When I was on the Zone Diet, I noticed that the size of some of the
carbohydrate blocks varied significantly between the various Zone
books. Barry Sears' explanation was that "they" keep changing their
minds about what constitutes fibre.

There does seem to be a problem here, not only for the Zone Diet,
but for any purpose where available carbohydrate content is to be
measured.

The problem is that most food composition tables (e.g. the USDA
tables) measure carbohydrate 'by difference', which brings the need
to subtract a figure for total dietary fibre to obtain a measure of
the available carbohydrate content. The need to correct for fibre
in this way introduces an impact of any confusion as to what
constitutes fibre.

I have found an authoritative source of food composition data where
the carbohydrate content is not measured by difference, but by
adding the analysed values of the various components of available
carbohydrate, i.e. glucose, fructose, galactose, sucrose, maltose,
lactose, oligosaccharides, dextrins, starch and glycogen.

That method seems to bypass the problems related to measurement of
total dietary fibre.

The data is in

McCance and Widdowson's the Composition of Foods by R. A. McCance,
E.M. Widdowson, and B. Holland

which is the food composition 'Bible' produced by the UK
government's Ministry of Agriculture, Fisheries and Food.

Another advantage that source of data is that the carbohydrate
values are expressed as the monosaccharide equivalent, which seems
more relevant for many purposes than the total weight of the
carbohydrate in its several forms.

Yet a further advantage over using carbohydrate figures obtained
'by difference' is that with the latter method any errors in the
measured content of protein etc. are carried forward into the
figure that is obtained for carbohydrate.

In the case of some foods (e.g. mushrooms) the carbohydrate content
given in McCance and Widdowson is considerably less than the
content calculated (by correcting for fibre content) from the USDA
tables, and with most vegetables is a little less.

Of course, the data in McCance and Widdowson is for the foods that
we eat in the UK, but I would think that although there may well be
debate about whether "a calorie is a calorie", wherever the food is
grown then (for example) an onion is an onion.

Perhaps the McCance and Widdowson figures would be better ones to
use for such purposes as calculating the amounts of various foods
needed to take one in or out of ketosis.

Peter Smith