RESTRICTIONS ON SALT INTAKE: ARE THEY REASONABLE FOR ALL DIABETICS?
Many diabetics have hypertension, or high blood pressure. About half of all people with hypertension will experience blood pressure elevations when they eat substantial amounts of salt for at least two months. This rarely occurs in those who are not already hypertensive. Hypertension accelerates glomerulopathy (destruction of the glomerulus) in people with chronically elevated blood sugars, but in type 1 diabetes, hypertension usually appears after, not before, the appearance of kidney damage as indicated by significant amounts of albumin in the urine. Is it therefore appropriate to ask all diabetics to lower their salt intake?
Your physician might find informative the following articles on this subject:
“Molecular and Physiological Aspects of Nephropathy in Type 1 Diabetes Mellitus,” by Raskin and Tamborlane, Jnl Diabetes and Its Complications, 1996, 10:31–37; “The Effects of Dietary Protein Restriction and Blood Pressure Control on the Progression of Chronic Renal Disease,” by S. Klahr et al., New England Jnl Med, 1994, 330:877–884; also, in the same issue of New England Jnl Med, the editorial
“The Role of Dietary Protein Restriction in Progressive Azotemia” (pp. 929–930). Another study, in the journal Diabetes Care, 25:425–430, in the year 2000, showed that obese people on a high-protein diet lost more fat and less muscle mass than those on a low-fat diet. They also showed more than double the reduction in LDL (the “bad” cholesterol).
Let us look at a few of the mechanisms involved in the hypertension that some diabetics experience.
People with advanced glomerulopathy will inevitably develop hypertension, in part because GFR is severely diminished. These people cannot make enough urine, and therefore retain water. Excessive water in the blood causes elevated blood pressure. There are many other ways hypertension can be caused by high blood sugars. The mere presence of high blood sugar will cause water to leave tissues and enter the bloodstream, even experimentally in nondiabetics.
It is not unusual to observe reduction in blood pressure concomitant with control of blood sugar. Studies have shown that many, and possibly most, hypertensive nondiabetics are insulin-resistant, and therefore have high serum insulin levels. In addition to causing elevation of serum triglycerides and reduction in serum HDL in nondiabetics, high serum insulin levels have long been known to foster salt and water retention by the kidneys. Furthermore, excessive insulin stimulates the sympathetic nervous system, which in turn speeds up the heart and constricts blood vessels, causing further increase in blood pressure.
Thus type 2 diabetics who eat lots of carbohydrate, and therefore will tend to make excessive insulin, can readily develop hypertension. Type 1 diabetics treated with the usual industrial doses of insulin to cover high-carbohydrate diets are likewise more susceptible to hypertension. One dramatic study showed that in hypertensive individuals, blood pressure is directly proportional to serum insulin level.
A report from Nottingham, England, showed that a brief infusion of insulin and glucose would increase blood pressure in normal men without changing their blood sugars. A 1998 study in Glasgow, Scotland, demonstrated that salt restriction increased insulin resistance in type 2 diabetics.
*A study of older individuals who were rotated between low-, moderate-, and high-salt diets demonstrated that those on low-salt diets experienced significantly more sleep disturbances, and had more rapid heart rates and higher serum norepinephrine (adrenaline) levels. An international study called Intersalt, covering 10,079 people in 32 countries, reported in 1988 that “salt has only small importance in hypertension. ”More recently, another study showed that salt restriction increases insulin resistance and thus can indirectly increase blood pressure. Large amounts of dietary salt can facilitate loss of calcium from bones of post-menopausal women, who are already at high risk for osteoporosis (bone weakening).
Why don’t all diabetics on high-carbohydrate diets or all poorly controlled diabetics have hypertension?
One reason is that the body has several very efficient systems for unloading sodium (a component of salt) and water. One of the more important of these systems is controlled by a hormone manufactured in the heart called atrial naturietic factor (ANF).When the heart is expanded by even a slight fluid overload, it produces ANF. The ANF then signals the kidneys to unload sodium and water. Hypertensive individuals, and the children of two hypertensive parents, tend to produce much lower amounts of ANF than do normal people. Nonhypertensive
diabetics apparently are able to produce enough ANF to control the blood pressure effects of high blood sugars and high serum insulin levels, provided they do not have moderately advanced kidney disease. Indeed, a study, in which some of my patients participated, showed that diabetics with high blood sugars produce significantly more ANF than those with lower blood sugars.
How does all this apply to you?
First, you and your physician should know if you have glomerulopathy. This is readily determined if the renal risk profile tests suggested in Chapter 2 are performed. If these tests are abnormal, your physician may advise you to reduce your salt intake because salt is much more likely to cause hypertension in people with diminished GFR.
Whether your renal risk profile is normal or abnormal, your resting blood pressure should also be measured. A proper measurement requires that you be seated in a quiet room, without conversation, for 15–30 minutes. Blood pressure should be measured every 5 minutes, until it drops to a low value and then starts to increase. The lowest reading is the significant one. If you get nervous in the doctor’s office, then you should measure your own blood pressure at home in a similar fashion. Repeated measurements, with low values just exceeding 135/ 85, suggest that your blood pressure is “borderline.” (The American Diabetes Association suggests that 120/80 be considered borderline for younger diabetics.) You then may benefit from dietary salt reduction. The only way to find out is to check your blood pressure while on your current salt intake, and again after following a low-salt (sodium) diet for at least two months.
Your physician can give you guidelines for such a diet, and you can consult nutritional tables such as those in the books listed in Chapter 3. I would suggest that resting blood pressures be measured several times a day, and at the same hours each day, throughout the study. Each day’s blood pressures can then be averaged, and the averages compared. If your blood pressure drops significantly on the low-salt diet, your physician may urge you to keep the salt intake down.
Alternatively, he may want you to take small amounts of supplemental potassium, which tends to offset the effects of dietary salt on blood pressure.Recent studies suggest that as many as 40 percent of hypertensive patients (the so-called low-renin hypertensives) may show lower blood pressures when they take calcium supplements.