Are these 6 weight loss diet supplements really worth your money?

1Green tea

Green tea has long been believed to help boost metabolism, either by drinking it regularly or taking green tea extract in supplement form. I don't know about you, but whenever I drink a mug of green tea, especially on an empty stomach, it makes me feel pretty nauseous – so does that indicate something is going on? In the case of green tea, there is research to suggest it may result in a small boost to fat loss in overweight individuals [1]. However, much of the research looks at green tea extract, rather than the drink, with the beverage unlikely to cause any significant changes. Most research concludes there could be a small effect, but it is not significant enough to be considered clinically relevant [1]. There are also slight safety concerns over the dosing suggested by some green tea extract supplements, with high doses potentially being linked to harmful side effects [2].

2Caffeine

It's well known that caffeine is a stimulant, and I'm sure many of us are familiar with the jittery feeling you get when you’ve had one too many high-strength coffees. This is because caffeine is a drug which stimulates the central nervous system making us feel more alert. Due to this stimulating effect, caffeine can have an impact on our metabolic rate and fat metabolism, and indeed some studies do show that caffeine intake is associated with a slight reduction in weight [3]. As with most things in life, you can have too much of a good thing, so while moderate intakes of caffeine (400-500mg per day) are thought to be fine, having excessive (>500mg per day) amounts can become harmful. There is also research to suggest that we build up a tolerance to caffeine over time [4], which I think many coffee lovers will have experienced, which means any positive effects on metabolism or weight also reduce with prolonged consumption.

3Raspberry ketones

I remember raspberry ketones were all the rage a decade or two ago, as this supplement was touted as a great weight loss aid. Raspberry ketone is a compound found naturally in raspberries but can also be found in other fruits such as grapes or apples. Although it's natural, only very small amounts exist in foods, so the raspberry ketones used in supplements are often biosynthesised.

Interestingly raspberry ketones are one of those products that gained a lot of hype with very little evidence to back up the claims. Much of the research was conducted in labs or on mice, and I can only find one study which has been done with humans. In this study, the supplement given included several other compounds associated with improved weight loss, so while the participants in the supplement group did experience increased weight loss, it's not possible to know if raspberry ketones played a role [5].

While there has been some evidence to support the effectiveness of raspberry ketones on improving weight loss in mice [6], this cannot be transferred to humans. The safety and dosing of the supplements is not well studied, so it's best to tread with caution.

4Conjugated linoleic acid (CLA)

CLA is essentially a fatty acid which is most abundantly found in dairy foods and meats. It's thought to play a role in fat metabolism, which is why it is considered a useful weight loss supplement. CLA supplements contain large doses of the fatty acid – much more than the amount found naturally in foods. There have been some studies in humans to support a modest effect on weight loss when supplementing with CLA [7], but conversely there have also been studies which find no such effect [8]. Even though CLA has been extensively studied over the years, the research is still quite mixed. The most common conclusion is that CLA supplements may cause a very small increase in weight loss, but the effects are not considered to be of any clinical significance.

5Apple cider vinegar

You may have heard of the Apple Cider Vinegar diet which promotes drinking 1-2 tbsp of apple cider vinegar each day to help curb appetite and boost weight loss. The belief is that the acetic acid – which is the acid found in most vinegars, not just apple cider varieties – can increase fat usage and help with appetite control. Interestingly most of the research in humans is looking at acetic acid, rather than apple cider vinegar itself – so there's little evidence to suggest this particular variety has any significant effect on weight loss.

There has been some research in humans looking at the effect of daily intakes of vinegar (i.e. acetic acid) on body weight. One study which involved 155 obese Japanese participants, supplemented them with either a placebo or high-dose vinegar drink for 12 weeks [9]. The study found that after 12 weeks, the group taking the drink lost on average 2kg in weight, whereas the placebo group remained the same weight. This was reported to be in the absence of any significant change in calorie intake between groups. Although it should be noted that the energy intakes were calculated from self-reported diet diaries, so there is some room for human error. That said, the findings of this study are interesting and do suggest daily vinegar intake may have a small effect on body weight.

It must be noted however that the effects are only small, and the study was limited to a 12-week period. It could be that ongoing use may not continue to produce such an effect. Also, there are a limited number of studies looking at the isolated effect of vinegar intake on body weight, so firm conclusions can't be drawn. The long term safety of consuming vinegar-containing drinks daily is also not clear, but we do know that neat vinegar can be harmful to our teeth and that it may interfere with insulin levels, which should be considered by anyone with diabetes particularly.

6Bitter Orange

This is a type of citrus fruit which you may find in your everyday marmalade. The interest with bitter orange and weight loss is due to a compound found within the peel called synephrine, which is thought to aid weight loss. While there are conflicting research findings regarding the impact of synephrine on weight loss with some studies supporting a connection [10], a recent review concluded that there isn't substantial evidence supporting synephrine's effectiveness in weight loss. Moreover, the review highlighted significant safety concerns associated with its use in certain supplements [11].

The main takeaway about this supplement is that it may have some adverse side effects such as causing anxiety, headaches and increased blood pressure [2], so it should be treated with caution.