The ways in which drugs alter nutritional balance tend to be subtler and take longer to develop than the effects of food on medication. Drugs may have an influence on food intake due to changes in taste and appetite, they may decrease the absorption of nutrients, change their metabolism or cause them to be excreted in increased amounts (accompanied by diarrhoea and vomiting).
Drugs that affect food intake
Sensitivity to taste is one of the more delicate senses which can be affected fairly easily. Some drugs dull the tastebuds and make eating a less pleasurable experience - which, of course, may have the effect of making you eat less. Three drugs which decrease the sense of taste are clofibrate (a cholesterol-lowering agent), levodopa (an anti-Parkinson agent) and penicillamine (a chelating agent).
One or more of the four basic tastes (sweet, sour, salty and bitter) may be affected. For example, amphetamines decrease sensitivity to sweetness, while fluorouracil (a cytostatic agent) increases it. Diabetics who have to use insulin over prolonged periods find that they experience decreased sensitivity to both salty and sweet tastes. Other drugs - such as allopurinol (an anti-gout agent) - simply leave an unpleasant taste in the mouth.
Drugs that alter appetite
As the table below shows, some drugs directly stimulate the appetite, while others depress it. (Oral contraceptives, for example, tend to have the former effect.)
Drugs that decrease the absorption of nutrients
Drugs can affect the absorption of nutrients in several ways (including changes in the time the food spends in the digestive tract, in the acidity of the stomach, bile acid activity, enzyme systems, mucosal cells, or through binding to nutrients).
Firstly, the absorption of certain nutrients may be affected when the digestion of food is accelerated - a side effect of certain pills given for high blood pressure (ganglion blockers) - or when micro-organisms necessary for digestion are destroyed. In particular, drugs used to prevent indirectly the absorption of cholesterol from food into the body are often accompanied by reduced absorption of other nutrients (especially fat soluble vitamins). As another example, antacids are given to 'mop up' the excess acid in the stomach, but thiamin (vitamin B1) needs an acid medium lower down in the digestive tract to be absorbed, so antacids may impair its absorption.
Drugs that affect nutrient metabolism
Interference with nutrient metabolism can occur when a drug is similar in structure to a vitamin and therefore displaces a vitamin in a metabolic pathway. One example of this is the interaction between methotrexate (amethopterin), a cancer chemotherapeutic agent, and folic acid. Methotrexate kills cancer cells by displacing folic acid
and is therefore called a 'folic acid antagonist'. The problem is that the healthy cells are also prevented from using folic acid, which results in a type of anaemia.
The Pill and vitamin deficiency
The effect of oral contraceptives on nutritional deficiences has received wide attention in recent years. It has been shown that usage of 'the Pill' can decrease the absorption of vitamin B12, vitamin C, folic acid, vitamin B6, riboflavin, magnesium and zinc. On the plus side,however, it increases iron and calcium absorption. Although the Pill does seem to affect absorption and metabolism of many nutrients, there is little scientific evidence that women using it require increased amounts of nutrients to prevent nutritional deficiencies, provided of course that they are healthy and are following a balanced diet. In these circumstances, nutrient levels do not fall to alarming levels.
Some researchers have however established that the depression suffered by certain women who use the contraceptive pill can be due to a deficiency in vitamin B6, which can be treated by supplementation with this vitamin. A number of theories have been proposed regarding the cause of the pre-menstrual syndrome (PMS) and its link to the Pillone of these being that women who suffer from PMS also have low levels of vitamin B6. However, there is as yet not enough scientific evidence to justify fortification of the Pill with vitamin B6, or to suggest that doctors should treat the problem routinely with vitamin B6 supplementation.