Two examples are Dr. Mercola and George Newman. https://articles.mercola.com/sites/articles/archive/2017/11/26/fasting-powerful-metabolic-therapy.aspx
Dr. Mercola was taking 1 g of magnesium per day and yet his RBC magnesium was only 3.5 which was below average. His fasting insulin level was 2. Improved insulin sensitivity is supposed to help our cells store magnesium. https://articles.mercola.com/sites/articles/archive/2016/11/30/how-to-lower-blood-pressure.aspx
Dr. Newman was able to treat his heart rhythm problems for at least two years by avoiding excessive exercise and taking magnesium. He took 5.5 g per day. Now he's taking 1.5 g per day. The shocking thing was that once he stopped taking it, within 48 hours, his AFib recurred, implying that he's excreting large amounts of magnesium.
It would seem like even if they ate foods grown in magnesium rich soil which seems like what Dr. Mercola was doing, they would still need to supplement.