- Click here for explanation of classification scheme.

To establish the reliability of a dietary protocol for the assessment of salt-sensitivity involving dietary salt depletion and repletion in normotensive subjects.

Healthy males aged 20 to 31 years old; diastolic pressure less than 85mm Hg and systolic pressure less than 140mm Hg

None specified

Recruitment

Volunteers; no further information provided

Design

In Part 1, each participant was given a low-salt diet plus a 190mmol supplement for seven days followed by a low-salt diet for seven days. A sub-set of participants underwent a randomized crossover trial of dietary salt loading and depletion.

Blinding used

• In Part 2, participants were blind to which arm of the dietary protocol they were in
• Those collecting the data do not appear to have been blinded.

Intervention

• Part 1
  • Low-salt diet (standardized diet-20mmol sodium; kcal per day estimated to keep body weight constant) given for 14 days; during the first seven days 190mmol NaCl added to diet; diet provided by the study and consumed in outpatient setting.
• Part 2
  • Low-salt diet, as above, with and without supplement of 200mmol per day, for seven days each in random sequence (cross-over study).

Statistical Analysis

• Mean ± standard error of the mean (SEM)
• Two-sided Student T-test to compare means
• Kappa statistics to assess reliability of salt-sensitivity classification
• Fischer test to assess relationship between familial history and salt-sensitivity.

 

Timing of Measurements

Measurements of blood pressure taken after a 30-minute resting period on days seven and 14 for both Part 1 and 2 of the study; blood pressure was measured over a one-hour period at two-minute intervals.

 

Dependent Variables

• Systolic blood pressure, diastolic blood pressure, mean arterial blood pressure, heart rate
  
  • Blood pressure was measured after a 30-minute rest period at two-minute intervals over the course of an hour using an automatic oscillometric device.

Independent Variables

 Dietary sodium intake, high and low

Control Variables

 Urinary sodium excretion was monitored to assure compliance with the diet

 

Initial N

Part 1, 40 young men; Part 2, 20 young men

Attrition (final N)

Part 1, 34 young men; Part 2, 15 young men

Age

Ages 20 to 31 years old

Ethnicity

No information; in Part 2, 10 had positive family histories of hypertension and 10 did not.

Other relevant demographics

No information

Anthropometrics

No information. The paper indicates there were no differences in age or body-mass-index between the subjects with positive family histories of hypertension and those without.

Location

Berlin, Germany

 

Part 1

Salt-sensitivity was defined as a having a decrease in 3mm Hg or more between the average of the 30 blood pressure readings taken between the high- and low-salt periods.

• 13 of 19 subjects with positive familial histories were classified as salt-sensitive
• Three of 15 subjects with negative familial histories were classified as salt-sensitive. (P<0.01)

Part 2 

Mean arterial blood pressure fell from 83.2±2.3 to 77.6±2.8 (P<0.001) mm Hg under the low-salt diet in the salt-sensitive group but was unaffected in the salt-resistant group (high salt 79.0±2.3 vs. low-salt 79.1±2.6mm Hg)

• All of the persons identified as salt-sensitive in Part 1 were again identified as salt-sensitive in Part 2
• One of the persons identified as salt-resistant in Part 1 was identified as salt-sensitive in Part 2
• The reliability of classification was very strong (kappa statistic 0.87)
• Changes in blood pressure in Part 1 were not significantly correlated for either the salt-sensitive nor the salt-resistant group.

The authors concluded that salt-sensitivity can be reliably tested in normotensive subjects using the dietary protocol specified here. They further concluded that salt-sensitivity is related to a familial history of hypertension.

The authors suggest that the lack of correlation between the blood pressure changes in the two parts of the study may be due to the different order of the dietary regimens.

University/Hospital:

Universitatsklinikum Steglitz, Freie Universitat Berlin

There is no explanation of the reason for attrition by six subjects in Part 1. Nor is there any description of the characteristics of those who failed to comply in Part 2.

No information was given about the ethnicity of the subjects. Nor was any information given about how they were recruited. There is no explanation of the reason for attrition by six subjects in Part 1. It is not clear how participants were selected to participate in Part 2 from among those meeting the criteria sought, i.e. salt-sensitivity or resistance. Nor is there any description of the characteristics of those who failed to comply in Part 2. These facts along with the very small sample size, 15, who completed Part 2, make it impossible to judge how pre-existing elements of their genetic make-up or environment may have influenced the outcome of the study.

The statistical analysis of the cross-over design did not address whether the order of the dietary treatments influenced the outcome variables. Therefore it is not clear whether there was carryover from one treatment to the next and whether a seven-day treatment was sufficient to control for this possible influence. In fact, the authors suggest that the lack of correlation between the blood pressure changes in the two parts of the study may be due to the different order of the dietary regimens.