Dietary guidance generally recommends that consumers obtain their nutrients from whole foods. The 2010 Dietary Guidelines for Americans indicated some at-risk populations, such as women of reproductive capacity and seniors, should consume fortified foods and specific dietary supplements to achieve nutrient adequacy. The Dietary Guidelines Advisory Committee report noted several nutrients of concern, including potassium and vitamin D. Yet those in the nutrition and health community know there are many other nutrients of concern, including choline, magnesium, iron, and even vitamin A, as suggested by the 2007–2010 NHANES data (Wallace et al., 2014).
There are many factors that influence nutrient uptake and utilization that extend beyond our fundamental understanding of absorption. Mechanisms that regulate the absorption or excretion of nutrients depend on various host factors (Krebs, N.F., 2001; Solomons and Slavin, 2001; King, J.C., 2001; Heaney, R.P., 2001). Some of those factors include age, sex, and physiological state as well as dietary sources and nutrient chemistry. This leads one to consider food factors, such as the presence and content of other nutrients, the presence of nutrient absorption inhibitors, food preparation procedures, and storage conditions and duration.
Many of the nutrient absorption inhibitors are found in plants (Hallberg et al., 1989; Sandberg, A.S., 2002). Three of those inhibitors are phytate, oxalate, and polyphenols. Phytates, which are found in seeds, nuts, vegetables, fruits, legumes, and cereal grains, contribute to an 18%–82% reduced absorption of minerals, such as iron, magnesium, zinc, and calcium. Similarly, oxalates can reduce the absorption of these nutrients. Polyphenols and related compounds, often heralded for their potential health benefits, represent a spectrum of substances that are native to these foods and beverages such as tea, coffee, and wine, as well as herbs and spices.
A recent review summarized the complexities associated with the absorption, distribution, metabolism, and excretion of flavonoids and interrelated phytonutrients (Del Rio et al., 2013). The culmination of evidence indicates the body treats phytonutrients as xenobiotics, for which the body rapidly processes them for urinary excretion (Spencer and Crozier, 2012). For example, the compound resveratrol found in red wine is quickly transformed into its metabolites. These metabolites achieve a maximum plasma nanomolar concentration, often a 1,000-fold diminution from dietary intake, in approximately an hour. Nearly 70% is absorbed, most of which is rapidly excreted (Valle et al., 2004). On the other hand, only about 3% of anthocyanin and related compounds are absorbed and achieve a maximum plasma concentration in about 10 hours, followed by limited loss (<0.1%) via urinary excretion (Del Rio et al., 2010; Wu et al., 2002). These dramatic differences in pharmacodynamics reflect the nature of the sugar moiety, the structure of the parent compounds, and the presence of specific digestion enzymes (McGhie et al., 2003; Prior and Wu, 2006).
Limited evidence indicates an increased number of sugar moieties, such as glucose, galactose, and xylose, associated with polyphenols may improve absorption. Phenolics with rhamnose are not absorbed in the small intestine, but may be subjected to hydrolysis by microflora in the large intestine (Karakya, S., 2004; Manach et al., 2005; Selma et al., 2009).
Dietary supplements represent a chemically hostile matrix (Kondo et al., 1982; Takenaka et al., 1997). Within this matrix and during storage, an array of vitamin analogues often develops. On the other hand, upon close examination of folic acid and vitamin B-12 in this matrix and evaluation of similar nutrients in a food milieu, observations indicate nutrient inequities. With respect to folic acid, the innate chemical forms in food are polyglutamated, reduced, labile, and low bioavailability (~50%). Yet the chemical forms of folic acid in dietary supplements are monoglutamate, oxidized, stable, and good bioavailability (85%-100%) (Caudill, M.A., 2010; Institute of Medicine, 1998; Bailey, L.B., 1998). With respect to the dynamics of vitamin B-12, the best sources are from animal-based foods. Only select bacteria and some algae can synthesize this complex nutrient (Watanabe et al., 2013; Stüpperich and Nexø, 1991). Other factors that impact bioavailability are particle size (Eible et al., 2011; Clemens and Mercurio, 1981), ionic state (Clemens and Mercurio, 1981), types of encapsulation (Zimmerman, M.B., 2003), and co-administration of scripted medications (Yetley et al., 2007).
There remain many questions at the bioavailability of dietary components and health interface. These questions extend into the clinical realm and encompass attributes such as the microbiome and bariatric surgical consequences. Bottom line, is the bioavailability of innate nutrients in foods better than those used in dietary supplements? At this juncture of our understanding of nutrient bioavailability, it just depends.
Roger Clemens, Dr.P.H., CFS,
Chief Scientific Officer, Horn Company, La Mirada, Calif.