To calculate the ketogenic diet, it is necessary to know how much fat, carbohydrate and protein there is in each food. Most foods are now labelled with their food values, but this is not always the best source of information.

In the UK (and the rest of the EC), food labelling could have been designed for the ketogenic diet. All food has to be labelled according to a well defined standard. Food must have a label specifying the food values per 100g and per "portion" or actual size, each to an accuracy of 0.1g. By contrast, food labelling in the US is not satisfactory. It is not controlled by the same strict standards, and it is usually done to far less accuracy.

Rather than using food labels, the best sources of information are tables of food values. There is some variation in food values between countries - eg the fat content of cream may vary - and there may be considerable variation in convenience foods. For this reason, you should consult local food values wherever possible.

(not available)   UK food values

Common UK food values are listed for your convenience. There may be minor variations in the food values for other countries.

Food labelling in the UK (and EC)

In the UK (and the rest of the EC), food labelling could have been designed for the ketogenic diet. All food (there are exceptions for things like fresh vegetables and fruit) has to be labelled according to a well defined standard. Food must have a label specifying the food values per 100g and per "portion" or actual size, each to an accuracy of 0.1g.

A typical label (for slightly salted butter) might be:

In addition, information must also be provided on any other ingredients for which nutritional claims are made.

The food labels must be calculated according to defined rules:

• Protein in g is defined as 6.25 times the nitrogen content of the food. Protein is normally measured in terms of the nitrogen content, but the conversion factor does differ for different types of protein, varying from 5.7 for flour to 6.4 for milk, so the notional protein content may differ from the actual by up to 5%

• Carbohydrates are declared as the actual weight, not the Monosaccharides equivalent. This affects the energy calculation, because polysaccharides deliver more energy per g, by up to 10%.

• In food labels, the metabolic energy is calculated by using the following defined metabolic factors to convert from grams (g) to kilo calories (kcal):

  Fat	9
  Protein	4
  Carbohydrate	4
  Alcohol	7
  All organic acids	3
  Sorbitol	2.4

The food labelling information in the UK is more than adequate for the purposes of calculating the ketogenic diet. However, you should always use the figures for 100g, and not those for a portion, because the latter are not so accurate.

Food labelling in the US

By contrast, food labelling in the US is not satisfactory. It is not controlled by the same strict standards, and it is usually done to far less accuracy. Typically, ingredients are only specified to an accuracy of 1g, and this may be for a relatively small quantity (20g or less). There is also a tendency to round up, or down, according to the perceived "goodness" of the ingredient - so protein may be rounded up while fat and carbohydrate are rounded down.

This can have a significant effect. Take the example of a 14g portion of butter. The label says:

This could contain almost a gram of carbohydrate, and still be labelled at 0g. If we multiply up to get the ratios for 100g, this means there could be as much as 7g of carbohydrate in 100g, a huge potential error. (Of course, we all know that there is no carbohydrate in butter.)

What should be done?

Let us look further at the example of butter. The first thing to notice is that the serving size was 14g, but the total weight of fat, protein and carbohydrate is only 11g, so we are missing 3g. Well, the answer is that it is probably mostly water, some of it may be inedible matter like collagen (fibre) which contributes no calories, and some of it could be due to rounding errors - usually the manufacturer will underestimate the fat and carbohydrate and overestimate the protein. The second thing to notice is that the total calories from the fat, protein and carbohydrate are only 99 kcal (11 x 9 + 0 x 4 + 0 x 4), although the total calories are listed as 100 kcal, so there appears to be a missing calorie.

Let us compare the 14g portion with the "true" 100g figures, by scaling up the smaller portion by multiplying by 100/14:

They look surprisingly different could they really be from the same data - the answer is yes. The portion was actually probably calculated not as 14g, but as100kcal, so let's go the other way and recalculate the "14g portion" from the 100g figures first by multiplying by 100/737, and then by rounding the figures. Now, we get:

Which are just the figures on the label. What you have seen is an example of how to lie with numbers. Our problem is how to go in the reverse direction and get to the truth. Unfortunately, it is just not possible, the label figures are too imprecise for our purpose.

Unless the information on the label is accurate to 1% or better, it is of no use for calculating the ketogenic diet. Accurate to 1% means that if the quantities are specified to an accuracy of 1g, then the portion size must be 100g or more or if the portion is smaller than 100g, that quantities are specified to 0.1g.

The best thing to do is to get the food values from a standard reference, which will give them to greater accuracy than the label. The next best thing is to ask the manufacturer for better information. If all else fails, then you may have to make an intelligent adjustment (ie guess). The principle should be to ensure that you err on the side of overestimating the carbohydrate, but you must always treat the results with caution.

So, in the US, it is better to rely on published food values rather than labels.

Food values

Rather than using food labels, the best sources of information are tables of food values. There is some variation in food values between countries - eg the fat content of cream may vary - and there may be considerable variation in convenience foods. For this reason, you should consult local food values wherever possible. This also means that any food values in ketogenic diet programs should be used with caution by non US people.

The standard reference in the UK is:
McCance and Widdowson’s The Composition of Foods
B Holland, AA Welch, ID Unwin, DH Buss, AA Paul and DAT Southgate
published by The Royal Society of Chemistry and the Ministry of Agriculture, Fisheries and Food
ISBN 0-85186-391-4

This information is also available as a CD; the information does not appear to be available on the web.

McCance and Widdowson first appeared in 1930, and has been updated regularly. It is now in the fifth edition, and there are a number of supplements.

Beware: the carbohydrate values in McCance and Widdowson are based on a metabolic factor of 3.75 not 4. This is because they present carbohydrate values in terms of monosaccharide equivalents. So, if you use food values from this source, the number of grams of carbohydrate in a food will look higher, but the calories will come out to be much the same, providing you multiply by 3.75.

It provides information on 1188 foods, from Aduki beans to Zucchini, with the following information about each (for interest, I have put in the actual values for (salted) butter:

The standard reference in the US is:
Bowes and Church’s Food values of Portions Commonly Used
Jean AT Pennington
Lippincott 1994

Sources of food values

Information about food values for the ketogenic diet available in books and on the internet.

Accuracy of food values

There will be a variation in values between one sample of a food and another. This applies particularly to raw ingredients - for example there is a lot of difference between a green apple and a ripe apple, or between samples of beef.

There are errors in the assay process. In the case of protein, the protein level is estimated from the nitrogen content. Since the nitrogen content varies for different proteins, and there are sources of nitrogen other than protein, (this is more of a problem with vegetables and things like mushrooms), the estimate may be in error by up to +/-10%. Most seriously, carbohydrate is often estimated by "difference", ie it is taken as the quantity left after everything else has been measured. This means that carbohydrate is often the most inaccurate measurement. Fortunately, it will probably be an overestimate, because when using "difference", the dietary fibre (ie cellulose etc) will not measured either, and so gets counted as carbohydrate.

It is difficult to establish the effect of these errors in variation between sample and assay, but they are unlikely to be more than +/- 1g in 100g for each of the fat, protein and carbohydrate content.

Finally, the conversion to energy is not very accurate, because the "standard values (9,4,4) are only approximations, and will vary from food to food. This does not affect the calculation of the ratio of the diet, but may mean that the total number of calories from the diet is slightly different from the expected value.

return to   Practical management of the ketogenic diet
continue to   Cooking the ketogenic way
return to   Calculating the ketogenic diet

(checked: )
(update 2.2: 31 January 2003)
(issue 2: 26 May 1998)