Jumat, 08 Juli 2016

Controversies Involving Nutrition and Health


source: www.listtoday.org/

Several recent meta-analyses illustrate the mixed messages that can surface
about what role, if any, diet has in health promotion [3–6]. While these
findings frequently reflect disagreements in interpretation among scientists,
they also lead to confusion among consumers and can erode the trust that they
have for the scientific enterprise. Thus, greater attention needs to be given to
the totality of the information rather than to individual studies in defining the
health significance of the diet. It would be truly disappointing if a simple summation
of evidence-based nutrition studies became the ‘gold standard’ since the
majority of case-control and cohort studies are not simple repeats of previous
undertaking and thus vary enormously in experimental design, tested populations,
and outcome measures; all of this can surely influence overall interpretations.
Unfortunately, conclusions based on a meta-analysis may even depend on
the method the authors used to select trials for inclusion in the analysis. Thus,
summaries of evidence which do not consider the biological response, plausibility
and consequences are doomed to create more confusion than they resolve.
What is increasingly clear is that inadequate long-term intervention studies
exist for making definitive conclusions about who will benefit and who might
be placed at risk by dietary change. Indisputably, well-controlled long-term
intervention studies which incorporate the newest technologies hold the greatest
promise for unraveling the complex interplay between diet and health.
Future clinical studies must incorporate genomics in the study design, and not
just use it as an analytic approach to confounders to data interpretation.
Considerable preclinical evidence linking diet to health outcomes centers
on the response to a single bioactive component as a modulator of a key cellular
process or series of critical processes [7, 8]. Both in vitro and in vivo studies
suggest that multiple targets are likely responsible for the phenotypic response
to foods and/or dietary supplements [8, 9]. These targets may be involved in cell
division, inflammation, apoptosis, compound bioactivation or a host of other
biological processes which influence the phenotype. Focusing on a process
which can be modified by one or more bioactive food components will help
with a systematic approach to understanding the role of diet in health promotion.
However, in some cases, research suggests that whole foods may offer
advantages over isolated components, possibly indicating that multiple food
components or multiple targets are needed to bring about a desired effect.
Nutrient-nutrient, as well as nutrient-drug interactions can be significant determinants
of the overall phenotypic response. For example, the ability of n–3 fatty
acids to increase the sensitivity to anthracyclines is dependent on vitamin E
intake [10] or the benefits of calcium are generally dependent on the intake of
vitamin D [11]. While not as frequently examined, negative interactions among
food components or nutrient-drug interactions are also possible and such lines
of investigation deserve added attention to assist with the identification of
potentially vulnerable individuals.

Since the quantity of exposure can markedly influence the outcome, it is
imperative that nutrition studies use physiologically relevant concentrations and
consider the totality of the diet as a factor influencing the overall response
[7, 8]. Sadly, multiple exposures in humans are hampered by the availability of
definitive biomarkers that reflect a long-term health outcome. Finally, it is
worth noting that studies which examine the impact of dietary change through
more than one phase of life are exceedingly rare, yet are desperately needed if sense is to be made out of the diet-health conundrum and the opportune time for
intervention to bring about a desired change. A narrow and simplistic view of
dietary patterns is obviously problematic and may contribute to misconceptions
and thus the confusion that exist today about the importance of functional foods
and health promotion and disease prevention.
Clearly, predicative biomarkers which can evaluate long-term consequences
are fundamental to resolving dietary issues which cannot be addressed
for practical or ethical reasons. At present, few validated biomarkers are available
for assessing the impact of diet in health promotion. Similar to environmental
toxicity research, it is likely that at least 3 different types of biomarkers
will be needed to assess the impact of diet (fig. 1) [12]. Foremost among these
is the need to accurately identify exposures to foods and their components.
Obviously, if the effective concentration does not reach the target tissue, there is
little hope that it will be effective in bringing about a desired effect. Likewise,
sensitive and reliable biomarkers for identifying the impact of bioactive food
components are in short supply. These ‘effect’ biomarkers should provide sensitive
and time-/dose-dependent information about how the food component(s)
modifies/modify one or more specific cellular processes [6–8]. Assuming this/
these molecular target(s) can be analyzed in the affected or surrogate tissue, it
can ideally be an effective biomarker for assessing the response to physiologically
relevant exposures to foods or their components. Finally, it is clear that we need to assess susceptibility factors including genetic and epigenetic markers
which can reflect an individual’s responsiveness to the biological response to a
food, food patterns or dietary supplements. The use of such information may
assist with improving the usefulness of standard 24-hour recall and food frequency
questionnaires by developing predictive models that take into account
genetic factors influencing absorption, metabolism and excretion. These susceptibility
biomarkers (fig. 1) will again provide important clues about responders,
both positive and negative, in the molecular target to diet-induced phenotypic
change.

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