Pick or mix

by Jennifer Bowden / 09 October, 2010
To prevent adverse reactions, is it a good idea to avoid combining different foods?

Question: It's been said the English don't get the antioxidant benefits from their tea because they add milk to it. If you turn your breakfast into a laboratory of chemicals, it seems even worse: fruit juice, cereal, bacon and eggs, milk and tea; there's a high chance of chemical reactions. I space out my breakfast - first juice, followed by cereal and milk, and then tea - to reduce interactions. Is this a good idea?


Simple messages like "eat more fruit" belie the complex nature of nutritional science. It's no easy task to understand how the multitudes of chemicals and compounds in our food supply are altered by food preparation methods, interact with each other during digestion and influence the health of genetically diverse individuals.

The antioxidants found in black and green tea, known as flavonoids, have been credited with improving the function of our artery walls. Three cups of tea a day are thought to reduce the risk of a heart attack by about 11%. All well and good, but does adding milk to tea really inhibit the absorption of these flavonoids? A 2010 study, published in Nutrition Research, found lower-fat milks added to tea reduced antioxidant absorption more than full-fat milks; antioxidant losses resulting from adding milk varied from 7-25%.

But before you switch milks or ditch milk altogether, consider this - many compounds in our food interact. For example, vitamin C in fruit enhances the absorption of non-haem iron in ­cereals, so adding peaches to your muesli is a good idea; drinking a cup of tannin-containing tea with your breakfast will decrease non-haem iron absorption.

What scientists know about food and chemical interactions, however, is just the tip of the iceberg. For example, the tea researchers acknowledged that anti­oxidant absorption from tea is also affected by how long the tea brews, how often tea is drunk and whether with a meal, and variations in our gut microflora. But they can't quantify these interactions yet.

Better then to put your time and effort into eating a healthy diet and not worrying about interactions that scientists don't yet fully understand. Unless, of course, you have a specific health condition that warrants further efforts; for example, if you're iron-deficient, learning about which foods and compounds enhance and inhibit iron absorption is a good idea. Otherwise, just relax and enjoy eating a healthy diet and know you're doing the best you can.

Question: What's a suitable age for child­ren to start drinking tea and coffee? I drank tea as a 9- to 10-year-old, but didn't have coffee until I was about 15.


Coffee has surged in popularity among teenagers in recent times, helped by the array of coffee houses that provide a welcome congregation point away from home and school. But are tea and coffee positive additions to a child or adolescent's diet?

From a safety perspective, the caffeine in tea and coffee must be considered. Caffeine can cause temporary adverse effects in some people starting at daily levels as low as 3mg of caffeine per kilogram of body weight, according to the New Zealand Food Safety Authority (NZFSA). Adverse effects include dizziness, rapid heartbeat, irritability, anxiety, tremors and insomnia.

A typical cup of tea contains between 25-50mg of caffeine, a can of cola 50-70mg, a cup of instant coffee 60-125mg and a cup of brewed coffee 125-250mg. So, just one cup of instant coffee could produce adverse effects in children aged 10-12, and one cup of tea could adversely affect preschool children. And if older children consume more than one cup of tea a day, they may also suffer from adverse effects.

From a nutritional perspective, a milky tea or coffee may be a useful way to increase calcium intake. On the downside, though, both tea and coffee contain tannins that inhibit the absorption of non-haem iron from plant-derived foods and meat. About 90% of our dietary iron intake is non-haem, so enhancing the absorption of it is important.

Iron is critical for children's healthy growth and development. An iron deficiency during childhood can reduce attention span and alertness, impair learning (possibly permanently) and weight gain, and cause gastrointestinal issues, fatigue and impaired immunity. We clearly don't want to risk a child's health and long-term well-being by reducing their iron status.

Unfortunately, iron deficiency is one of the most common micronutrient deficiencies in the world. For starters, 13-23% of New Zealand infants and toddlers are thought to have iron-deficiency anaemia. And a 2003 New Zealand study, published in the Journal of Paediatrics and Child Health, found tea consumption by toddlers was associated with iron deficiency. This research formed part of the evidence for a 2007 policy statement for the Royal Australasian College of Physicians regarding iron deficiency among preschool children. The researchers concluded tea should not be given to young children because this could lower iron levels.

A 2006 review of evidence on tea and health, published in the European Journal of Clinical Nutrition, recommended that groups vulnerable to iron deficiency - such as children, the elderly, pregnant women and those with low iron stores - avoid drinking tea at mealtimes. The Ministry of Health's Food and Nutrition Guidelines for children and adolescents recommend tea and coffee not be served at mealtimes for this reason. However, we have no guidelines on how much tea, coffee or caffeine is nutritionally sensible for children and adolescents - except for the NZFSA's advice that children and young teenagers should avoid caffeine-containing energy and smart drinks, because of the potential for adverse effects.

The Ministry of Health is preparing a new Food and Nutrition Guidelines for Healthy Children and Young People. The update will contain more information on caffeinated/energy drinks/shots, tea and coffee, according to team leader and senior adviser (nutrition) Elizabeth Aitken, and have more detailed advice for the 13- to 18-year age group, including recommendations on daily tea and coffee consumption. These guidelines are expected to be released for public comment by next month.

In the meantime, Health Canada has caffeine guidelines that provide a good foundation for New Zealand parents to use when overseeing their children's tea, coffee or cola intake.

Children aged 12 and under should have no more than 2.5mg of caffeine per kilogram of body weight per day. This equates to a maximum daily caffeine intake of:

45mg for children aged 4-6;

62.5mg for children aged 7-9; and

85mg for children aged 10-12.

This equates to less than a can of cola a day for younger children, and although a cup of tea might be acceptable for 7- to 12-year-olds, most coffees contain too much caffeine for the under 12s. For adolescents aged 13 and over, Health Canada recommends taking a conservative view and sticking with the daily caffeine limit of 2.5mg/kg body weight as a precaution.

It would also be prudent for adolescent girls to avoid drinking tea/coffee with meals, because their menstrual blood losses mean they need more iron than adolescent boys.

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