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Medications and Nutrition Online Drug and Nutrition Overview

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About | User Information | Abbreviations | Drugs & Nutrition Overview

Icebergs are an excellent metaphor for our knowledge of the effects of drugs on nutritional health –we’re aware of their existence, but we don’t really know the size of the underwater or “hidden” component.

Drugs can both affect our nutritional status and be affected by various foods, nutrients and our nutritional health. This overview will briefly address some of these aspects.

How Drugs Affect Nutrition

Drugs can affect nutrition at many points.

1) Alteration To Food Intake

Food intake can be affected both directly and indirectly:

  • the primary direct effects are either increased or decreased appetite

  • indirect effects are the adverse reactions such as nausea, constipation, dry mouth

Consequences of the adverse reactions include

  • change in food intake to include or exclude certain foods

  • refusal to eat

  • use of over-the-counter drugs (OTCs) for relief of symptoms

  • refusal to take the drugs

Altered food intake and the consequences of adverse reactions can seriously compromise the general well-being of the consumer.

2) Alteration to Nutrient Absorption

Nutrient absorption can be affected by both direct and indirect mechanisms of interference

  • the primary direct mechanisms are related to inhibition of absorption of nutrients by drugs eg carbamazepine competitively inhibits the absorption of biotin and pantothenic acid

  • indirect mechanisms include altered gastro-intestinal (GI) tract pH:

    • altered nutrient absorption due to increased gut pH. Some nutrients are pH sensitive for absorption, and increased pH will decrease their absorption, for example maximal folate absorption is pH 6.0-6.2 and there is no absorption at pH 7.0.

    • bacterial overgrowth. As gut pH increases, gut microflora survive further up the GI tract, and access nutrients before the host, resulting in reduced availability of nutrients for absorption by the host.

3) Alteration To Nutrient Metabolism

Some drugs alter nutrient metabolism and thus alter nutrient requirements. For example phenytoin increases the metabolism, and therefore the requirements, of vitamins D, K, and folate.

Some drugs inhibit the conversion of a vitamin to its active form, and may therefore cause a resultant deficiency eg isoniazid and vitamin B6.

4) Alteration To Nutrient Excretion

Nutrient excretion can be increased or decreased

  • increased excretion – eg frusemide and potassium, and a potassium supplement is typically prescribed to compensate for the drug-induced loss

  • decreased excretion – eg thiazide diuretics and calcium, which has a positive effect on bone health

Polypharmacy Effect?

The degree of effect of polypharmacy (multiple medications) on the status of vitamins and minerals is difficult to ascertain. Some unanswered questions

  • is nutrient depletion dose dependent?

  • is there a time factor?

  • is the depletion additive, synergistic, a combination?

  • is there an adaptive response?

  • if there is an adaptive response then at what point does the adaptation cease and irreversible change commence?

  • are some of the debilitating effects in the elderly due to sub-clinical or overt clinical mal-nutrition?

There is a continuum of nutrient depletion that includes a reversibility/irreversibility component. A classic example is thiamine deficiency in people who are alcoholics. Alcohol intake is associated with decreased thiamine availability. Initial thiamine deficiency manifests as Wernickes syndrome and is reversible with appropriate thiamine intervention, however if the deficiency is not addressed then there is progression to Korsakoff’s Psychosis which is essentially irreversible even with thiamine intervention.

The clinical manifestations of drug-induced disorders - anaemia, diarrhoea, weight loss, etc - are often attributed to the underlying disease.

Nutritional Factors That Alter Drug Effect

Many factors alter drug effect including food intake, nutrient supplements and one’s nutritional status. These factors include:-

  • the presence of food in the gut - which may alter
    • i.     gastric secretion
    • ii.    GI motility
    • iii.   gut transit time
  • recent changes in diet – which can be voluntary or involuntary
    • i.     voluntary changes
      • philosophical beliefs such as vegetarianism
      • dietary change for pragmatic reasons such as health benefits eg reduced salt intake may alter the necessity or dose of some drugs
    • ii.    involuntary changes
      • new diagnosis necessitating significant dietary change eg coeliac disease, diabetes
      • change in domicile such as from home to residential care – results in altered meal provision – type of food consumed, serving times of meals, meal split eg whether the main meal is at lunch or the evening meal
  • malnutrition – the plasma proteins are typically indicators of nutritional status and are generally reduced in malnutrition. If there is ≥ 90% binding of drug to plasma proteins, then it is likely the plasma proteins are the primary transporters for those drugs. Inadequate protein status may result in altered drug availability and consequently altered therapeutic effect(s) and possible exacerbation of side effects - for example INR is significantly altered by hypoproteinaemia.
  • weight change –some drug dose calculations include weight directly; more commonly weight is included in the calculation of renal function. Change in weight may necessitate change in drug dose for example a loss of weight will result in increased thyroxine effect and a gain in weight will result in reduced thyroxine effect.
  • food-drug interactions – can be either positive or a negative
    • negative interactions include
      • the stimulatory effect of caffeine opposes the effect of sedatives
      • intermittent intake of grapefruit products alters the effectiveness of a range of drugs
      • goitrogenic foods interfere with thyroid function
    • positive interactions include
      • regular daily intake of grapefruit products
      • a low salt diet which may result in decreased dose for a range of drugs
      • riboflavin as an adjunct therapy in the management of malaria
    • Foods may alter urinary pH which can alter the half-life of some drugs. Foods such as milk, vegetables and citrus fruits can alkalinize the urine; and meats, fish, cheeses and eggs can acidify the urine.
  • nutrient-drug interactions – concomitant administration of a range of drugs with mineral supplements is likely to decrease the availability of both drugs and mineral supplements; an example is iron and thyroxine.

    Nutrient supplements can also affect drug utilisation. For example pyridoxine can reverse the anti-parkinsonism effect of levodopa, and vitamin K counteracts the anticoagulant effect of warfarin.

Nutritional factors have a profound impact on the overall effect of drugs.

Strategies For Some Interactions

Strategies can be developed to utilise the positive interactions and manage the negative interactions, and could include

  • a low salt diet – not adding salt during cooking, and choosing low salt products (< 120 mg sodium (Na)/ 100 g product)
  • a caffeine-free period from evening meal onwards – to minimise the caffeine-stimulatory effect on prescribed sedatives and sleep
  • mineral supplements and multivitamin & mineral supplements to be administered at a different time from most drugs – to minimise nutrient-drug interactions
  • a sunshine policy – sunshine increases vitamin D uptake and melatonin availability. Vitamin D is associated with an improvement in health factors such as bone health, and reduced risk of falls; melatonin is particularly associated with regularising the circadian rhythm and therefore improved sleep patterns
  • stop dates when nutrient supplements such as iron, zinc are prescribed. Long term single nutrient supplements may interfere with the status of other nutrients that share the same absorption mechanism


The potential for drugs to affect nutritional status, and for nutritional status to impact on drug action is steadily gaining recognition. Drugs can significantly affect the intake, absorption, metabolism and excretion of nutrients. Drug effect can be significantly affected by factors such as malnutrition, voluntary or involuntary change in diet, and nutrient supplements. The interactions can be both positive and negative.

The inclusion of the effects of drugs on nutrition in the clinical management of the people in our care can only enhance their overall well-being. Although there are many issues that require further research, there is sufficient knowledge for application in the clinical setting today.

Maybe in 5 years time the iceberg will seem smaller!



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