For people who have good metabolisms…do they need extra nutrients to stay healthy and gain energy?
It seems that logic would dictate that the faster one metabolizes food the less time, the body has to absorb nutrients.
It seems to me that if there were a downside to having a “good” metabolism, that would make it more of a “bad” metabolism. So, no. 
I’m thinking that by ‘good’ the OP means ‘fast’.
A faster metabolism would suggest greater calorie intake. Studies show increased longevity for restricted calorie intake, but these are not human studies. The results are often used to suggest this correlation in humans, but it the jury is still out.
A restricted calorie diet can make it difficult to take in all the needed nutrients for longevity. A faster metabolism in theory would allow more food choices and opportunities to take in needed nutrients, but do they get absorbed well enough?
I can think of one downside to a fast metabolism because I experience it. That is that it cost me more to eat then the next three average people. I burn significant calories just bringing in the groceries, not to speak of the extra utilities and time to cook everything.
Metabolism of food into energy (and other things) takes place in the cells throughout the body. Absorption of nutrients takes place in the gut. Except for the fact that the latter is necessary for the former, they have little to do with one another, and their relative speeds (whether good or bad) have little to do with one another.
If your food passed through your gut too quickly, then you might not have time to fully absorb all the nutrients from it. But that is not a “good” metabolism, that is diarrhea.
Like most things in biology, “good” and “bad” depend on the environment. In our environment where food is cheap and plentiful, the people with fast metabolisms have lower rates of obesity and all of the problems that go with that, and so a fast metabolism is good. If we had a famine, though, where food was harder or impossible to come by, those folks would be the ones who starved first, and the folks who had managed to get fat before the famine struck would be in the best shape. So in that situation, a fast metabolism would be bad.
I have a really fast metabolism in that I have to eat a lot to maintain my weight. Since thin is in and I like food, it’s a “good” thing, but I always think about how I’d be doomed if I was ever a victim of a shipwreck, a downed plane, concentration camp, etc. As** Chronos** said, it’s only "good"because we live in an environment where food is plentiful and readily available.
As Homer Simpson once proudly proclaimed, “I am both drought and famine resistant.”
Possibly more resistant to hypothermia as well, though I’m not sure. That would probably be related to the proportion of total volume to surface area, and I don’t know which would grow faster as a person gained weight.
It also matters how the fat is arranged on the body; women are more resistant to hypothermia than men in part because they carry more of their body fat right under the skin and more evenly distributed over the body. A big gut won’t keep your arms and legs from losing heat.
If you have a big enough and flabby enough gut, you can push your hands into it to keep them warm. 
Both as you get bigger in general, and as you get closer to spherical, your surface area to volume ratio decreases. And a fat body will help keep your core temperature up, which is after all most important.
Does a fast metabolism mean “more cell division” (I don’t know)? If so, it probably means more cancer.
It seems a faster metabolism today is beneficial as you can easily consume sufficient calories on quite a low income. The likelihood of being shipwrecked, put in a concentration camp or experiencing (and surviving) a plan crash onto some Island like the one in ‘Lost’ is, in my humble opinion, lower than the chance of suffering from weight-induced illnesses.
Of course having to eat more to maintain weight means you’re putting more of whatever stuff into your body than is ‘normal’, and not being overweight doesn’t mean you’re immune to high levels of cholesterol or toxins, so eating ‘bad’ food (if this were all that was available to you, again a pretty low risk) only would surely be more beneficial to those with a slow metabolism who don’t need to eat as much.
And physics calculations of you become more accurate.
But when the survivors turn to cannibalism, they will be looking at my luscious, mouth-watering tenderloins and leave you until the end. So, you may be in the best situation after all. 
I’m going to question the whole concept. I haven’t seen any evidence that there even is such a thing as “fast (or “good”) metabolism”. I think there are just people using more calories daily, and people using fewer calories daily. If you use more calories daily, you have to eat more to maintain the same weight. I think a higher calorie intake will only make it easier to consume the necessary vitamins and minerals.
I’ve had a fast metabolism all my life. Y’know that heroin thin look.
I would say that is the number one downside right there, being under weight. If you’re a woman it can affect your menses, getting pregnant could become problematic.
Being stick thin made me very prone to bruising. The slightest knock and I look like I’ve been in a fight.
Also, if the bus gets a flat or the train is delayed, you better pray to god there is some biscuits in my bag (there always is!), without food, every few hours, a change in my disposition occurs that is dreadful to see.
I would also like to add that while the world is delicate to the overweight, in some ways, they are most outspoken to the stick thin. Everyone feels free to opine, should they see me eating. Saying, ‘Eat something!’, is not amusing to people with the metabolisms of squirrels. Not the first time, not the 50th time.
Visiting a doctor, no matter your complaint, often strays into what you eat. Suddenly you sense the Dr thinks you have an eating disorder. The more you protest the more suspicious he seems. It’s annoying.
So yeah, I think there are downsides.
I’ve studied molecular biology, and I’m doubtful that this is a thing in itself.
I think that maybe you move around more, or think more, than average people, and therefore burn more energy. On the other hand, if you are very thin you must have less muscle mass than other people, so that would make you burn less energy.
I rather doubt that there is something molecular that makes you need more energy than others somehow. Hypothetically a defective pathway somewhere in the glycolysis pathway, that would make it give less output per input, but that seems unlikely to me, and I have never heard of it.
So, I think you just have to eat maybe somewhat more than other people of your own weight, due to higher activity level, and that’s all.
Just what do you think a “fast metabolism” means? That’s like saying “I haven’t seen any evidence for gravity. I think there’s just a force that pulls massive objects together”.
He means the difference between, like, walking an extra 10 miles a day, or maintaining huge muscles, or having restless leg syndrome, and just having “good genes” that cause you to, all other things being equal, burn more calories. From my observations in life, I would side with the “good genes” being real; some people can eat a lot without maintaining muscle or doing exercise, and yet stay thin. I have also seen people develop hormone disorders (like what happened to steve jobs) where they maintained a healthy appetite but lost weight.