hey there, measuring serum zinc allows you to compare your values to those reported in the scientific literature. it is not as valuable a test in terms of accuracy, but when you need replicated data for comparison, you have to go with what's out there in the majority of research.
washout periods are good for assessing baseline levels, and background levels (storage). when treatments are compared in multiple groups in studies, washout periods for true clearance (eg between treatment a and treatment b) tend to be much longer than one week, more like 4-6 weeks.
first time i ever did a washout, i wanted to get a clear read on my vitamin b12 status. i had been taking supplements before my tests for ages, and consistently coming up with low normal results. i wanted to know the consequences of my taking the supplements just for the day or two prior to the test. would i still be low normal? or would it be something more drastic? when i did the washout and had my serum vit b12 tested, my b12 levels were undetectable (the test was sensitive down to 75 pmol/L, so on the lab printout my result was 'less than 75'). the low level was purely a result of my dietary choices. anyone on a specialty diet alone, never mind a supplement regimen, needs to be able to check the effects on nutrient status. especially ms patients who have a known constellation of poor nutrient levels. and especially ms diets, which always seem to limit or cut out food sources of important nutrients.
i have heard of the taste test. http://www.msrc.co.uk/index.cfm/fuseact ... pageid/653
although i find this article well written, it bothers me that it is unreferenced. also the test is highly subjective, and i haven't seen ANY studies out there that would allow for meaningful comparison of your case to published data. there is plenty of data out there on serum zinc, however.
i agree that serum/plasma zinc levels are *usually* a poor assessment of body stores, but i can tell you that problems show up WELL before 0.6 ug/dL, which converts to 0.09 umol/L. !!!!!!!!!!!!!!!!!!!!!!!!!!! this is actually making me pretty angry. the 'normal' range is 11.5-18.5 umol/L, and my original baseline test value (you can bet i didn't take any zinc beforehand, thank you!) was 8.6 umol/L. i was so screwed up i almost lost my drivers' license!! just because docs don't *understand* the signs of suboptimal zinc (eg, MS diagnosis!!) doesn't mean they aren't THERE. in the literature, healthy controls serum zinc levels are repeatedly up in the top end of the normal range.
the reason you stop taking the vitamins before the test is EXACTLY to determine absorption! i'm sorry to say so, but it doesn't sound like you have the right doc in the mix.
tell him you want serum values because you want to be able to compare your data to the decades of published nutrition science which use serum data. if he still says no, it's time to find another doctor, or go outside the NHS for private assessment.
as of now, i've reached the level of steam which tells me it's time to take a breather. learned my lesson a few years back, something about leading a horse to water. there has to be a perfect constellation of critical components for success in this approach, and here we seem to have everything in place except the medical professional. i can't do anything about that, but i hope you can. best of luck finding someone cooperative!