearance of food
Occupation	Dental health, e.g. ill-fitting dentures, sensitive teeth, gum disease	Odour of food
Media and advertising	Physical disabilities making it difficult to shop for fresh produce, prepare food, self-feed, chew and/or swallow food	Availability of nutrients in foods, e.g. binding to other food components
Economic influences, e.g. household finances, economy of community/country	Gastrointestinal symptoms, e.g. nausea, vomiting
Medication use (see Table 10.2)	Psychosocial factors, e.g. isolation, confusion, loneliness
Cooking techniques, e.g. up to 100% of vitamin C can be lost through cooking	Everyday moods and emotions, e.g. comfort eating

Source: adapted from Braun & Cohen (2007)¹

One factor that tends to be overlooked is the effect of medication use on nutritional status and the possibility of inducing deficiency with long-term use. Table 10.2 gives examples of some commonly used drugs and the nutrients that can be affected. In particular, clinicians should consider the nutritional result of chronic medication use in individuals who have a barely adequate diet, take multiple drugs or are elderly and frail.

TABLE 10.2 Examples of drugs and their interactions with nutrients

Drug or drug class	Nutrient(s) affected
Loop diuretics	Increased urinary excretion of vitamin B₁, magnesium and zinc
Oral contraceptive pill	Reduced levels of folate, vitamins B₂, B₃, B₅, B₆, B₁₂ Increased serum levels of vitamin A
Corticosteroids	Reduced calcium, vitamin D, calcium and iron
Antibiotics	Reduced endogenous synthesis of vitamins B₁, B₅
Proton pump inhibitors and H₂ antagonists	Reduced dietary absorption of folate, iron, vitamin B₁₂
Orlistat	Reduced dietary absorption of vitamins A, D, E, K
L-thyroxine	Insoluble complexes formed with iron, magnesium, calcium and zinc, resulting in reduced drug and nutrient absorption

Source: Braun & Cohen 2007.¹

RDAs AND RDIs

The concept of recommended daily allowances (RDAs) originated in the United States in the 1940s as a basis for setting the poverty threshold and food stamp allotments for the military and civilian populations during times of war and/or economic depression.² At this time, the first RDAs were determined for vitamins A, C, D, E, thiamine, riboflavin, niacin, energy, protein and the minerals calcium and iron. These levels were established by observing a healthy population’s usual dietary intakes and extrapolating from this information.

Over the subsequent 50 years, new ideas about and scientific research into health and nutrition emerged, and as a result the original concept of RDAs required modification. The key ideas to emerge were as follows:

• Many other nutrients and food components are important for health, so the list of substances with recommended levels needed to be expanded.

• There was a need to delineate the appropriate uses of the derived nutrient standard recommendations—that is, nutrient intake levels specified for a variety of uses.

• Clearer endpoints by which to set adequacy levels were required.

• Nutritional intake levels were required to prevent not only deficiency, but also future disease.

• Risk assessment was needed.

As a result of these developments, a new framework was set up in the mid-1990s. It aimed to establish new nutrient intake recommendations to meet a variety of uses and to base nutrient requirements on the reduction of chronic disease risk, with a clear rationale for the endpoints chosen. The new guidelines still contain RDAs but have been expanded to include three new intake recommendations: estimated average requirements (EARs), adequate intake (AI) and upper level (UL) intake.

Revisions were also afoot in Australia and New Zealand, and in 2006 the National Health and Medical Research Council (NHMRC) published its newly adjusted nutritional guidelines.³ These new guidelines are far more comprehensive than previous versions and have incorporated some of the new initiatives developed in the United States.

The guidelines are for healthy people and have been differentiated into gender and age requirements, assuming an average body weight for the adult male as 76 kg and female as 61 kg. Four key terms used in the document are defined here:

• estimated average requirement (EAR)—a daily nutrient level estimated to meet the needs of 50% of the healthy population in a particular life cycle and gender

• adequate intake (AI) when RDI cannot be determined—based on observed or experimentally derived estimates of daily nutrient intake to meet the needs of healthy individuals; produced when the available evidence is insufficient or conflicting

• upper level (UL) of intake—the highest daily nutrient intake estimated to pose no risk to almost all people in the general community.

In some instances, the new RDI values for specific nutrients have increased substantially since the previous guidelines (e.g. for iron and folate), whereas others have increased marginally (e.g. for calcium) or decreased (e.g. zinc requirements for adult females).

PITFALLS OF THIS SYSTEM

Setting nutrient reference values presents many challenges. Russell,² from the Human Nutrition Research Center on Aging at Tufts University in the USA has outlined eight obstacles.

• Many of the databases used when determining reference values provided information from studies that only evaluated dietary intakes and ignored supplemental intakes.

• Most studies fail to consider the differences in bioavailability of nutrients depending on the food matrixes they are in.

• Few long-term studies are available, so information is extrapolated from short-term studies (e.g. 1–2 weeks).

• There is a paucity of information about children, adolescents and the elderly from which to establish reference values with any degree of certainty.

• Most experiments fail to consider the interaction between nutrients.

• There is a lack of information about individual variability in response to the indicators in question.

• There is a lack of dose–response information—that is, responses to multiple levels of the same nutrient in the same individual.

• There are doubts about the purported action of the nutrient on the indicator being considered—e.g. multiple factors affect an outcome, so a cause and effect relationship can be uncertain and only speculative.

Overall, for many nutrients and food components there is no adequate database to confidently set an RDI or UL, and the scientific basis for many of the reference values is weak. Clearly the reference values we now use can only be a general approximation and loosely relevant to the general healthy population, with even less relevance to the individual with comorbidities or special needs.

To create a set of reference values with greater accuracy and relevance, substantial research funding is required. Even then, some values such as ULs may never be entirely accurate, as ethical considerations would prevent such research from ever being conducted.