Electrolytes include potassium, sodium, calcium, magnesium, phosphate, chloride and bicarbonate. Electrolytes are required for ion channels / transporters, signalling pathways, blood pressure regulation any many important metabolic pathways.

When electrolytes are deficient, neurological symptoms, fatigue, muscle spasms and pain/inflammation may occur (skeletal, smooth and cardiac tissues). Digestion and nutrient absorption may be impaired. Serum levels of electrolytes are tightly regulated and often fail to help indicate intracellular or systemic levels until an emergency hospital visit is required.

Electrolytes can “exchange” at the cellular membrane as pairs by pumps called "ATPases", which consume approximately 2/3 of ATP produced inside the cell, to maintain functional levels of each electrolyte inside and outside the cells.

For example:
The Na+K+-ATPase exchanges sodium : potassium.
The Ca2+-Mg2+-ATPase exchanges calcium : magnesium.
Under extreme metabolic conditions, alternate exchangers, such as the Na+/Ca2+ exchanger (NCX) can exchange sodium : calcium, too.
This is not an exhaustive list.

For this reason, a deficiency of one electrolyte in the pair can create issues absorbing and retaining the other, creating a secondary deficiency. Ideally, deficiencies for electrolytes should be addressed as pairs, or all at once, to prevent exacerbating an existing imbalance.

Intracellular deficiencies for each electrolyte can come from various influences other than simple dietary deficiency of it and / or its "exchanging partner", eg.