The first thing you need to realize is that supplements are nutrients – they're essentially food, as they're providing your body nutrients it needs to function properly and recover from the intense workouts you did on previous days. Do you still eat food on your rest days? Of course you do. For the same reason, you should definitely take supplements on these days, too.
Take Pro JYM, my protein powder. As you'll see in all of my meal plans, my general protein recommendations don't change from training days to rest days; I still emphasize getting in at the very least 1 gram of protein per pound of bodyweight on rest days, preferably 1.5 grams. For most people, it's tough to reach these levels with regular food alone – that's a lot of eggs, chicken and steak! – so taking at least a couple scoops of Pro JYM will get you there much easier.
And then there's the JYM Support Stack: Omega JYM, Vita JYM and ZMA JYM. These three products provide you the omega-3 fatty acids you need (Omega JYM) as well as critical vitamins and minerals (Vita JYM and ZMA JYM) for proper functioning and recovery. Essential fatty acids, vitamins and minerals aren't nutrients that correspond to a particular training session or day – these are things you need everyday.
So what about Pre JYM and Post JYM, which you'd normally take before and after a workout? Similar to the other supplements I've listed, Pre JYM and Post JYM contain important nutrients (amino acids, mainly) you need to maximize recovery from previous days' workouts and see the best results in muscle growth and strength. These ingredients include BCAAs, creatine, beta-alanine, betaine, taurine, glutamine and carnitine (the latter two are only in Post JYM). Nutrients like creatine, beta-alanine and glutamine are "stored" up in your muscles, and it's best to keep these muscle stores fully stocked at all times.
I recommend taking both Pre JYM and Post JYM on rest days. You can sip on them whenever you like throughout the day, either with or without meals.
However, I realize many of you are on tight budgets, and taking both Pre JYM and Post JYM seven days a week can be cost-prohibitive. In this case, take one or the other (Pre JYM or Post JYM) on rest days instead of both. Or, you can skip them altogether on rest days. Research has shown that creatine stays at reasonably high levels in the body if you take full doses at least half of the days during the week – basically, four days a week or more. Personally, I think it's best to take both Pre JYM and Post JYM seven days a week to at least keep levels of creatine and beta-alanine maximized, but I also realize that people have different financial situations and need to make their supplements last as long as possible before re-upping.
For those taking Alpha JYM and/or Shred JYM, I recommend taking these on rest days as well because they provide beneficial plant-based phytonutrients that aren't necessarily tied to any one particular training session. You take Alpha JYM to raise your testosterone levels and Shred JYM to maximize fat loss. You want those T levels to remain high and you want to drop body fat on rest days, right?
To see me answer the "supplements on rest days?" question on video, watch this: