Impact of Sodium Restriction on Body Weight in Patients with Ascites

 

Runyon makes the following correlations in patients with ascites (Table 1, page 250):

(1) sodium content of ascites fluid is 130 mmol sodium per kilogram fluid

(2) nonurinary excretion of sodium is about 10 mmol/day (assumes little tendency to exercise or sweat)

(3) urinary excretion of sodium may range 0 to 1,000 mmol per day

 

If we assume that weight change in an ascites patient is due to changes in amount of ascites fluid and not other factors (probably an acceptable assumption for short term assessments), then loss of ascites fluids requires sodium restriction so that excretion exceeds intake.

 

Sodium Intake	Effect
exceeds excretion	weight increases
matches excretion	weight stable
less than excretion	weight decreases

 

A 2 gram sodium requires patient cooperation, and a diet with less sodium can be difficult to achieve outside of the hospital.

 

The molecular weight of sodium is 22.99 and of chloride is 35.45. Sodium chloride has the molecular weight of 58.44.

 

Restriction of salt intake to 5 grams per day represents 85.6 mmol/day, or about 1.98 grams (86 mmol) sodium (2 gram sodium restriction).

 

For excretion > intake

 

(10 mmol/d nonurinary loss) + (measured urinary loss in mmol/d) > 86 mmol/d

 

NOTE: My analysis differs from that given by Runyon in that (1) my estimate of a 2 gram sodium diet is 86 mmol/d compares with his reported 88 mmol/d, and (2) his sodium loss equations have (urinary loss minus 10 mmol/d). In the examples sometimes the 10 mmol/d is added while at other times subtracted.

 

Since 1 liter of ascites fluid represents about 130 mmol per kg sodium:

 

change in weight per day in kilograms on 2 gram sodium diet =

= (86 - ((urinary sodium excretion in mmol/d) + 10)) / 130

 

change in weight per day in kilograms on sodium diet with n grams sodium =

= (((n grams) / 22.99) - ((urinary sodium excretion in mmol/d) + 10)) / 130

 

Weight gain while on a sodium restriction diet indicates either:

(1) nonadherence to the diet

(2) very low urinary excretion

 

Greater weight loss than expected may indicate incomplete 24 hour urine collection with underestimation of urinary loss. This can be checked by calculating daily urinary creatinine excretion.