I'm trying the leangains approach (16 hours fast, 8 hours feed) where I typically don't have my first meal of the day until 1 or 2 pm. The first few days have worked out great and I really like this way of eating, however I began to think.... am I doing more harm than good? I know that people have reported huge weight-loss gains with this approach, but I can't stop thinking about the conventional wisdom surrounding the frequency of meals. Is it not better to eat 5-6 small meals throughout the day so your metabolism keeps on trucking? I can't seem to shake free of this long accepted piece of "wisdom".
Leangains: A Critique of the ISSN Position Stand on Meal Frequency has lots of links in the article leading to other articles on the topic.
That 5-6 meals thing is just not true. Studies generally show no differences in weight gain/loss comparing 3-6 meals. In short-term (within-day) studies, increasing meal frequency appears to have beneficial effects on appetite control, though these results have generally not been supported by long-term studies.
Some studies have shown body comp changed favorably with fewer meals (1vs3 and 3vs6)
Metabolic rate is actually increased during the first couple days of fasting, and doesn't slow down until 72+ hours of fasting due to increased adrenalin hormones.
There are a lot of studies to support lower meal frequency, have a look around leangains.com for more
If you can't relax and be comfortable with it this may not be the time for intermittent fasting (IF.) You always have the option of trying it at a later time.
I admit I love the fact that my days aren't restricted by fixed meal times. If I'm busy I just cancel the hassle of fixing salads and cooking meals. I also love days when (in my 4-6 hr window) I eat lots of home-fixed foods. It's about what you want and what brings you serenity.
If you like having a consistent and spread-out pattern of meals, what's the harm?
Now, if you reach a point where you're not satisfied with your results and want to try something different, you can take another look at IF.
With regard to the title of your question, my metabolism does not slow down from IF. Nor does it slow down in periods when I eat several meals every day. I do think it slows if I go more than 2 days without a brisk walk.
PaleoGal, everyone posted some awesome responses and studies already, so I can chime in with a little n=1. I've been IFing not knowing what it was for years, and for about a year knowing the details and following a LeanGains approach, and I can attest to the amazing benefits and that your metabolism doesn't slow down at all. On the contrary, I noticed it sped back up (it slowed down for me when I went vegan...booo) and I generate a ton of body heat after I eat. And I agree with you - it's so liberating!
Edit: I also do the 16/8 window, though some days I extend it to 18-20 if it happens that way. And this may be TMI but it might be helpful: a few days before a period, myself and a lot of other IFing women (check out Fierce Fit Fearless on Facebook) get super hungry, so the fasting window shortens to 12-14 hours instead. There's no medical reason to do that, it's just that you might find you are ravenous beyond belief. ;P
The only test I think on the subject on meal frequency ran from 1 meal to 17 meals, and found no effect. Essentially, the 'added burn' nets out as the same for the quantity (and type) of food being digested. So you can spread the meals out and be revving slowly all day, or eat larger meals, have a bigger increase in metabolism which then drops until the next meal. There's no gain trying to take advantage of this, a properly functioning metabolism is designed to handle this. The only difference I think is if a different meal pattern leads you to eat different amounts. The little and often approach has worked for many people, but I think mostly through limiting what they eat or lending greater awareness to their food choices. But it doesn't really fit with a practical paleo lifestyle for most.
The research on fasting in the short-term (such as the first 16 hours) shows that the metabolism increases. I'm not sure what can be done to give you the necessary confidence other than recommend reading the literature. Or just keep going since you feel good, what have you got to lose? :o)