Stomach acid is an important part of digestion. Most of us try to avoid having too much, because it can cause heartburn and acid reflux, but too little stomach acid can also be problematic. Hatty Willmoth writes.

Most of us have heard of having too much stomach acid – with heartburn, acid reflux, and all the other uncomfortable symptoms that come with it. It’s a very common and well known condition. 

But did you know that low stomach acid can also be problematic, with knock-on effects for the rest of the body? 

Katie Clare, registered nutrition practitioner and module coordinator at the Institute for Optimum Nutrition, says: “It’s not a disease to have low stomach acid; it’s not recognised as a condition in the same way that high stomach acid is. But it’s seen as a function of your body that might be working sub-optimally.” 

Hypochlorhydria (another way of saying ‘low stomach acid’) is the subject of disagreement among scientists, healthcare professionals and nutrition experts. Namely, it’s not clear how prevalent low stomach acid is, how best to test for it, or how to treat it. 

But there are some things we do know – starting with what low stomach acid is.

What is low stomach acid, or hypochlorhydria?

Stomach or ‘gastric’ acid is an important piece of the digestion puzzle. Its main active component is hydrochloric acid (HCl), but it also contains potassium chloride (KCl) and sodium chloride (NaCl). 

Along with these acids, the stomach secretes bile, digestive enzymes, and a protective mucus to stop these corrosive liquids from damaging the stomach lining. 

That’s because stomach acid is seriously acidic. Healthy stomach acid has a pH somewhere between 1 and 3; for context, lemon juice and vinegar are about 2, while 7 is as neutral as water. A pH value above 3 suggests possible hypochlorhydria, i.e. low stomach acid. 

Stomach acid needs to be that acidic in order to activate digestive enzymes in the stomach. That helps with digestion, enabling us to break down proteins and absorb nutrients. But it also kills potentially harmful bacteria, thereby protecting us from infection.  

Eating is a signal to the body to start producing more stomach acid so we can cope with digestion. That means its levels change – and on average, a human stomach secretes about 1.5 litres of stomach acid per day. 

But people with low stomach acid don’t produce enough of the active substances needed to digest food efficiently. (Here we’re talking about HCl, although some people don’t produce enough of other components of gastric acid either, such as the digestive enzyme pepsin.) 

With insufficient acidity to break it down, food may sit in the stomach for longer than it should, allowing for bacteria to build up. The food may travel to the intestines insufficiently digested, leading to fermentation as it passes through the gut, which can cause bloating and gas. Proteins and micronutrients (vitamins and minerals) may stay locked in the food intact, and harmful bacteria may travel into the intestines. 

So, it’s bad news all round.

"Things that can lead to suboptimal gut health, like low stomach acid, can impact your immune system, your gut bacteria, your brain health… The gut is important for everything!"

What can hypochlorhydria do?

Because low stomach acid disrupts digestion, it can come with a lot of symptoms generally associated with poor digestion – some of which overlap with symptoms of high stomach acid. 

People with low stomach acid may experience uncomfortable bloating, tummy cramps and symptoms of an upset stomach. They may feel a desire to eat that isn’t hunger, or get heartburn and acid reflux. Low stomach acid may lead to gas, diarrhoea, and poo that contains undigested food in it. And finally, people with low stomach acid may burp a lot, get bad breath, or feel fatigued. It’s an unpleasant business. 

But that’s not all. Protein is a macronutrient that we need to consume consistently in order to maintain good health, and we need sufficient stomach acid to digest it. 

Clare explains: “Proteins need a lot of digestion by acidity, because protein denaturing (the unfolding of the protein structure) is a really key part of protein digestion. So protein is a concern with low stomach acid.” 

If we can’t digest protein properly, we can develop a protein deficiency – no matter how much protein we’re eating. That can manifest itself in a number of ways, from hair loss to brittle fingernails and persistent fatigue. 

But protein is not the only nutrient dependent on stomach acid for digestion. Low stomach acid may also disrupt the absorption of vitamins, such as vitamin C, B6 and B12, and minerals, especially calcium, iron, folic acid and magnesium. 

“You need those nutrients for every system in the body,” says Clare, “from your hormones to your brain function, to the cardiovascular system [heart and blood vessels], so suboptimal digestion is going to affect all of those things.” 

In the long term, deficiencies in those nutrients can lead to conditions such as pernicious anaemia and osteoporosis.

Bacterial infections and gut health: knock-on effects of low stomach acid

Stomach acid is an important immune defence, killing off potentially harmful bacteria in our food before it can cause trouble. With this defence weakened, the bacteria can replicate, and then pass through to the small intestine. 

It follows that low stomach acid can increase the risk for bacterial infections (e.g. Clostridium difficile) and small intestinal bacterial overgrowth (SIBO). 

More generally, low stomach acid is a ticket to poor gut health, and that in itself is significant, as Clare explains. 

“It used to be that suboptimal gut function wasn’t seen as that important,” she says. “But now we’ve learnt about how much of the immune system is in the gut – about 70%. 

“Things that can lead to suboptimal gut health, like low stomach acid, can impact your immune system, your gut bacteria, your brain health… The gut is important for everything!” 

Age, stress and fatigue: three routes to low stomach acid

Given the far-reaching effects that low stomach acid can have on the rest of the body, it’s not something we want. But how do people develop low stomach acid? 

There are several different routes, the most common of which is old age. 

Clare says: “[Stomach acid] is thought to be something that deteriorates with age anyway, so in a lot of older people, it’s known that they might not be able to absorb things as well.” 

Consequently, older people are more at risk of developing digestive issues as a result of inadequate stomach acid. 

But low stomach acid is also often brought on by stress or fatigue – leading some to believe it’s more common than many think. 

Clare explains: “Stress is definitely going to have a big impact. It takes a lot of chemical energy to make stomach acid. 

“When you’re stressed, a lot of your resources are being diverted, and if someone’s fatigued, they might not have the energy spare to make enough stomach acid.” 

Clare also explains that stomach ulcers – usually associated with high stomach acid – might be caused by low stomach acid, due to stress. “People talk about stomach ulcers caused by stress but, digging into that, it’s not so much the stress but the rebound for stress. 

“When you’re stressed, you’re not making as much stomach acid, so your body’s also not going to be making as much mucus because you don’t need as much protection from that stomach acid. 

“And then, if you change your job, or have some time off, or go on holiday, your body is rested, and it can go back to making more stomach acid for digestion – but your mucus might not be there to cope with it. 

“So, it tends to be rebound from stress when you tend to be most vulnerable to get an ulcer. Or, in the words, of Stanford University’s celebrity neurobiologist, Robert Sapolsky, ‘the stomach gets caught with its pants down’!” 

Other causes of hypochlorhydria

So age, stress and fatigue are all common causes of low stomach acid, but they’re not the only ones. It can also be part of an autoimmune condition, says Clare, or triggered by a bacterial infection such as Helicobacter pylori. 

Low stomach acid can be brought on by stomach surgery, notably gastric bypass surgery, and it can be prompted or exacerbated by eating lots of sugar, eating too quickly, or eating foods to which you are sensitive or allergic. 

But the other notable group who develop low stomach acid are people on medications that reduce stomach acid. 

In particular, people who have experienced high stomach acid may be prescribed antacids called proton-pump inhibitors (PPIs) to combat heartburn, acid reflux and stomach ulcers. 

Studies show that people taking PPIs commonly have a resting stomach acid pH of between 5 and 7, leading some experts to warn against taking PPIs in the long term, unless really necessary. 

It starts at the root

So, you think you might have low stomach acid; what do you do? Number one: address the root cause. 

If you’re stressed or fatigued, this is much easier said than done, but recognising and tackling the issues at the heart of your stress or fatigue should have positive ripple effects on its physical manifestations, whether that be low stomach acid or something else. 

And if your low stomach acid has been brought on by medication or an infection, then a conversation with your doctor should be your next port of call. 

Of course, hypochlorhydria is not always brought on by a fixable issue; older people can’t just become younger. 

In that case, there are alternative ways to boost your stomach acid production. Because digestion starts in the mouth, Clare says taking the time to chew your food thoroughly and take smaller bites will make a difference, helping to get the processes of digestion started. 

Similarly, she says: “If someone is struggling with digestion, then things like soups and stews and slow-cooked meals, where a lot of that digestion has already happened, can be a good way to support it.” Foods such as these may be particularly helpful for those who find thorough chewing difficult, such as elderly people. 

Bitter foods to stimulate stomach acid

Another tactic is attempting to boost stomach acid production naturally, through the diet. Clare says bitters are a particular interest of hers, and they have the potential to help here. 

“Bitters are just the bitter components in food,” she says. “Bitter tastes are thought to help stimulate stomach acid. 

“Our culture is very anti-bitter – bitter flavours are often taken out of foods – but in place like Italy or China or India, it’s more part of the diet.” 

The research on bitter flavours and their link to stomach acid is not exactly exhaustive, but there is evidence that tasting bitterness prompts the body to secrete more stomach acid, digestive enzymes and bile, supposedly in preparation to digest a challenging food. 

“You could have a bitter salad of rocket and radicchio before a meal,” suggests Clare, “or black coffee after a meal. All of these things are seen as good for the digestive system.” 

Other bitter foods include green leafy vegetables such as spinach and kale, herbs like dandelion, and fruits such as grapefruit and bitter melon. Swedish bitters can also be bought from health food shops, says Clare. 

Some of us may find bitter flavours difficult to enjoy, she says, but it is possible to wean onto them gradually. 

Apple cider vinegar: a myth?

A common recommendation for people with low stomach acid is drinking apple cider vinegar – in small amounts, diluted with water. 

Apple cider vinegar is a fermented liquid made from crushed apples, bacteria and yeasts that has been associated with a wide range of health benefits, some with more scientific backing than others. 

Clare warns that a lot of the ‘miracle cures’ associated with apple cider vinegar are not backed up by much evidence. 

However, in this instance, she says: “It has a low pH, so it makes sense that it helps, along with foods like lemon and other vinegars.” 

Plus, studies support the fact that apple cider vinegar has antimicrobial properties, which may help with bacterial issues that can result from low stomach acid. Most apple cider vinegar is also probiotic, so it should help cultivate a diversity of gut bacteria too; key to gut health. 

Clare says she sometimes makes herself “a little digestive aperitivo” of hot water with apple cider vinegar, lemon and bitters to drink before meals. 

Supplements: pills to the rescue?

Of course, there’s always the supplement route. Stomach acid can be supplemented with HCl, which can be taken before mealtimes to quickly heighten the acidity of the stomach and thereby facilitate digestion. 

There’s not a wealth of evidence on the efficacy of HCl supplements, but the studies that do exist indicate that it’s a safe and often successful method of combating hypochlorhydria. 

But Clare warns: “With those, there’s always a concern that you could give somebody a stomach ulcer, if they don’t have enough mucus. They’re always an option, but you should be cautious with them.” 

Clare also suggests addressing other components of digestion too. “If someone’s stomach acid is low, it might be that their digestion is sluggish in general, so you might think: how is their bile production? 

“Some supplements might be more complex, containing hydrochloric acid [HCl] alongside bile and digestive enzymes.” 

And if digestive enzymes would be helpful, then there are ways of getting those from food, she says. “Certain foods, like pineapple – particularly the core – and green papaya; those things raw are thought to contain digestive enzymes, which can help. 

“They’re the sorts of things you’d get in a supplement, but you can get them in foods as well.” 

And, on the subject of supplements, some people may benefit from addressing specific nutrient deficiencies that they may have developed due to low stomach acid, either with food alone or with supplements too. 

Gut health and inflammation

Other recommendations commonly touted on the internet include eating probiotic foods and reducing inflammatory foods in the diet. 

It is difficult to trace an exact link between probiotic foods and stomach acidity in the research, but probiotics are generally good for gut health. 

Since low stomach acid levels disrupt the gut microbiome and may introduce harmful bacteria to the gut, so it follows that probiotics – which introduce beneficial bacteria to the gut – would be useful. 

As far as inflammation goes, the link is even more tenuous, but eating more anti-inflammatory foods (e.g. lots of fresh fruit and vegetables, and healthy fats) and fewer pro-inflammatory foods (e.g. ultra-processed foods, refined carbohydrates, and damaged fats) can only be a positive. 

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