As a physician, one of my hobbies is reading books on health written for laypeople. I first read this book a few years ago. The author is a Pharm D (doctor of pharmacology) who has both clinical and research experience. There are some things in this book that surprised me completely.
Surprise #1: For some people, cutting salt will increase blood pressure.
First off, normal blood pressure (BP) is less than 120/80. A BP of 120/80 is not normal and considered prehypertensive but 119/79 is normal. In people with normal BP, prehypertensive, and hypertension, respectively 80%, 75%, and 55% will not have an increase in BP with a high salt intake.
Now, let’s look at the flip side of lowering salt intake and the BP result: in people with severe hypertension (defined differently over the years, approximately >160 or 180/>90 or 100), only 25% had any benefits, however, harms were found including worsening kidney function. People also had to severely restrict salt intake to about 1/11 teaspoon to get any benefit. Studies have consistently found that lowering salt only reduces BP by an average of 3.6/1.6 mmHg in people with a BP of 140/90 or higher. Would you be happy if you cut out all your salt, ate bland food and your BP went from 140/90 to 136-137/88-89?
Of 52 populations studied in the Intersalt study, only 5 had a low salt intake of less than 2,400mg of sodium daily. Only 1 of the 5 were not a primitive group and they were found to have higher systolic BP (top number) compared to several high salt groups. When the primitive groups were removed from the analysis, there was a clear decrease in BP as salt intake increased.
Higher salt intake also causes blood vessels to relax resulting in lower peripheral vascular resistance (fancy term or relaxed arteries and arterioles). In a study of people with normal BP, prehypertension, and mild hypertension, when restricting salt 41% had an increase in. BP! When looking only at those with hypertension, 37% had an increase in BP. The author summarizes this study and the increase in BP to occur in salt-restricted normal BP, prehypertensive, and hypertensive as 3/5, 2/5, and 1/3 respectively who will have an increase in BP!
Surprise #2: Low Salt Diet Health Risks
The author discusses several health risks and I will only focus on a few.
A low salt diet results in increased heart rate resulting in increased cardiovascular risk. In animal studies, it has been shown to increase triglycerides, LDL, decrease HDL, and Total cholesterol: HDL ratio (you want that ratio low or higher HDL).
A low salt diet compromises kidney function by increasing peripheral vascular resistance which essentially increases the risk of hypertension. Further, kidney hormones are increased. Every day, our kidneys filter 3.2-3.6 lbs of salt, or about 150 times the amount we ingest. The kidneys use a lot of energy to reabsorb all of the salt. A low-salt diet stresses the kidneys.
Reduced fertility. In pigs, a low salt diet results in a doubling of time from weaning offspring to fertility and reduced successful mating. In mice, it triggers reproduction failure. In humans and animals, a low-salt diet acts to lower sex drive, weight of infants, animal litter size, and increase erectile dysfunction and time to female sexual maturity.
Metabolic problems have been found with low salt diets including reduced adiponectin. Low-salt diets have been found to cause insulin-resistant blood vessels leading to constriction. Worse, a low-salt diet will cause you to make more insulin. Think of insulin as one of the kidney hormones, when it is high, the kidneys hold onto salt, when it is low, they dump it. The increase in insulin is a direct result of salt restriction and the body’s way of trying to hold onto salt. Low salt diets also result in less brown fat and this is not good as this fat burns glucose to produce heat and helps with weight loss.
Surprise #3: The Relationship Between Salt and the Thyroid
Iodine deficiency is more common than we think. Most people get their iodine from table salt. Cutting salt intake significantly increases the risk of thyroid problems. People who sweat, especially due to exercise in hot environments, lose a significant amount of sodium and iodine in their sweat. A study of soccer players practicing in a hot environment found 50% of the players to have an enlarged thyroid! Sodium and iodine loss in sweat was found to be high in all of them with some having losses of 50% or more than the daily intake of either mineral.
People with hypothyroidism, even if treated with thyroid hormone replacement frequently have chronic and persistently low blood sodium levels. This is caused by salt-wasting in the kidney due to not enough thyroid hormone. Over time, this can be permanent.
Not included in this book
My only critical critique of this book would be the lack of mention of POTS or Postural Orthostatic Tachycardia Syndrome. That said, the author did discuss the increase in heart rate of people on low salt diets as well as symptoms experienced by people on medications that would cause a POTS-type response.
DiNicolantonio, Dr. James (2017).The Salt Fix: Why the Experts Got it All Wrong. -- and how eating more might save your life. Harmony Books in US, Piatkus in UK
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