22 July 2008

Diet supplements in the spotlight

Green tea and an essential fatty acid called CLA are often combined in supplements as a powerful one-two punch to combat excess weight. But can the claims be believed?

Green tea has been touted as a miracle beverage – it reportedly treats arthritis, boosts fertility, and prevents a range of conditions from tooth decay to cancer. Recently it was reported that an extract of the bitter beverage could even be useful in treating genital warts.

It’s also said to promote weight loss. Now we’ve got your attention!

Another secret weapon in the weight-loss war is said to be an essential fatty acid called conjugated linoleic acid (CLA), and the two are often combined as a powerful one-two punch to combat excess weight.

The claims
Manufacturers of supplements that contain extract of green tea and CLA claim that:

  • green tea thwarts the absorption of fat, activates fat-burning mechanisms in the body, stabilises blood-sugar levels and increases energy expenditure;
  • CLA reduces body fat, increases lean muscle mass, increases energy expenditure, lowers blood lipids, increases insulin sensitivity and enhances immune function.

Green tea
Among several small studies looking at the use of green-tea extract for weight management are:

  • A study linking positive results to high volumes of specific antioxidants, called catechins, found in green tea.
  • Another that showed that green-tea extract increased fat oxidation (which means the fat is used as an energy source) during moderate-intensity exercise. It also indicated improved insulin sensitivity and glucose tolerance in healthy young men.
  • Yet another study in which green tea not only increased fat oxidation, but also increased thermogenesis (that is, generation of heat associated with greater energy expenditure) in 10 study participants.
  • Several epidemiological studies that showed heart and metabolic benefits from drinking five to six cups of green tea a day.

On the surface, it looks like these results provide back-up for weight-loss claims. Sadly, though, there are also a number of studies which show no effect, and experts say it’s too early to get excited.

"Better research is needed before a strong recommendation can be made," says the US National Institutes of Health. So beware of labels on products that make any firm weight-loss claims.

Perhaps future studies will provide certainty, but in the meantime take note of the risks associated with drinking large volumes. There’s caffeine in green tea, so you could set yourself up for:

  • Insomnia
  • Worsening incontinence, due to caffeine's diuretic effects
  • Worsening of stomach-ulcer symptoms
  • Increased heart rate and blood pressure
  • Increased blood-sugar levels

Also, if you're currently on medication, you shouldn't drink green tea or take the extract without consulting your doctor. The tea can interact with several medications such as aspirin, warfarin, ephedrine and more.

If you like green tea, however, and experience no ill effects or aren't on any medication, it's probably safe to continue drinking it. According to the American Academy of Family Physicians (AAFP), five cups a day are generally regarded as safe.

CLA is an essential omega-6 fatty acid that occurs naturally in red meat, dairy products and safflower oil, and which our bodies can't produce on their own. Research shows that CLA is a powerful antioxidant, and there's good evidence that it supports weight loss too.

According to the AAFP, it's been found that CLA reduces fat deposition in obese mice, possibly through increased fat oxidation and decreased fat storage. What's more, positive results have also been found in humans.

In a study conducted by our very own UCT/MRC Research Unit for Exercise Science, a sample group of 64 men and women, aged between 21 and 45, who were already physically active, of normal weight and leading healthy lifestyles, were given a CLA supplement. The scientists then checked the participants' blood-lipid levels, glucose tolerance and insulin resistance, metabolic rate and the distribution of body fat and lean tissue.

What they found was startling: positive results were seen in the women, but not in the men. Interestingly, the women's weight wasn't affected, but their fat percentage was. Plus, their insulin sensitivity increased slightly.

But the verdict really lies in a comprehensive meta-analysis that looked at 18 studies on CLA, which appeared in a 2007 edition of the American Journal of Clinical Nutrition. Here, the researchers note that a dose of 3.2g of CLA per day produces a modest loss in body fat in humans. "When the body of evidence is considered as a whole, CLA does have a beneficial effect on human body composition. Although this effect is modest, it could be important if accumulated over time...," they write.

So, at last, there's good news.

But there's a snag: CLA comes in different chemical forms or "isomers", with the t10, c12 isomer being the one most likely to help with weight loss. CLA products often contain more than one type of isomer, which influences their safety and efficacy, explains Prof Vicky Lambert from the UCT/MRC Research Unit for Exercise Science, based at the Sports Science Institute of SA (SSISA). For example, some isomers have a positive effect on insulin sensitivity, while others can have a negative effect. This makes identifying a safe and effective CLA supplement tricky.

Another word of caution: excessive intake of omega-6 fatty acids such as CLA could interfere with the uptake of the omega-3 fatty acids, of which most of us get too little. If you use margarine and sunflower oil in your kitchen, then chances are that you're getting more than enough omega-6 fatty acids already. Adding more to your diet through CLA could be detrimental to your health.

According to registered dietician Dr Ingrid van Heerden, omega-6 will be preferentially metabolised and your body will not be able to use the omega-3s efficiently. "Such an imbalance can cause all kinds of metabolic disturbances and contribute to certain diseases," she says.

Our position
At this stage, it seems that too little is known about green tea to firmly recommend it for weight loss. And while the results on CLA are quite positive, it's important to note that supplements are still poorly regulated in South Africa. This means you can't be 100% sure that what you're taking contains the critical amounts of the key ingredients. Hopefully, this will change in the near future.

In the meantime, if you're struggling to lose weight, it's probably safer to spend your money on a consultation with a professional, such as a registered dietician, a nutritionist or a doctor; or to sign up for a reputable weight-loss programme such as Weigh-Less or Weight Watchers.

- (Carine van Rooyen, Health24, July 2008)


  • A.D.A.M. (28 April 2008) Weight Management – In-depth report: Medications. Website of The New York Times (
  • Saper, RB, Eisenberg, DM, Phillips, RS. (November 2004). Common Dietary Supplements for Weight Loss. Website of the American Association of Family Physicians. (
  • Author unknown. (1 March 2008) Green tea (Camelia sinensis). US National Institutes of Health – MedlinePlus. (
  • Whigham, LD, Watras, AC, Schoeller, DA. (2007) Efficacy of conjugated linoleic acid for reducing fat mass: a meta-analysis in humans. Am J Clin Nutr 2007;85:1203–11. (
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