The bicarbonate habit: an antacid that switches off pepsin
Mr Ngwa, a 42-year-old teacher in Buea, has had a burning pain in the upper abdomen for two months. Food relieves it for an hour or so, then it returns. A friend told him the pain comes from 'too much acid', so for the last three weeks he has taken two heaped teaspoons of bicarbonate of soda in half a glass of water after every meal, and sometimes before meals as well. He now reports that he belches constantly and feels bloated, that his stools are loose and contain recognizable shreds of meat, and that the pain now wakes him at about two in the morning. He has not gained weight. He asks whether he should double the dose, and says the powder must be harmless because it is 'just natural salts'. He takes no other medicine apart from occasional ibuprofen for backache and drinks two bottles of beer at weekends.
1. Explain, using the pH of the stomach and the properties of pepsin, why Mr Ngwa's protein digestion became less efficient after he started taking bicarbonate of soda. (6 marks)
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What the stomach normally provides
- Parietal (oxyntic) cells secrete hydrochloric acid, holding gastric juice at about pH 1.5-2.0.
- Chief cells secrete the inactive proenzyme pepsinogen, which becomes pepsin only when the pH is low.
- Pepsin works best near pH 2. Above about pH 4-5 it is inactivated, and the loss becomes permanent at higher pH.
- The acid also denatures food protein, unfolding it so peptide bonds are exposed, and kills most swallowed bacteria.
What his remedy does
NaHCO3 + HCl -> NaCl + H2O + CO2. The carbon dioxide is the cause of the belching and bloating. The acid is neutralised and gastric pH rises far above pepsin's optimum, so pepsin is idle for much of the time the meat is in the stomach. Because the protein is also poorly denatured, fewer bonds are exposed for attack. Undigested fibres therefore reach the stool.
Do not overstate it
Digestion is reduced, not abolished: pancreatic proteases (trypsin, chymotrypsin, carboxypeptidases) act at alkaline pH in the duodenum and partly compensate. That is why he has loose stools containing fibres rather than gross malabsorption with wasting.
2. Explain why the stomach wall does not digest itself, and why Mr Ngwa's pain has become worse at night despite the neutralising powder. (6 marks)
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Why the wall survives
- Surface mucous cells secrete a thick mucus layer that also traps bicarbonate, keeping the epithelial surface near neutral pH while the lumen is at pH 2.
- The epithelium is joined by tight junctions, so acid does not leak between cells.
- Pepsin is secreted as inactive pepsinogen, so it is only activated in the lumen, not inside the cells that make it.
- Prostaglandins maintain mucus and bicarbonate secretion and mucosal blood flow, and the lining is replaced rapidly by mitosis in the crypts. Ibuprofen inhibits prostaglandin synthesis, so his occasional painkillers weaken this defence.
Why night pain is worse
Acid secretion is inhibited by acid itself: when pH falls below about 3, gastrin release from the antrum is switched off. Neutralising the acid removes that brake, so gastrin rises and the stomach secretes more acid once the bicarbonate is used up. There is then a rebound of acidity. At 2 a.m. the stomach is empty, so there is no food to buffer the acid, and it irritates an already inflamed or ulcerated area. Distension from the CO2 can also cause reflux. Night pain that wakes him, plus weight loss, needs medical assessment, not a bigger dose.
3. Advise Mr Ngwa on what he should do, justifying each point in terms of what you know about antacids, timing and ulcer disease. (6 marks)
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Stop the home-made high-dose powder
The dose is uncontrolled. Large sodium loads are undesirable, prolonged alkalosis can occur, and the rebound acidity makes the problem worse. 'Natural' is not the same as harmless.
Correct timing if an antacid is used at all
One hour after meals and at bedtime, not with the meal. Food slows gastric emptying, so an antacid taken an hour after eating stays in the stomach for up to three hours; taken on an empty stomach it is emptied within about half an hour and is largely wasted. Tablets are preferable to spoonfuls of powder because the dose is known.
Get the diagnosis settled
He needs to be seen at a clinic: test for Helicobacter pylori, and possibly an endoscopy if there is weight loss, night pain, anaemia or vomiting of blood. A proton-pump inhibitor or H2 blocker prescribed by a clinician suppresses acid far more reliably than bicarbonate and allows the mucosa to heal.
Remove the things that slow healing
Stop the ibuprofen where possible (it inhibits protective prostaglandins), reduce alcohol and stop smoking. Eat smaller, regular meals. Explain that the pain waking him at night is a warning sign, not proof that the dose is too small.