8.1 Drug-Nutrient Interactions and Nutrient Depletion
Key Takeaways
Patients taking MAO inhibitors must avoid tyramine-rich foods such as aged cheeses, cured and fermented meats, soy sauce, and fermented fish pastes, because tyramine can trigger a hypertensive crisis.
Isoniazid, used to treat tuberculosis, antagonizes vitamin B6, so pyridoxine is given to prevent peripheral neuropathy.
Grapefruit juice inhibits the intestinal enzyme CYP3A4 and can raise blood levels of some statins, calcium channel blockers, and cyclosporine.
Tetracyclines and fluoroquinolones bind calcium, iron, magnesium, and zinc, so they should not be taken with milk, antacids, or mineral supplements.
Long-term metformin use reduces vitamin B12 absorption, and loop diuretics increase urinary losses of potassium, magnesium, and thiamin.
The table of specifications asks examinees to identify drug-nutrient interactions and depletions. Nutritionist-dietitians review medication lists during assessment because interactions can cause treatment failure, toxicity, or deficiency.
Types of Interactions
| Type | Mechanism | Example |
|---|---|---|
| Food affects drug absorption | Binding, changes in gastric pH or emptying | Calcium and iron bind tetracycline; food reduces levothyroxine absorption |
| Food affects drug metabolism | Enzyme inhibition or induction | Grapefruit juice inhibits CYP3A4 |
| Food affects drug action | Opposing or additive effects | Vitamin K opposes warfarin; tyramine with MAO inhibitors |
| Drug affects nutrient absorption | Binding, malabsorption, changes in pH | Orlistat reduces fat-soluble vitamin absorption; PPIs reduce B12 absorption |
| Drug affects nutrient metabolism | Antagonism or increased breakdown | Isoniazid antagonizes vitamin B6; phenytoin increases vitamin D breakdown |
| Drug affects nutrient excretion | Increased urinary loss | Loop diuretics increase potassium, magnesium, and thiamin loss |
| Drug affects intake | Appetite, taste, nausea, dry mouth | Chemotherapy causes anorexia; corticosteroids and megestrol increase appetite |
High-Yield Interactions
| Drug | Interaction | Nutrition advice |
|---|---|---|
| Warfarin | Vitamin K opposes its anticoagulant effect | Keep vitamin K intake consistent; avoid sudden large changes in leafy vegetables, and report new supplements |
| MAO inhibitors (some antidepressants) | Tyramine causes norepinephrine release and severe hypertension | Avoid aged cheese, cured or fermented meats, fermented fish and shrimp pastes, soy sauce, tap beer, and overripe or spoiled foods |
| Levothyroxine | Calcium, iron, soy, and fiber reduce absorption | Take on an empty stomach 30-60 minutes before breakfast; separate from calcium and iron by about 4 hours |
| Tetracycline, ciprofloxacin | Chelation with calcium, iron, magnesium, zinc, and aluminum | Avoid milk, antacids, and mineral supplements around the dose |
| Isoniazid (tuberculosis) | Vitamin B6 antagonist | Give pyridoxine supplements to prevent neuropathy |
| Metformin | Reduced vitamin B12 absorption; GI upset | Monitor B12 in long-term use; take with meals |
| Methotrexate | Folate antagonist | Folic acid supplements as prescribed |
| Phenytoin | Lowers folate and vitamin D; enteral formula reduces drug absorption | Monitor folate and vitamin D; hold tube feeding before and after the dose as per protocol |
| Loop and thiazide diuretics | Increased urinary potassium and magnesium (and thiamin with loop diuretics) | Potassium-rich foods or supplements as ordered |
| Potassium-sparing diuretics, ACE inhibitors, ARBs | Potassium retention | Avoid potassium-based salt substitutes and excess potassium supplements |
| Corticosteroids | Hyperglycemia, sodium retention, potassium and calcium loss, protein breakdown, increased appetite | Control sodium and simple sugars; ensure calcium, vitamin D, and protein |
| Proton pump inhibitors | Reduced acid lowers B12, iron, calcium, and magnesium absorption | Monitor in long-term use |
| Orlistat | Blocks fat absorption, including vitamins A, D, E, and K | Take a multivitamin at a different time of day |
| Bile acid sequestrants (cholestyramine) | Bind fat-soluble vitamins and folate | Separate supplements from the drug |
| Lithium | Low sodium intake raises lithium levels | Keep sodium and fluid intake consistent |
| Levodopa | Large neutral amino acids from protein compete for absorption and brain uptake | Spread protein evenly or follow the prescriber's protein timing advice |
| Grapefruit juice with some statins, felodipine, cyclosporine | CYP3A4 inhibition raises drug levels | Avoid grapefruit juice with these drugs |
| Metronidazole | Disulfiram-like reaction with alcohol | Avoid alcohol during treatment and for a few days after |
| Broad-spectrum antibiotics | Reduce gut bacteria that make vitamin K | Watch for bleeding in poorly fed patients |
| Digoxin | Low potassium or magnesium increases toxicity | Maintain potassium and magnesium |
Important
For warfarin, the goal is consistency, not avoidance. A patient who suddenly eats large amounts of malunggay or pechay may lower the drug's effect, while one who stops eating green vegetables may bleed.
Drug-Nutrient Interactions in Tube Feeding
- Phenytoin binds to formula proteins and its absorption falls; feeding is often held for about 1-2 hours before and after each dose, with drug levels monitored.
- Warfarin dosing must account for the vitamin K content of the formula.
- Crushing enteric-coated or extended-release tablets is not allowed; ask the pharmacist for a liquid form.
- Flush the tube with water before and after each medication to prevent clogging.
- Liquid medications that contain sorbitol can cause diarrhea that is wrongly blamed on the formula.
Drugs That Change Appetite, Taste, or Body Weight
| Effect | Examples |
|---|---|
| Increased appetite and weight gain | Corticosteroids, megestrol acetate, some antipsychotics and antidepressants, insulin |
| Decreased appetite | Chemotherapy, stimulants, GLP-1 receptor agonists, digoxin toxicity |
| Taste changes | Metronidazole (metallic taste), ACE inhibitors, chemotherapy |
| Dry mouth | Anticholinergics, antihistamines |
| Nausea and GI upset | Metformin, iron supplements, antibiotics |
The Nutritionist-Dietitian's Role
- Review all prescription drugs, over-the-counter drugs, herbal products, and supplements during assessment.
- Identify interactions and nutrient depletions with the highest clinical risk.
- Give practical timing advice in coordination with the physician and pharmacist.
- Monitor relevant laboratory values, such as potassium, INR, B12, or drug levels.
- Document the interaction and the counseling given.
A patient taking an MAO inhibitor antidepressant asks which meal is safest. Which choice should the nutritionist-dietitian recommend?
Aged cheddar cheese sandwich and salami
Rice with bagoong and longganisa
Fresh grilled fish, rice, and steamed vegetables
Tofu with soy sauce dip and fermented black beans
A patient starting treatment for pulmonary tuberculosis is prescribed isoniazid. Which supplement is usually given with it, and why?
Vitamin B6, because isoniazid antagonizes pyridoxine and can cause peripheral neuropathy
Vitamin K, because isoniazid causes bleeding
Vitamin B12, because isoniazid blocks intrinsic factor production and causes pernicious anemia
Iron, because isoniazid causes iron loss in urine
Which instruction is correct for a patient taking a tetracycline antibiotic?
Take it with a glass of milk to protect the stomach
Take it with your iron and calcium supplements so you remember all three
Take it with an antacid to reduce stomach upset
Avoid milk, antacids, and mineral supplements around the time of the dose
Sections you finish are checked off in the contents.