I don't take a multi-vitamin. I eat a wide range of veggies and meat and have never noticed a difference in how I felt, long or short term, when I've taken a multi in the past.

I do take a calcium/magnesium/Vit D supplement, potassium, and B-complex every day, plus liquid supplement for electrolytes in the water I drink.

Since boosting my electrolyte intake significantly, I've started to physically feel much better. Gotten rid of foot and leg cramps, muscles in my neck are much more relaxed, etc. So I notice a big difference with that.

I take vit D at the suggestion of my doctor who says everyone who lives in the Pacific NW is low on vitamin D (maybe not fair-skinned people who work outdoors, but everyone else). And I take calcium in hopes of warding off my family history of osteoporosis (yes, I'm aware of it's concern re: heart disease, but that's less of concern with my family history and it's more of a problem for men).

The B-complex seems like an odd outlier for someone eating plenty of beef on a low carb diet. However, I notice significant differences in my energy level and mental capacity (I have ADHD) when I take it. Since starting this I've read that many neurological disorders are associated with the MTHFR gene mutation and people with that mutation are often deficient in B vitamins, in turn causing problems with synthesizing neurotransmitters (along with a host of other issues). I don't know that I have that gene mutation (I've never been tested). But given my apparent need for extra B vitamins and my neuro history, it wouldn't surprise me.