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Dropping pounds is best achieved by altering food intake and exercising, says an Oregon researcher who studied weight-loss supplements. (IStock)

Weight-loss supplements advertised as fat blockers, appetite suppressants or body-composition changers usually have little effect — especially over the long term — and may even harm you, according to a new U.S. study.

Of the hundreds of products reviewed by Oregon State University researcher Melinda Manore, many were scrutinized in randomized clinical trials to prove their effectiveness, and only a few products — such as green tea, fibre and low-fat dairy supplements — resulted in a modest weight loss compared to placebo groups.

Manore, whose study was published in Tuesday's online International Journal of Sport Nutrition and Exercise Metabolism, also notes that most of the supplements that had any weight-loss benefit were also tested as part of a reduced-calorie diet, meaning purchasing such products usually amounted to a waste of money.

"There is no strong research evidence indicating that one specific supplement will produce significant weight loss, especially long term," her study concludes. "Some foods or supplements … may complement a healthy lifestyle to produce small weight losses and/or prevent weight gain over time."

In the U.S., one in three adults is obese, compared to one in four Canadian adults, according to a Statistics Canada study released last March. The study, released jointly with the U.S. Centers for Disease Control, said 24.1 per cent of adults in Canada were obese between 2007 and 2009. In the U.S., the figure was 34.4 per cent.

"For most people, unless you alter your diet and get daily exercise, no supplement is going to have a big impact," Manore, a professor of nutrition and exercise sciences who is on the science board for the President's Council on Fitness, Sports and Nutrition, said in a release.

In the U.S., weight-loss supplements are a $2.4-billion-a-year industry.  The overall weight-loss industry in Canada and the U.S. is estimated to be worth $44 billion US.

Health Canada notes that an increasing number of Canadians are using health products for weight loss, including prescription drugs and natural health products. However, since 2007, Health Canada has issued warnings involving at least 170 weight-loss products, including the supplement Hydroxycut, which in the U.S. has been linked to at least a dozen cases of liver toxicity and one death.

Exercise, healthy eating urged

Although some weight-loss supplements may provide benefits when used properly as part of a weight management program, misuse can pose serious health risks, Health Canada warns. It urges anyone considering use of weight-loss products — especially people under age 18, pregnant or breastfeeding women, or those who have any medical conditions or serious diseases such as heart problems, high blood pressure or diabetes — to discuss any potential risks with a health-care practitioner.

The supplements examined in Manore's study fell into these categories:

  • Chitosan, which blocks absorption of fat or carbohydrates.
  • Stimulants such as caffeine or ephedra that increase metabolism.
  • Products such as conjugated linoleic acid that claim to change the body composition by decreasing fat.
  • Appetite suppressants such as soluble fibres.

The study warns supplements containing metabolic stimulants, such as caffeine, ephedra or synephrine are most likely to produce adverse side-effects and should be avoided.

Manore stresses the key to weight loss is to eat whole grains, fruits, vegetables and lean meats, reduce calorie intake of high-fat foods, and to exercise regularly.

"Adding fibre, calcium, protein and drinking green tea can help," Manore said. "But none of these will have much effect unless you exercise and eat fruits and vegetables."