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  • Updated:5 Dec 2005

03.The top 10 ingredients

Unproven, but worthy of further study

Capsicum annum / Frutescens

  • What is it? Hot chillies and red peppers.
  • What does it contain? Capsaicin is the active ingredient.
  • How is it thought to act? By increasing metabolism and fat burning, and decreasing appetite.
  • How good is the evidence? Studies on an increase in metabolism are interesting, but mixed — some show food intake is reduced after eating spicy food containing capsaicin. Most studies only looked at short-term effects on metabolism after a test meal in small numbers of people, not at weight loss. One which looked at long-term effects on weight regain in a large group of people who’d lost weight found it made a difference to metabolism, but that this didn’t translate into a difference in weight regain. One study found obese women were less responsive to capsaicin than lean ones. It’s not clear whether you need to taste the spiciness to get a significant effect, and this will obviously affect whether or not supplements might be effective. Good-quality long-term trials on weight loss are needed.
  • What are the risks? No safety studies were found.

Citrus aurantium

  • What is it? An extract of the rind of Seville oranges, also known as bitter orange.
  • What does it contain? Synephrine.
  • How is it thought to act? By increasing metabolism and fat burning, and decreasing appetite.
  • How good is the evidence? There’s little evidence it’s effective in weight loss. Larger and longer trials are needed. One recent study found an increase in energy burning in women, but the authors concluded the increase may not be enough to result in a significant amount of weight loss over time.
  • What are the risks? There’s the possibility of blood pressure and cardiovascular effects and drug interactions — more research is needed.

Garcina quasita / cambogia

  • What is it? The rind of the fruit of a tropical plant; also called brindleberry or Malabar tamarind.
  • What does it contain? Hydroxycitric acid (HCA).
  • How is it thought to act? By modifying metabolism, and reducing fat storage and food intake.
  • How good is the evidence? In animals it suppresses weight gain, but there’s little good evidence in people. A number of trials in people have shown conflicting results. A positive study in humans of one form of the substance is interesting, but has been criticised in terms of its analysis and possible conflicts of interest. More good-quality, independent studies are needed.
  • What are the risks? There’s a general lack of studies, but it appears to be relatively safe for short-term use in the doses tested in people. Long-term safety is yet to be investigated.

Phaseolus vulgaris

  • What is it? An extract of white kidney beans, proprietary name Phase 2.
  • What does it contain? An alpha-amylase inhibitor (alpha-amylase is an enzyme that digests starch in the body).
  • How is it thought to act? By inhibiting the digestion of starch and so reducing the sugars absorbed.
  • How good is the evidence? In the test tube, it stops starch breakdown; in animals, the evidence is mixed. A recent clinical trial with 50 obese people didn’t find a significant effect.
  • What are the risks? Little data are available. The clinical trial raised no safety issues over eight weeks.

May have an effect, but it appears small

Camellia sinensis

  • What is it? Green tea.
  • What does it contain? Antioxidants, including epigallocatechin gallate, and some caffeine.
  • How is it thought to act? By increasing fat burning, reducing fat synthesis, blocking fat digestion and decreasing appetite. It’s not clear to what extent green tea catechins may act independently of the caffeine content.
  • How good is the evidence? In the test tube, catechins increase fat burning, and weight loss effects have been shown in animals. One animal study suggests weight may be regained when the treatment is stopped, and that the effect is less when it’s swallowed, rather than injected. One study in 10 people found capsules of green tea extract increased energy burnt by about 330 kJ (80 Calories) a day in the short term, but it didn’t look at whether it resulted in weight loss over time. A recent study in 35 Japanese men found a significant difference in weight after 12 weeks, but it was fairly small (on average about an extra 350 grams per month). Another 13-week study of 104 overweight people who had lost weight found no significant difference in weight regain with green tea capsules. There are other interesting studies in people, but there aren’t any high-quality clinical trials, or those that exist are of combination products. Overall, more research is needed, but any possible effect appears to be small.
  • What are the risks? In theory, green tea catechins may affect hormone systems, so there’s a possibility of consequences in pregnant women and young children, for example. However, none of the handful of studies in people has reported adverse effects.


  • What is it? A compound from the exoskeleton of crustaceans (prawns, crabs, lobsters).
  • What does it contain? Chitosan is the active ingredient.
  • How is it thought to act? By binding fat in the intestines, so less is absorbed.
  • How good is the evidence? In the test tube it binds fat, and a few animal studies have shown an effect on weight. But the evidence it works in people is not convincing — most studies have problems with their methods and the results are conflicting and short-term. A meta-analysis of the clinical trials (a statistical analysis of all their results taken together) found that it’s doubtful whether chitosan is effective in people. It concludes that if there’s an effect it’s minimal and unlikely to be significant. Studies looking at the amount of fat excreted found it to be insignificant in terms of weight loss
  • What are the risks? Little is known, but gastrointestinal disturbances, such as constipation and flatulence, are the most commonly reported side effects. People with an allergy to shellfish might want to avoid taking it. It’s also possible that the absorption of essential nutrients could be affected.

Little supporting evidence


  • What is it? The essential trace element chromium, most commonly used in the form of chromium picolinate.
  • What does it contain? Chromium.
  • How is it thought to act? Enhances insulin activity and fat burning; it may increase lean body mass and decrease body fat.
  • How good is the evidence? Its effectiveness is controversial and in general experts conclude there’s insufficient evidence. An analysis of 10 clinical trials suggests that if there’s an effect it’s very small — 80 to 200 grams a week of weight loss — and it’s not enough to make a significant difference compared to diet and exercise. The US Federal Trade Commission concluded in 1997 that there was no basis for weight-loss claims.
  • What are the risks? No human studies have reported side effects of concern. However, a possibility of DNA damage has been raised in test tube and animal studies, though some experts have refuted this. The data suggests high doses (more than 400 µg) may be of concern, but further data is needed on the possible effect of lower doses over a longer terme. These concerns have been raised about chromium picolinate, not other forms of chromium, such as chromium chloride. Of the chromium-containing supplements we looked at, only OPTISLIM 2000 Plus and SLIMIST didn’t use the picolinate form.

Fucus vesiculosus

  • What is it? A type of seaweed.
  • What does it contain? Iodine.
  • How is it thought to act? Iodine increases thyroid activity, increasing metabolism and energy burnt.
  • How good is the evidence? There’s one inconclusive study of it for obesity in the literature. There’s no evidence that iodine will increase metabolism unless you’re deficient in it.
  • What are the risks? An excess of iodine could cause an overactive thyroid.

Gymnema sylvestre

  • What is it? The leaves of a climbing plant.
  • What does it contain? Gymnemic acids.
  • How is it thought to act? In the mouth, it stops you tasting sweet foods as sweet for several hours. It may also reduce the absorption of both sugars and fats.
  • How good is the evidence? No trials on people that looked at weight loss were found, except when mixed with other weight-loss herbs. There’s test tube and some animal evidence that it affects fat and sugar absorption, but no good evidence it’s effective in weight loss.
  • What are the risks? There’s little data on possible interactions with other medications and minerals in the diet. It may interact with diabetes medications.

Paullinia cupana

  • What is it? A South American vine also known as guarana.
  • What does it contain? Caffeine.
  • How is it thought to act? By increasing metabolism and fat burning, and decreasing appetite.
  • How good is the evidence? Some evidence in animals shows caffeine affects metabolism. Several small, short-term metabolic studies have shown an increase in metabolic rate after taking caffeine, but these haven’t looked at effects on weight loss. One good-quality study looked at weight loss and found that caffeine and a weight-loss diet had no more effect than just the diet.
  • What are the risks? Caffeine may contribute to insulin resistance. Large doses of caffeine can cause anxiety, gastrointestinal discomfort, insomnia, heart arrhythmia and problems in pregnancy, and can lead to dependence.

This article last reviewed December 2005


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