Diet Shakes for Weight Loss

Last updated June 9th 2015

There is a large market for meal replacement shakes. These offer an easy-to-use, low-calorie alternative to one or two meals of the day, but the idea of having the same drink for two meals every day may not appeal to everybody.

The term ‘diet shakes’ generally refers to low-calorie shakes that are designed to replace meals. This type of meal replacement shake is readily available to buy from high street stores as well as online.

Although the ingredients in each individual product will vary depending on the brand, some similarities can be drawn between the products. The shakes are usually made to replace breakfast and/or lunch each day and are high in protein with added vitamins and minerals to make for a nutritious ‘meal’.

Here we will discuss what diet shakes are, how they are supposed to work, and whether or not they are suitable for everybody.

What are Diet Shakes?

Diet shakes usually come in the form of powder that can be mixed into water or milk to make a drink, although some are available that are already in liquid form. The products tend to be high in protein and fibre, but low in carbohydrates and calories. They also usually have a selection of vitamins and minerals in the blend to ensure that the drink is nutritional. Since they are made to replace a meal, the shakes should be filling and prevent you from snacking.

Diet shakes come in a variety of different flavours in an attempt to cater for everybody’s taste buds; flavours range from strawberry to cappuccino. Some companies even produce protein soups and puddings. Depending on the brand that you opt for, the shakes may be made primarily of different types of protein and vitamins, or they may contain ingredients that are thought to boost weight loss, such as guarana and green tea extract.

Some products may also come with a diet plan. A diet plan usually offers advice on how to lose weight, what foods to eat, provides you with targets, and teaches you how to control calorie intake and portion sizes. Those that come with diet shakes are also likely to tell you how often to replace your meals with shakes, which meals to replace, and what to eat for the meal(s) that are not being replaced.

How do Diet Shakes Work?

The theory behind diet shakes is very simple. By drinking a low-calorie shake instead of eating a calorie-rich meal, you should consume fewer calories. Consuming fewer calories should mean that you lose weight, providing that you undertake regular exercise too. In order for diet shakes to be effective, they must fill you up and prevent you from snacking between meals and eating more during the meals that have not been replaced by shakes. The majority of diet shake products contain large portions of protein in order to achieve appetite suppression.

Some products may also contain some ingredients that are believed to aid weight loss in other ways, for example through the processes of metabolism boosting and fat burning. These are usually herbal ingredients that are common to weight loss supplements. Both fat burning and metabolism boosting are thought to be achieved in similar ways – through the process of thermogenesis.

If an ingredient can induce thermogenesis, it causes an increase in the internal body temperature. By doing this, it may help the cells in the body to work harder and so burn more calories in the process. It is also thought to produce a more optimal environment for the enzymes that are involved in the breakdown of fat molecules (lipolysis); thus possibly helping them to burn fat more efficiently.

Whether or not ingredients have the potential to work in these ways is debateable, and the efficacy will vary depending on the exact ingredient. Some herbal ingredients have been tested for their use as metabolism boosters and/or fat burners in scientific studies, whilst others have not. The clinical studies behind those ingredients that are sometimes found in diet shakes will be discussed in more detail below.

It is clear to see why fat burning and metabolism boosting ingredients would be added to a diet shake, provided that they have been proven to work. The slight additional effect that could be provided by these ingredients could help people to lose weight when taken alongside a diet plan and regular exercise.

Common Ingredients in Diet Shakes

Whey Protein

Whey protein is a type of protein that is obtained from whey – a bi-product of the cheese-making process. Although it can be used in the treatment of conditions such as eczema and asthma, its main use is in the supplement industry, and whey protein can be found as a major constituent of many protein powders and athletic supplements, as well as meal-replacement shakes. It is a good way to fulfil the body’s protein requirement and offers a great source of amino acids that can be used in the production of muscle.

Source: WebMD

Soy Protein

Similar to whey protein, soy protein can also form the main constituent of a meal-replacement shake. In contrast to whey protein which is obtained from animal sources, soy protein is sourced from soybean. Soy protein is found in a number of different foods including, for example, breads and cereals. The protein is commonly added to protein powders and meal-replacement shakes because it provides all of the essential amino acids that are required by humans. Proteins have a number of functions in the body, including helping with the production and repair of muscle.

Source: Wikipedia

Vitamins and Minerals

Different meal replacement products contain different ingredients, but the majority contain a selection of vitamins and minerals. Those that are commonly added to the products include: vitamin B6, ascorbic acid, folic acid, nicotinamide, riboflavin, and chromium picolinate, amongst others. These substances will all help with the maintenance of general health and are a particularly important addition to meal-replacement products as they would otherwise be obtained from a person’s diet. Some, such as chromium, have also been claimed to help aid weight loss in the past.

Sources: NHS, NIH

Flaxseed Powder

This extract is taken from flaxseeds – the seeds of the Linum usitatissimum plant. Flaxseed powder is not universally found in diet shakes, but has been added to a few branded products. The ingredient has been used in traditional medicine in the past to treat a range of health conditions, but is best known for its potential effects on the gastro-intestinal tract. The substance is thought to treat conditions of the colon and digestive tract in general, although evidence for these effects is lacking. It has not been shown to be of any benefit to weight loss.

Source: NIH

Guarana Extract

Guarana extract is another herbal ingredient that is only found in a few meal-replacement products. This substance is extracted from the seeds of the guarana plant and is primarily used in weight loss and athletic supplements, though it has been used in traditional medicine in the past for the treatment of conditions such as low blood pressure and malaria. Guarana is believed to have the potential to induce weight loss and boost athletic performance due to its stimulant properties.

Source: WebMD

Green Tea Extract

Green tea extract is obtained from the Camellia sinensis plant – the same plant that is used to make traditional tea. The process by which green tea is made should mean that the ingredient retains its antioxidant capacity, and so it is believed to have a wide range of health benefits. People believe that green tea can help to treat stomach disorders, genital warts, and several types of cancer, amongst many other conditions. In terms of weight loss, green tea is thought to be able to boost the metabolism and possess fat burning qualities.

Source: WebMD

Garcinia Cambogia Extract

This ingredient is extracted from the green, pumkin-like fruit of the Garcinia cambogia plant – a plant that is native to Indonesia. As with guarana, green tea extract and flaxseed powder, Garcinia cambogia extract is only present in a few meal replacement products available on the market. The ingredient has been touted for its weight loss benefits, particularly after it was featured on the Dr. Oz show. It is believed to have the potential to bind to fat molecules, preventing them from being absorbed by the body.

Source: WebMD

Clinical Studies on Diet Shakes

A number of clinical studies have been undertaken in an attempt to determine whether or not meal replacement shakes offer an effective and safe method of weight loss. A clinical study is a trial that is performed by scientists under controlled conditions to test a product or ingredient. For it to be deemed reliable, the clinical study should have used randomised, double blind, placebo controlled methods on a large sample of human subjects; it should also have been published in a peer-reviewed scientific journal. Other types of study include those on animals (such as mice and rats) and studies on individual cells, which are performed in a laboratory.

A handy meta-analysis of studies performed to determine the effects of meal replacement shakes on weight management was published in the International Journal of Obesity in 2003. For this study, the results of six scientific trials were pooled. All of these trials were randomised and controlled, and continued for at least three months using subjects who had a BMI of 25kg/m2 or more. The trials all involved the replacement of two meals with a commercially available, low-calorie, liquid meal replacement product that was supplemented with vitamins. Statistical analyses were performed on the data from these six trials.

The results indicated that subjects in both the meal-replacement and standard reduced-calorie diet groups lost significant amounts of weight during the trials. It was also found that those taking the meal replacement products had lost considerably more weight than those following the standard diet. This difference was also seen to remain at the one year period. The dropout rates for the two groups were similar for the three month trial; no side effects were reported, and at the one year point, fewer people had dropped out from the meal replacement group. It was therefore concluded that meal replacement plans could be a very effective and safe method of sustainable weight loss.

A second study published in 2003 also produced positive results. For this study, 100 obese adult participants took part in a randomised, controlled trial that was 12 weeks long. The subjects received either a soy-based meal replacement, or one session of dietary counselling and a pamphlet regarding weight loss. The results showed that those consuming the meal-replacement lost significantly more weight than those in the control group, and also had statistically higher changes in total and LDL cholesterol levels. The authors concluded that the soy-based meal replacement offered an effective method of reducing body weight, fat mass, and LDL cholesterol.

In another trial, the effects of soy protein-based meal replacements were compared to casein protein-based meal replacement shakes. A total of 43 obese, female participants were involved in this trial, with BMIs varying from 30 to 40kg/m2. The women were randomly allocated to receive either three soy shakes or three casein shakes per day for 16 weeks, alongside an energy-restricted diet. A number of measurements were taken at the eight and sixteen week marks.

The results of this trial indicated that subjects in both groups lost a significant amount of weight, and that the weights from baseline were not significantly different between the two groups. The authors found that whilst both soy and casein meal replacement shakes formed an important part of a successful weight loss programme, neither was seen to be significantly more effective than the other.

Diet Shakes Side Effects

Diet shakes are generally considered to be safe and unlikely to cause adverse side effects. Studies do not tend to report any adverse side effects with the consumption of meal-replacement shakes. Nevertheless, it is important to note that the potential side effects of a meal-replacement shake will be dependent upon the precise product. Each product contains different ingredients and ingredient quantities, and in turn, side effects will undoubtedly differ.

For some people, the consumption of shakes high in protein will cause stomach upset to begin with. Side effects of shakes high in whey powder may include stomach cramps, headaches, thirst, loose stools and diarrhoea, but is likely to subside with time as the consumer gets used to the product. Those low in carbohydrates and fibre could cause constipation.

Some products may also contain caffeine (or caffeine-containing ingredients, such as green tea extract). Depending on the quantities, caffeine may cause adverse effects such as nausea, insomnia, vomiting, anxiety, headaches, jitteriness, increased heart rate and increased blood pressure, amongst others.

Some have also noted a concern that consuming protein-rich meal replacements could be harmful to liver or renal function and bone density compared to meal replacements with a standard amount of protein. One study was undertaken to determine whether or not this was the case. One hundred obese men and women were involved in this randomised, placebo-controlled trial. The subjects were divided to receive either a meal replacement that was high in protein (2.2g/kg lean body mass), or one containing a standard amount of protein (1.1g/kg lean body mass).

They consumed the meal replacements twice a day (one meal, one snack) for a total of three months, and then once daily for a further nine months. Both groups had lost weight by the end of the trial and no significant difference in weight loss between the two groups was noted at the one year mark. There was also no significant changes in liver function, renal function or bone mineral density in either of the two groups. It was concluded that protein-rich meal replacements are not likely to cause side effects associated with the liver, renal function, or bone density over the period of one year.

Is there anybody who shouldn’t take Diet Shakes?

Clinical studies do not appear to have highlighted any groups of people who should not take diet shakes. The potential side effects associated with the gastro-intestinal tract may mean that those who suffer from a digestive disorder, such as irritable bowel syndrome, should avoid diet shake products as they may cause discomfort. It is recommended that you consult a doctor before using meal-replacement shakes if you are under the age of 18, pregnant, breastfeeding, or suffer from any pre-existing health condition.

Diet Shakes vs Diet Pills

Diet shakes offer a completely different product to other weight loss supplements on the market (such as diet pills, drops, and patches). Whilst diet pills are created as supplements to a healthy diet, diet shakes are actually made to replace some aspects of the diet (one or two meals a day). Rather than targeting areas of weight loss such as fat burning, metabolism boosting, and carbohydrate blocking (although some still do target these too) their primary function is to fill you up on a drink that contains few calories, thus reducing your daily calorie intake.

The use of diet shakes has been backed by a number of clinical studies – something that cannot be said for the majority of weight loss pills. Having a shake for breakfast and lunch reduces the amount of food preparation that you have to do, and means that you don’t have to worry too much about counting calories – that is done for you (apart from your evening meal).

Diet shakes will however not suit everybody. Some people may find that the shakes are not substantial enough to keep them full, some will dislike the texture and taste, and others will not be able to cope with the lack of variety in their diet. Substituting two meals a day with a shake could become a little samey for some people and others may simply miss their cereal and sandwiches.

Although dependent on the products, diet shakes are generally associated with fewer side effects that diet pills (particularly those that are high in stimulants), and there are fewer groups of people who should not take them. Diet shakes are however less convenient for consuming on the go than diet pills, and they are much less subtle if you are consuming them at work, for example.


Diet shakes are a method of weight loss that is backed by a selection of scientific studies. The shakes are generally low in calories and supplemented with vitamins and minerals; this means that they should not be detrimental for health and can be consumed alongside a healthy diet as part of a weight loss plan. Whilst they may have the potential to boost weight loss, diet shakes will not be for everybody. Some people may suffer from mild digestive side effects, and others may simply dislike the taste of shakes, or the idea of skipping one or two meals every day.