Comments about a few items in the Low Protein Food List for PKU (2nd edition, December 2002)

By Virginia E. Schuett

Sweet Potatoes vs. Yams

Someone in our PKU Listserv group told me she was confused over which phe value to use for sweet potatoes. In the new food list, on p. 63, sweet potatoes are listed as Sweet Potatoes (yams) with a phe value of just over 1 mg phe per gm of food for all three versions given (raw and cooked). But then on p. 64, there is an entry for Yam, cooked that has a value of 0.69 mg phe per gm of food. She wanted to know why there were this descrepancy and which value she should use.

At the time, I myself was unsure about the descrepancy. Sweet potatoes and yams are names which are often used interchangeably. But when I did some research, I finally found the answer.

The vegetable that we eat for Thanksgiving in the U.S. is a SWEET POTATO (botanical name: Ipoeoea batatus). A certain type of sweet potato (a moist-fleshed sweet potato) is also called a yam but is still botanically a sweet potato. All of the potatoes that resemble what you know to be a sweet potato or yam in the grocery store, whether yellow, orange or white fleshed, is a sweet potato whether they are called sweet potatoes or yams.

So, the value to use for your Thanksgiving sweet potatoes is one of the ones listed on p. 63, using the appropriate figure depending on whether you are counting it in the raw or cooked state.

The TRUE YAM (botanical name: Dioscorea spp.) listed on p. 64 is from an entirely different plant and is unrelated botanically to the sweet potato. It is actually a tropical vine related to a morning glory, with a large, thick, sweet tuberous root that is cooked and eaten as a vegetable. But it is usually only found in tropical countries and my guess is that you would have to go out of your way to find it in a specialty grocery store in the U.S., if you could find it at all.

Bacon Fat and Other Meat Fat

Because of many queries about the content of meat fat, I had various amino acid analyses performed in March 2003 by a laboratory. This included bacon fat drippings (strained liquid drippings from bacon with all visible meat removed in advance of cooking), crisp fried bacon fat (all visible meat removed in advance of cooking), solid raw bacon/salt pork fat (no visible meat) and beef and lamb fat drippings (from fat trimmings taken from beef and lamb).

Please see the Food List Corrections for crisp fried bacon fat (the food list incorrectly states that 15 gm is phe-free). Please see the Food List Additions for information on the other items.

Also please note that the figures refer ONLY TO MEAT FAT DRIPPINGS, NOT TO MEAT DRIPPINGS. Fat drippings refers to the liquid fat rendered from pieces of solid fat; meat drippings are the drippings that are formed when a meat is roasted, consisting of fat plus blood and meat juices which contain a significant amount of protein and phenylalanine. The phe content of meat drippings cannot be quantified by analysis, since the amount of phe depends on the meat used and the concentration of the drippings (which will vary greatly depending on many factors). So, you cannot roast a meat and use the resulting juices to make gravy or prepare vegetables, etc. without adding an unknown amount of extra phenylalanine.

French Fries: Why the phe content changed from the first edition of the food list to the second edition

The phe value per gm of French fries is uniform throughout the second edition of the food list book, unlike in the first edition. Fran Rohr and I made a very conscious decision to make all of the phe values per gm of French fries the same: 1.3 mg/gm of food. The reason for this is the following:

Several years ago (1998), an experienced PKU nutritionist and a parent did some actual weighing of French fries before and after cooking at fast food restaurants. They found that the 1.3 mg phe/gm of food value fit the data they obtained, using reliable USDA data for raw potatoes as the basis for their calculations.

We do not really have good data on all of the French fries that are produced. What appeared in the previous food list was often based on less accurate data. Therefore, we decided that the 1.3 value was probably as good as any other and that we should use it throughout. We felt that this was the proper decision to make, rather than possibly misleading people into thinking that each restaurant's French fries or each brand of French fries were somehow different when we had less accurate data than we truly needed to make this judgement.

Many people have felt for a long time that the higher values for French fries in the first edition of the food list were too high, and Fran and I agree. As you know, almost all of the phe values in the food list are estimates (with major exceptions of the low protein products and formulas), since actual phe analysis is rarely available. Therefore, we have to make the best estimates possible. We think that the 1.3 mg phe per gm value is a very good estimate for French fries.

Kroger White Almond Bark

This product now contains a tiny bit of nonfat milk, but the company says that it has only .02 mg phe per 100 gm, making it still phe-free (1 cube is 57 gm).