Friday, June 29, 2012

Six Degrees of Botanical Separation

Why do allergic parents avoid way more than they need to avoid? If you hang around any of the chat boards for any length of time, it can seem like traumatized moms are looking for more stuff to take out of their child's diet.

I think the impulse is understandable. Something bad happens on your watch -- you never want it to happen again. So, locking down the hatches (and locking up the cupboards) seems sensible. But then, over time, more and more foods can get added to the list until the child's diet is far more limited than it needs to be.

Are you doing any of the following? Time for a gut check...

Avoiding foods because of sound-alike names.

CocoNUT. NUTmeg. Water chestNUTS. 

Coconut, despite the scary name (and the unfathomable FALPCA designation as a tree nut) is a drupe, essentially a fruit with a big seed. Coconut allergy is very rare; in addition, while possible, cross-reactivity with the seed storage proteins in tree nuts is also very rare (only two reported cases). Doctors do not recommend avoiding coconut solely because a child has a peanut allergy. Coconut is an excellent source of fat for skinny allergy kids!

Nutmeg is a seed. Water chestnut is an aquatic, grasslike vegetable!

Avoiding based on botanical family

This is a wonderful resource I use all the time from the Calgary Allergy Network.  HOWEVER...what I hate to see is people using this list to avoid ALL the items in a botanical family, simply because their child is allergic to one item. 

Plants that have similar botanical characteristics do not necessarily share the same proteins at the molecular level. And, if they do, those proteins may have very subtle differences that can completely change how the body reacts to them. 

The body reacts to proteins based on two characteristics:
  • The actual string of amino acids that makes up the protein. 
  • The SHAPE the protein takes as a result of the bonds between the different amino acids.
To use an analogy, consider this blog post. It has words in it that are analogous to the sequence of amino acids in a protein. Words always contain the same letters in the same order. However, while writing my blog post, I also choose the order the words go in. The words are important, but the order I use is equally important and can make a huge difference in meaning. Consider these two sentences, which contain most of the same words:

The woman danced well for a little while.

While a little woman can dance, she cannot dance well.

Caraway seeds

In a similar way, how the parts of a protein are ordered can vastly change the shape, and therefore the "meaning" to the immune system.

More years ago than I'd like to admit, I worked in the field of chromotography, a science devoted to separating out the parts of a mixture into its components. One of the really difficult challenges in the field is separating out something called enantiomers. These molecules have exactly the same chemical structure; however, shape-wise, they are mirror images of each other.
Spearmint leaves

You would think these molecules would have very similar -- maybe even identical -- characteristics, wouldn't you? After all, the difference in their molecular make-up is essentially only in the way they twist at the end. But the reality is they're often not even remotely similar. For example, the taste in caraway seeds and spearmint leaves are both from the chemical carvone. The molecule that makes up both has exactly the same chemical structure. Go chew on both to see just how different they are as a result of that little twist!

Avoiding based on categorization or type. 

"I don't give my child any seeds or nuts" is a common way to cope with potential allergens. But does it make sense?

Tree nuts are the trickiest decision. Somewhere around half (some studies say 30%, some more) of kids with a peanut allergy may also have a concurrent tree nut allergy. Are the two related? In many cases, the answer seems to be yes.

As I discussed in the post about component testing, each allergen has several different "hot spots" that can cause an allergic reaction. Each of these hot spots has a different sequence of amino acids AND a different shape in the body. Some scientists do think that, in at least some cases, the body "sees" the three-dimensional shapes of peanut and tree nut proteins as similar enough to cause a cross-reaction. However, only some hot spots appear to have this potential and the science is not well-enough explored to fully confirm this allergic pathway yet.

So how do you know if your child is allergic to a part of the peanut that might have a similar-shaped protein as another tree nut? You don't. It comes down to testing: clinical and challenge. (Here's what Sampson and Sicherer say on the topic.)

We pushed hard for this testing, as our son's food was so very limited. He showed some sensitivity to many nuts (Class 1 or even a little higher in some cases). However, after challenge testing for all of them, he now eats ALL tree nuts except hazelnuts (and we're still hoping that one is a pollen allergy). He had mild reactions to cashews that he outgrew. We buy our nuts whole, from a peanut-free source if possible, and wash them thoroughly before use. Nuts have become a wonderful source of protein in our house. 

I don't expect many other peanut parents to travel the same path, but I do think it's important to acknowledge that, for as many as half of you, it is possible. And, since keeping foods in a child's diet may have a protective effect with regard to developing new allergies, we try to use nuts frequently in our cooking. 

There are "clusters" of allergens that seem to travel together, probably the result of homologous proteins or some cross-reactive recognition at the molecular level. However, the bottom line is that the science of molecular categorization of these clusters is in the very early stages. (If you're really curious, this article about protein family classification is interesting bedtime reading.) We know that:
  • Many kids with milk allergy also end up with egg and peanut 
  • Peanut and sesame can go together, and reactions can be very severe when allergies to both are present
  • People allergic to latex can have cross-reactivity to kiwi, banana, hazelnut or other fruits
  • 1 in 20 people who have an allergy to one legume are allergic to others in the family  
  • Many milk-allergic children (studies say about 1 in 10) will have a concurrent allergy to soy.
Some day, your child's molecular allergy fingerprint may help to identify which other foods are a risk. Until then, it's important to recognize that every child is different and there are no certainties when it comes to cross-reactivity between allergens.

Does your child have an interesting "cluster" of allergies? Post in the comments!


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