This article was first published in the December 2019/January 2020 edition of NHD magazine.
Liquid diets and meal replacements come in various forms, and are often promoted for weight loss.
There are some medical indications for liquid diets, such as before or after certain types of surgery, or the use exclusive enteral nutrition for inducing remission in Crohn’s disease.
The focus of this article will be the nutritional content and evidence related to five liquid diets which are popular with the general public.
Juice diets involve only consuming juiced fruit and vegetables for about 3-10 days. These diets are often promoted as ‘detoxes’, ‘cleanses’ or ‘juice fasts’.
But of course no diet can ‘detoxify’ our body, as this is done by our lungs, gut, skin and kidneys.
A popular type of juice diet is called “the master cleanse” or the “lemon-cayenne pepper diet”. This extreme diet involves consuming mainly lemon juice, cayenne pepper, maple syrup and water for 10 days.
Raw juice diets are also popular, some of these involve consuming up to 2 litres of raw fruit and vegetable juice per day!
As juice diets are very low in calories they can lead to short-term weight loss, although weight tends to be regained when normal eating is resumed.1
Juice diets are also tend to be deficient in: calories, protein, fat, iron, calcium, vitamin B12, iodine and selenium.
Juice ‘cleanses’ can lead to unpleasant side effects, including: bloating, cramping, diarrhoea, dizziness, low energy levels and erratic blood sugar levels. This is especially unsuitable for those who are nutritionally vulnerable – such as: children, pregnant or breastfeeding women, those who are malnourished or those who have a history of an eating disorder.
A high intake of juice may interact with certain medication, such as the blood-thinner Warfarin. It can also be harmful to undertake a diet like this for those with diabetes, liver disease, kidney disease and thyroid issues (as these juices are often high in goitrogens).
Cabbage Soup Diets
This fad diet has re-emerged many times over the years, and was very popular in the 1980s. The cabbage soup diet involves consuming (you guessed it!) mainly cabbage soup for around a week, although some fruit, vegetables or low fat milk is also allowed.
No studies have investigated the impact of the cabbage soup diet.
However, the health impact is likely to be very similar to juice diets as it is also very low in calories and provides little to no protein or fat.
Therefore it is also an extreme and unbalanced approach, with the likelihood of gut irritation from the high intake of cabbage.
This usually involves only consuming water for 24 – 72 hours, although black tea and coffee are also sometimes allowed. Worryingly water fasting is sometimes promoted for 5-7 days, which would clearly lead to malnourishment!
Water fasting for 24 hours may be associated with similar health benefits to other types of daily intermittent fasting, such as improvements in metabolic health and inflammation levels.2 3 Fasting might also have benefits for healthy aging, as short periods of fasting may improve the way our body recycles damaged parts of cells (which is known as ‘autophagy’).4-5
However, we don’t have much evidence about how this impacts health in the long-run, and many of the studies have been carried out in animals.
There are also many risks to consider with avoiding food for 24 hours, such as: low energy levels, irritability, low blood glucose levels, likely increase in cortisol levels and impacting ability to exercise and carry out daily tasks.
Some studies have highlighted further harmful side effects of water fasting, including: nausea, low energy levels, headaches, high blood pressure, low sodium levels and even serious cases of dehydration.6 The risk of harm from this diet increases depending on the length of the water fast.
Overall, water fasting is an unnecessarily extreme and risky approach for most people.
Low Calorie Meal Replacement Shakes
Meal replacement shakes can be used as the sole source of nutrition, or to replace one or two meals per day.
The nutritional content of these shakes varies between different products. For example, some meal replacement shakes can be low in fibre.
It is safe to say that they can’t provide the range and combination of phytonutrients which is found in whole food.
Meal replacement shakes also can’t replicate the ‘food matrix effect’ which is the way that the structure and nutrient content of food interacts with one another. For example, consuming omega-3 supplements or a product which is fortified with omega-3 isn’t seen to have the same health benefits as compared with eating oily fish.7
Some meal replacement shakes use certain sugar alcohols (polyols) as a low calorie sweetener the product. While not inherently bad, this may cause gut issues or trigger IBS symptoms for some people.8
Very low calorie diets usually involve consuming meal replacement shakes and soups to provide around 800 – 1200 calories per day for up to 12 weeks. These are associated with significant weight loss up to a year after the diet has taken place (although some weight tends to be regained).9
These very low calorie diets may also have useful applications in type 2 diabetes.
For example, the DiRECT trial investigated whether following a very low calorie diet could put people with type 2 diabetes into remission.10 This involved consuming only 850 calories per day, in the form of four meal replacement shakes or soups, for 3-5 months followed by food reintroduction and ongoing healthy lifestyle support. By the end of the trial almost 50% of participants had gone into remission, and the success rate rose to 86% for those who lost more than 15 kg of weight. This is a promising new area, but it is important to remember that this diet is very restrictive so it wouldn’t be appropriate for many people, and a lot of support is needed to follow this in a safe way.
A very low intake of calories may also slow down metabolic rate due to processes like adaptive thermogenesis and relative energy deficiency (which is often referred to as ‘starvation mode’).11-12
A slightly less extreme approach is replacing one to two meals per day with a meal replacement shake or soup. This has been found to promote weight loss and lead to improvements in blood pressure, cholesterol, blood glucose and insulin levels, when compared with a traditional calorie-reduced diet.13
Some people find meal replacement shakes to be a convenient way to reduce their calorie intake. However they can also be expensive when used over a long period of time. They also do not replicate all of the goodness found in whole food, and are not advisable for those who struggle with disordered eating.
New Generation Meal Replacement Shakes
In the last 5 years a new type of meal replacement shake has emerged which is aimed at busy people who want a nutritionally complete meal in the form of a shake. So the focus isn’t on weight loss, but convenience and sustainability. Examples of these include Huel and Soylent.
These shakes tend to have a good nutritional profile and are suitable for many dietary needs; for example they are free of common food allergens, and suitable for vegans and vegetarians.
However, as mentioned above these are unlikely to match the nutritional benefits of a diverse whole food diet.
There is also very little evidence related to the health impact of these specific products.
Although one randomised controlled trial from 2016 reported a favourable impact of Soylent on the balance of gut bacteria in 14 participants.14-15
For more information about Huel, check out the NHD written by Alice Fletcher in issue 137 (August/September 2018 edition).16
Faddy liquid diets like juice diets and the cabbage soup diet are not nutritionally balanced and are likely to cause gut issues.
There may be some benefits to a 24 hour water fast for some people, but this is a new area of research and there can be serious side effects to water fasting.
Low calorie meal replacements shakes and soups may be useful for some people who are trying to lose weight, and more research is emerging related to their use as part of very low calorie diets to promote diabetes remission. However, close support and monitoring is needed with this.
The new generation of meal replacement shakes (which don’t focus on weight loss) are nutritionally balanced, convenient options. However, using meal replacements of any kind is unlikely to match the food matrix effect of whole food.
Furthermore, getting into the habit of drinking meals rather than eating food may encourage an unhealthy relationship with food for some people, especially those with a history of disordered eating.
- Obert et al. (2017) “Popular Weight Loss Strategies: a Review of Four Weight Loss Techniques” [accessed June 2019 via: https://link.springer.com/article/10.1007/s11894-017-0603-8]
- Horne et al. (2013) “Randomized cross-over trial of short-term water-only fasting: metabolic and cardiovascular consequences.” [accessed June 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/23220077]
- Chakravarthy & Booth (1985) “Eating, exercise, and “thrifty” genotypes: connecting the dots toward an evolutionary understanding of modern chronic diseases.” [accessed June 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/14660491]
- Rizza et al. (2014) “What are the roles of calorie restriction and diet quality in promoting healthy longevity?” [accessed June 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/24291541]
- Alirezaei et al. (2010) “Short-term fasting induces profound neuronal autophagy” [accessed June 2019 via: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3106288/]
- Finnell et al. (2018) “Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting.” [accessed June 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/29458369]
- [accessed June 2019 via: http://cochranelibrary-wiley.com/doi/10.1002/14651858.CD003177.pub3/full]
- Abdelhamid et al. (2018) “Omega‐3 fatty acids for the primary and secondary prevention of cardiovascular disease” [accessed June 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/28710145]
- Franz et al. (2007) “Weight-loss outcomes: a systematic review and meta-analysis of weight-loss clinical trials with a minimum 1-year follow-up.” [accessed June 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/17904936]
- Lean et al. (2018) “Primary care-led weight management for remission of type 2 diabetes (DiRECT): an open-label, cluster-randomised trial” [accessed June 2019 via: https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(17)33102-1/fulltext]
- Muller et al. (2016) “Changes in Energy Expenditure with Weight Gain and Weight Loss in Humans.” [accessed June 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/27739007]
- IOC (2018) “IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update” [accessed June 2019 via: https://bjsm.bmj.com/content/52/11/687?utm_source=trendmd&utm_medium=cpc&utm_term=0-A&utm_content=consumer&utm_campaign=bjsm]
- Heymsfield et al. (2003) “Weight management using a meal replacement strategy: meta and pooling analysis from six studies” [accessed June 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/12704397]
- Clinicaltrials.gov “Impact of Soylent Consumption on Human Microbiome Composition” [accessed June 2019 via: https://clinicaltrials.gov/ct2/show/NCT03203044]
- Hsu et al. (2017) “Impact of Soylent Consumption on Human Microbiome Composition” [accessed June 2019 via: https://www.biorxiv.org/content/10.1101/200881v1]
- “Huel: Is This The Future Of Food?” by Alice Fletcher. NHD Issue 137 (August/September 2018 edition) [https://issuu.com/nhpublishingltd/docs/issue_137_digital]