High salt intake increases urinary calcium loss

Background

Peat supports adequate salt intake. He has plenty of reasons to do so, given that salt restriction increases aldosterone, which promotes potassium loss and has degenerative effects when chronically activated.

However, he does not talk much about the effects of too much salt. Some people may conclude from his work that the more they salt their food, the better it is. But there are some caveats that you should know.

One caveat is that an increase of dietary salt increases the loss of calcium through the urine. This has been shown in both controlled animal and human studies. A possible explanation for this effect is that channels, that are important for reabsorbing calcium back into the body, are blocked by the excess sodium molecules secreted into the urine on higher-salt diets.

Studies where salt intake was increased

1. Increasing dietary salt increases calcium loss in rats (1). In rats fed a low calcium diet, this results in activation of the parathyroid gland and osteoporosis (2) (3). In one study bone loss was also observed in rats on a normal calcium diet (4).
2. In postmenopausal women, adding 3-6g of salt daily significantly increased calcium excretion (along with increased sodium and chloride excretion) and increased total urine volume (5).
3. In healthy young women, NaCl but not KCl increased urinary calcium excretion (6).
4. In a cross-over study on postmenopausal women, only a high-calcium/low-salt diet lead to a positive bone calcium balance, while on a high-calcium/high-salt diet bone calcium was unaffected. The low-calcium/low-salt and low-calcium/high-salt diets lead to a negative bone calcium balance (7).
5. In 23-76-year old adults a high-salt diet increased urinary calcium and PTH compared to a medium-sodium diet (8).

Studies where salt intake was restricted

1. In healthy subjects, a 4 day low sodium diet reduces urinary calcium excretion (13).
2. In patients with urinary calcium stones and high calcium excretion, a 3 month long salt-restricted diet reduced urinary calcium excretion (9).
3. In healthy 21–39 year-old adults, a salt-restricted diet didn't significantly decrease calcium absorption, while leading to a slight increase in PTH (10).
4. In healthy postmenopausal women with a sufficent calcium and vitamin D intake through supplements, overrestricting sodium under 1500mg/day lead to unfavorable effects on bone density compared to women who ate ~3000mg/day (11).

Association studies

1. In an association study, sodium excretion was the leading dietary factor associated with urinary calcium excretion (12).

Conclusions

1. Overconsuming sodium will increase urinary calcium loss and this can be especially problematic for people that have a low calcium intake (i.e. the general public that doesn't consume much dairy but plenty of salted processed foods). Excessive urinary calcium loss may promote osteoporosis, hypertension and the development of kidney stones. This relationship supports the notion of eating salt in balance with the other minerals (calcium, magnesium and potassium).
2. The mechanism may explain why some people tolerate high amounts of milk better with added salt (the increased urinary calcium excretion could regulate excessive calcium intake caused by the milk).
3. On the other hand, over-restricting sodium in healthy people with an adequate calcium intake is also problematic because it can lead to an increase in stress hormones, most notably aldosterone, which could also impair bone health.
4. Keeping salt in a normal range, by salting to personal taste, and balancing salt with adequate calcium, magnesium and potassium is probably the best option. Aldosterone has been shown to control salt intake, as high aldosterone levels correlate with higher salt pleasantness (13). There are no good reasons that we need to override this biological mechanism by overconsuming or over-restricting salt.