[ASC-list] Fourth message on obesity
forbes-ewan at tassie.net.au
Fri Apr 4 20:27:49 EST 2003
In three earlier messages I discussed the nature of the obesity epidemic,
health problems associated with obesity and possible causes (especially the
reduction in physical activity that is occurring as we increasingly allow
computers and other machines to carry out physically-demanding jobs).
This message continues the discussion of possible causes of obesity.
The claim has been made (sometimes by people who should know better) that
obesity is determined by 'genetics', so there is little hope of reducing
body fat levels in those predisposed to become obese.
Anyone who has seen photographs of Australian POWs emerging from Changi at
the end of World War II will know that this is a specious argument. Despite
having the same mix of genes as their parents, 60 years later about 60% of
the offspring of those horribly mistreated and extremely emaciated POWs are
above the healthy weight range (HWR). Clearly, the environment plays a major
role in determining to what extent the inherited potential for obesity is
realised. As far as we know, today's environment is the most obesogenic that
has ever existed.
In my last message I stated that a reduction in energy expenditure is almost
certainly contributing to the obesity epidemic. I also pointed out that food
intake may not be reducing to compensate for the lowered expenditure, hence
leading to a small but steady trend towards positive energy balance (i.e.,
accumulation of body energy reserves in the form of body fat). What hasn't
been discussed yet is the possible effect of the quality of diet on body
fatness. In this context, the word 'quality' means the nutrient mix,
particularly the relative contributions of protein, fat and carbohydrate to
total energy intake.
For many years it was accepted by the vast majority of nutritionists that
dietary fat was the major culprit--high fat diets were considered to be
obesogenic, while diets rich in carbohydrate (at the expense of fat) were
thought to be protective against weight gain.
One well-known nutritionist from the UK believed that sugar was the main
culprit, but there was little support for this idea from epidemiological
studies and his ideas were not taken seriously by mainstream nutritional
scientists for long.
A decade or two ago, another school of thought emerged--that it isn't total
fat intake that is critical, rather it is the type of fat, combined with the
predominant type of carbohydrate eaten, that is the major dietary culprit.
According to this group, consumption of too much 'saturated fat' and too
many 'high glycaemic index (GI) foods' leads to obesity and also to
increased risk of a host of chronic degenerative diseases--including
diabetes and its complications--and heart disease.
By way of explanation, 'saturated fat' is the type of fat found in dairy
products, beef and lamb, and in many cakes, pastries and takeaway foods.
High GI foods are those carbohydrate foods that are rapidly digested and
absorbed, leading to high blood sugar levels. They include potatoes, bread
and rice, among many other foods. By way of contrast, legumes, pasta and
milk products (e.g., yoghurt) are among the relatively low GI foods, and are
therefore believed by these nutritionists to be protective against obesity
and degenerative diseases.
The most recent variation on this theme suggests that 'omega 3' fats (found
in large quantities in fish and some foods of plant origin) are especially
protective, while grain foods (e.g., bread, rice, pasta)--particularly those
that are refined--increase the risk of obesity and chronic disease.
As is probably clear by now, consensus hasn't been reached on the role diet
has played in contributing to the obesity epidemic, or the role it might
play in attempts to overcome the epidemic. The debate between these various
schools of thought (and several others not discussed in this message) is now
being played out in nutrition journals, at nutrition conferences and in
cyberspace, with each group convinced that the others can't see the forest
for the trees. This debate is sure to continue for many years.
Next message: More on possible causes of obesity.
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