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while considering their digestion and digestibility, a whole
gut approach should be used.
Conclusion
Being quantitatively the most important dietary energy
source for most populations, carbohydrates have a special
role to play in energy metabolism and homeostasis. The
overview provided here deals with only selected physiological
effects of energy metabolism and gastrointestinal effects
of carbohydrates and their health implications. Several
carbohydrate-specific theories of appetite regulation have
been proposed but none are universally accepted, although
high-carbohydrate, low-energy-density diets rich in fruit,
vegetables and fibre are often recommended for weight
reduction or prevention of weight gain. Appetite and
hunger, which are of fundamental to survival, appear to
have many layers of control, with one layer compensating or
dominating another in some circumstances. The recent
growth of overweight and obesity throughout the world is
related to lifestyle changes, which have placed the human in
the unusual situation (at least in evolutionary terms) of
having to defend against a combination of persistent
abundance of tasty food and reduced physical activity. There
is a need to better understand the interaction between
energy density, GI/load, palatability and other factors and
their effects on feeding behaviour. At the same time, there is
room to establish more rational national and international
dietary energy systems and to consider greater application of
the NME system, which has some advantages over the more
commonly used, ME system.
The other physiological effects of many carbohydrates
depend on the site, rate and extent of their digestion in and
absorption from the gut. The majority of mono- and
disaccharides, together with maltodextrins and most starch,
are hydrolyzed by pancreatic enzymes in the small bowel
and at the epithelial surface, and the resultant monosaccharide
mixtures absorbed transported to the liver and then
stored or metabolized. These carbohydrates are primarily
energy yielding. The non-a-glucan oligosaccharides have
other properties, in that they increase calcium absorption
and some selectively modify the composition of the large
bowel microbiota to one dominated by bifidobacteria and
lactobacilli, known as the prebiotic effect. Studies to
demonstrate proven health benefits of prebiotics are
awaited.
The prebiotic carbohydrates, along with NSP, RS and some
polyols, reach the large bowel where they are fermented. The
principal end products, SCFA, are absorbed and provide a
further energy source to the tissues. Fermentation benefits
bowel habit, although the effects can be very small, and
provides mechanisms that could be important in cancer
prevention in the colon.
In the digestion of carbohydrates, the gut acts as an
integrated organ with distinct regions of function. The
European Journal of Clinical Nutrition
Physiological aspects of energy metabolism
M Elia and JH Cummings
nature of carbohydrate, both its chemical and physical
properties, together with the physiological responses of
absorption and secretion, local and visceral neuro-endocrine
reflexes, enzyme section and microbial activity, determine
the varied responses to carbohydrate in our diet.
Acknowledgements
We thank Professor Nils-Georg Asp, Professor Arne V Astrup,
Professor Nancy Keim, Dr Geoffrey Livesey, Professor Ian
MacDonald, Dr Gabriele Riccardi, Professor A Steward
Truswell and Dr Ricardo Uauy for the valuable comments
they provided on the earlier manuscript. We also thank Dr
Klaus Englyst and Dr R James Stubbs for helpful comments.
Conflict of interest
During the preparation and peer review of this paper in 2006,
the authors and peer reviewers declared the following
interests.
Authors
Professor John Cummings: Chairman, Biotherapeutics
Committee, Danone; Member, Working Group on Foods
with Health Benefits, Danone; funding for research work at
the University of Dundee, ORAFTI (2004).
Professor Marinos Elia: Since undertaking this work,
Professor Elia has joined a committee to undertake a
systematic review on the role of fibre in enteral tube feeding,
with an unrestricted grant from Numico.
Peer-reviewers
Professor Nils-Georg Asp: On part-time leave from university
professorship to be the Director of the Swedish
Nutrition Foundation (SNF), a nongovernmental organization
for the promotion of nutrition research and its practical
implications. SNF is supported broadly by the food sector;
the member organizations and industries are listed on the
SNF home page (www.snf.ideon.se).
Professor Arne V Astrup: Research grants from Arla Foods,
Danish Diary Association, Danish Meat Industry, Dutch
Diary Association, Schulstad (Bakery), Unilever and Weight
Watchers; speaker for Campina/Dutch Diary Association and
Suikerstichting, Holland.
Professor Nancy Keim: None declared.
Dr Geoffrey Livesey: Director and shareholder of Independent
Nutrition Logic Ltd, which also employs him as a
consultant to work with commercial, governmental and
educational establishments and to undertake research on
commissioned works on matters regarding health and
nutrition. Received payment or other support covering a
period during 2002–2006, and has an expectation of support
for the future from several commercial entities with an
interest in the subject matter of the FAO/WHO scientific
update, even if it does not convey any benefit to him
personally, but which benefits his position or administrative