This past week I’ve been taking in more salt than what I usually do. Not because I think I’m doing my body a favor by upping my intake of sodium chloride, but rather because so much of the food I’ve been served at the various social events I’ve attended during the holidays have been heavy on salt. I don’t feel like all of this salt has given my health a positive boost. Quite the opposite. I feel like my body isn’t performing at its best when its salt levels are as high as they’ve been lately. This isn’t surprising, seeing as the consumption of a lot of salt adversely affects blood pressure levels and cardiovascular health (1, 2, 3, 4). This is something almost all nutritionists, dietitians, and nutritional scientists recognize.
What’s not as widely known though, is that it’s not just the cardiovascular system that suffers in the presence of a lot of salt; many other systems of the human body do as well. Excessive salt consumption has been implicated not just in hypertension and heart disease, but also in the pathogenesis of various forms of cancers and autoimmune diseases, among other things (5, 6, 7). Accumulating research suggests that one of the primary reasons why salt has such a destructive impact on our health is that it lights up our immune systems (5, 6, 7, 8, 9, 10, 11, 12, 13)
How do gut bacteria react when they are exposed to a lot of salt?
The microbes that colonize our bodies regulate our immune systems; hence, it makes sense to start our discussion about the proinflammatory potential of salty diets by shining some light on the microbiome.
It’s well known that salt has antimicrobial properties. It blocks the growth of many bugs, which is why we use it to preserve food. What is often overlooked is that salt doesn’t leave its antimicrobial properties at the door sort to say when we consume it. It’s still capable of interfering with microbial growth when it’s inside of our bodies. Hence, it’s not surprising that recent research suggests that salt is a potent microbiome-modulator. Friendly gut bacteria such as Lactobacillus murinus don’t seem to fare so well when they are exposed to a lot of salt (14, 15, 16).
This research was done in animals and involved very high salt intakes; hence, it should be taken with a grain of salt 🙂 That said, I find it highly likely that our microbiotas, just like the microbiotas of mice, become more inflammatory in nature when they are exposed to salty conditions. Obviously, you won’t wipe out all of your friendly gut bugs by eating a lot of salty foods. You may inhibit the growth of some of them though, and thereby give other, more salt-resistant, and perhaps less friendly, bacteria a better foothold.
When we think about it, it’s not really surprising that recent research hints that salty diets don’t do a microbiome good, seeing as salt blocks microbial growth. If salt only inhibited the growth of bad bugs, this wouldn’t have been a problem; however, its effects are obviously not isolated to human pathogens. If you’ve ever tried making fermented vegetables and by mistake have added a little too much salt the mix, you’ve probably observed this for yourself. Lactobacillus plantarum and other so-called probiotics that are found in fermented foods don’t do so well when they are bombarded with salt. The environment of the human gut obviously differs from that of fermented vegetables or some other type of food; however, it’s possible that salt could exert some similar antimicrobial effects in the gut as it does outside of the body.
The proinflammatory potential of salt
Considering that the consumption of a lot of salt puts a strain on the cardiovascular system and likely shifts the composition of the gut microbiota into a more proinflammatory state, it’s not surprising that accumulating research suggests that high-salt diets are proinflammatory (5, 6, 7, 8, 9, 10, 11, 12, 13). Some of the most interesting findings to date come from a 2015 longitudinal study that looked into how three different salt intakes (6 g/day, 9 g/day, and 12 g/day) affected monocytic cells and immune responses in healthy humans (12).
The study, which was conducted in the frame of a long-term spaceflight simulation study, found that participants on the high-salt diet of 12 g/d displayed a significantly higher number of immune cell monocytes compared with the same subjects on a lower-salt diet. In addition, decreased salt intake was accompanied by reduced production of the proinflammatory cytokines interleukin (IL)-6 and IL-23, along with enhanced producing ability of anti-inflammatory cytokine IL-10. The study had a low number of participants, but it’s interesting nonetheless.
Here’s what a 2016 review paper entitled Over-salting ruins the balance of the immune menu had to say about the link between dietary salt and inflammation:
High salt levels interfere with alternative activation of macrophages (M2), which function in attenuating tissue inflammation and promoting wound healing. High salt also impairs Treg function by inducing IFNγ production in these cells. Together, these results provide evidence that environmental signals in the presence of high dietary salt enhance proinflammatory responses by interfering with both innate and adaptive regulatory mechanisms. (13)
Salt is obviously not solely responsible for fueling the inflammatory fire that’s burning inside the body of the modern man. There’s no doubt in my mind that it contributes to keeping the fire burning though. It definitely doesn’t act as a firefighter. It’s more like a dry piece of wood.
The preagricultural, natural diets that contributed to sculpting the human body and brain were low in salt; hence, it’s not surprising that salty diets don’t agree with the human biology
I don’t find it surprising at all that recent research studies indicate that salty diets are proinflammatory, given that it’s a very novel behavior, from an evolutionary perspective, for a human being to consume a lot of salt on a daily basis. The nutritional environments of our primal ancestors were low in salt. Hunter-gatherers don’t eat potato chips, crackers, bacon, sausages, or any of the other processed, salty foods that line the shelves of the modern supermarket and they don’t add salt to their food; hence, it’s not surprising that their diets tend to be very low in salt (1, 17).
The diets of modern, industrialized people, on the other hand, tend to be very high in salt. It’s not uncommon for a westerner to take in around 10 grams of salt a day, which is, from an evolutionary perspective, an extremely high salt intake. It’s more than 10 times as much salt as a typical hunter-gatherer consumes. Natural selection has never gotten around to preparing the human body for such an extreme salt intake; hence, it’s not surprising that a wealth of scientific research indicates that salty diets don’t do a body good.
Salty diets are unhealthy. They bring with them multitudinous negative health effects. The science on salt and cardiovascular health is crystal clear: high-salt diets are highly problematic in terms of their impact on blood pressure levels and heart health. The science on salt and microbiome/immune function is not as clear. It’s still in its infancy and is heavy on in vitro and animal studies. More human studies are needed to elucidate exactly how the human microbiome and the human immune system respond when they are exposed to large quantities of salt.
That said, I would argue that there’s more than enough evidence to conclude that the consumption of a lot of salt adversely affects immunity and microbiome composition. The fact that salt is problematic in this respect is undoubtedly one of the reasons why there’s a link between high salt intakes and various chronic diseases, including cancer, cardiovascular disease, and autoimmune illness.
There’s no doubt in my mind that we would be wise to follow the lead of our ancient ancestors, in the sense that we would be wise to limit our consumption of salt, which was not abundantly available prior to the time when we humans started mining salt and creating processed, salty foods.