They’re normally used to give a fruity smell to cosmetics and foodstuffs but, because you only get between one and four milligrams from every kilo of fresh fruit, a synthetic version is often used to get the same result. And now these ketones are being sold online, in supplement form, as a slimming aid.
The key lies in the fact that raspberry ketones appear to boost levels of a hormone called adiponectin, which regulates metabolism. Higher levels of this hormone are associated with fewer fat stores. Millions of Americans have become convinced of their powers to ‘melt’ away fat after they were recommended by Dr Mehmet Oz, a cardiothoracic surgeon featured on The Oprah Winfrey Show. He says raspberry ketones are ‘the number-one miracle in a bottle to burn your fat‘.
Do they really work?
Ann Ashworth, a spokeswoman for the British Dietetic Association, urges caution, pointing out the only published research into the effects of raspberry ketones on weight loss is small studies on mice. ‘Although this sounds very exciting, this evidence cannot be transferred to humans without further clinical trials,’ she says. ‘We need to know if it’s safe, if there are side-effects, or if it interacts with other medicines.’
Consultant nutritionist Helen Bond agrees. ‘People want a quick fix, especially at this time of the year, but when it comes to weight loss, no supplement can take the place of a healthy diet and exercise,’ she says.
‘It is not very glamorous but it will save you quite a few pennies along the way and will help you keep the weight off in the long term.’
Raspberry ketone is a natural phenolic compound that is the primary aroma compound of red raspberries. It occurs in a variety of fruits including raspberries, cranberries and blackberries. It is biosynthesized from coumaroyl-CoA. Extraction of pure raspberry ketone is usually 1–4 mg per kg of raspberries.
Since the natural abundance of raspberry ketone is very low, it is prepared industrially by a variety of methods from chemical intermediates. One of the ways this can be done is through a crossed aldol-catalytic hydrogenation. In acetone and sodium hydroxide, 4-hydroxybenzaldehyde can form the α,β-unsaturated ketone. This then goes through catalytic hydrogenation to produce raspberry ketone. This method produces a 99% yield.
Raspberry ketone is used in perfumery, in cosmetics, and as a food additive to impart a fruity odor. It is one of the most expensive natural flavor components used in the food industry. The natural compound can cost as much as $20,000 per kg.
Putative health effects
In mice, raspberry ketone has been shown to prevent high-fat-diet-induced elevations in body weight when given in very high doses, up to 2% of body weight. However, in another study in rats, no effects on body weight were observed with doses up to 200 times greater than the estimated intake in humans. The high dose effect is reported to stem from the alteration of lipid metabolism, increasing norepinephrine-induced lipolysis.
Although products containing this compound are marketed for weight loss, there is no clinical evidence for this effect in humans. Nutritional supplement manufacturer Andrew Lessman criticized and countered sensationalized media and marketing claims about raspberry ketone’s purported weight-loss benefits and strongly cautioned against its use.
Little is known about the long term safety of raspberry ketone supplements. In 1965, the US Food and Drug Administration placed raspberry ketone on generally recognized as safe (GRAS) status for the small quantities used as a food additive.