As a nutritionist, I often get asked what the fastest way is to lose weight. Many people understand that a calorie deficit is the only way to ensure weight loss, but some don't realise the complications that can arise from pursuing a very low calorie diet, as well as how unsustainable the results can be!
This week we are going to explore very low calorie diets as an option for weight loss, as well as some other options that can be much more sustainable in the long term!
What is a Very Low-Calorie diet?
Very low-calorie diets (hypocaloric diets based on 800 calories a day) represent a viable strategy for weight loss in obese individuals with a BMI over 30. They are generally used as part of an integrated intervention that includes medical monitoring and they are considered by professionals to be safe and effective for losing weight.
Compared to low-calorie diets which consist of approximately 1200 calories a day, very low-calorie diets are more extreme, but at the same time provide more rapid short-term weight loss. There are four main features that characterise very-low calorie diets:
- They are hypocaloric but relatively enriched with protein (0.8-1.5 g/kg) of high biological value.
- They are designed to contain the full complement of the recommended daily allowance (RDA) for vitamins, minerals, electrolytes and fatty acids, but not for calories.
- They are given as replacements for one or two meals a day and they are found in the form of shakes, soups, bars or porridge.
- They are recommended to be followed for a maximum of 12 weeks.
Research studies show that obese patients lose 1.5 to 2.5 kg/week and an average total loss of 20 kg after 12-16 weeks. However, a drawback that characterises very-low-calorie diets is that people tend to regain the weight lost after the 12 weeks of treatment.
For this reason, to avoid a yo-yo effect, researchers recommend to divide the program into four phases which include an initial phase in which patients consume a balanced low-calorie diet consisting of 1200 to 1500 calories for 1 to 4 weeks; a modified fast phase during which patients consume a very low-calorie diet consisting of 800 calories a day; a refeeding phase in which solid foods are reintroduced and a maintenance phase in which patients receive nutritional education to help sustain the weight loss.
These latter ones are important phases, as it has been observed that after 12 weeks, the rate of weight loss decreases due to the body’s metabolic adaptation. Therefore, it is important to reintroduce solid food and allow the body to continue losing weight.
The Benefits of a Very Low Calorie Diet
The primary benefit of this dietary approach is that it produces large weight losses within a 12 week to 24-week period in a significant proportion of patients. There is also a positive impact of the diet on body composition and metabolic state.
A randomised control trial conducted by G Merra et al 2017 shows that the diet produces a reduction in waist circumference as well as a reduction in resting metabolic rate which leads to a decrease in energy expenditure (reduced energy outputs). The diet also causes a reduction in total cholesterol anywhere between 5% to 25% and a decrease in lean body mass (e.g., muscle, bones). The percentage of weight loss composed of lean mass is generally higher if the initial body weight is lower. For this reason, the diet is specifically recommended for obese individuals, as it would cause unsustainable lean mass loss in individuals with lesser degrees of obesity. Finally, the diet’s results have also been associated with improved mood and a reduction in the number of people suffering from depression.
Although these metabolic changes are beneficial for a number of comorbid conditions including type-2 diabetes, hypertension and hyperlipidemia, very-low calorie diets can cause some adverse effects including fatigue, constipation, nausea and diarrhoea. Another common side effect is gallstones.
Gallstones are small stones usually made of cholesterol that form in the gallbladder. The reason for the formation of gallstones may be that rapid weight loss appears to decrease the gallbladder’s ability to contract bile; but it is unclear whether it’s the diet that causes gallstones or the weight loss itself.
Moreover, even though the diet is supposed to be nutritionally complete except for the calorie content, it can still lead to nutrition deficiencies and fatigue, as regularly eating fewer calories than your body requires leads to fatigue and poor nutrition.
Exercise vs How Much You Eat
Several studies have investigated whether it is safe to embrace a very-low calorie diet along with physical activity. Results show that 30 minutes of aerobic activity a day is helpful to lose weight and doesn’t compromise energy levels. In a randomised control trial, 79 participants with a BMI of 30-40 kg/m2 who were physically inactive, were randomised to exercise only, to a very-low calorie diet only and to both (exercise and very low-calorie diet). Those in the exercise-only group lost the lowest amount of weight, whereas those who were randomised to exercise and follow the diet achieved the highest weight loss. Similar studies show that light/moderate exercise while on a very-low calorie diet is beneficial for losing weight and it doesn’t cause a severe energy deficit.
However, it is important to recognise also that somebody on an 800-1000 calorie diet will have significantly lower energy levels than somebody on a more sustainable calorie plan. If you are somebody undertaking a very low calorie diet, we would not recommend exercising for longer than 30 minutes at a time, and that this exercise is something light (such as brisk walking). This is to ensure that you don't suffer burnout from putting out too much energy compared to the tiny amount that you are giving your body!
If you are simply trying to shed a few extra pounds, exercise paired with a sustainable, balanced diet in a slight calorie deficit is a surefire way to get you to your goals! The key to weight loss after all is exerting more calories than you take in. You can use our calorie calculator to work out the best method for you, as everybody's BMR (Base Metabolic Rate) is different. For example, if you work an office job and don't have time to get to the gym, you wouldn't need to consume as many calories as say a construction worker or personal trainer (who have very active lifestyles) in order to maintain your weight.
However, if you like to indulge a little and still want to lose weight; that's completely fine! We would just recommend trying to get in a couple of workouts a week to offset those extra calories, through a combination of strength training (muscle helps to burn fat!) as well as trusty cardio (if you're not sure of where to start, we have a great beginner's cardio guide here!).
Maintaining your energy throughout the day
As previously mentioned, your energy levels will be considerably lower when undertaking a very low calorie diet, which is another reason as to why they are unsustainable entirely in those who are only overweight, and only temporarily sustainable in the obese!
If you are able to consume just a slight calorie deficit (a few hundred below your maintenance calorie levels) you will find your energy levels to be considerably higher, meaning that you will still be able to enjoy exercise alongside your day to day life. Some studies find that exercise can boost your energy levels, due to it increasing your body's oxygen circulation, therefore promoting your cells to produce more energy (using their mitochondria) and function more effectively!
As well as this, in order to maintain high energy levels throughout the day we also recommend getting your full 8 hours of sleep at night (we know, sounds like a no brainer!). However the reason for this is to do with calories! When we sleep, we conserve energy which means that although we still do burn a small amount of calories as we sleep, it is considerably fewer than we do when we are awake. When we sleep we also repair muscles and blood vessels, which keeps them operating in the most efficient way! If you don't get enough good quality sleep, your red blood cells won't be able to transport oxygen to your muscles as effectively. On top of this, your energy levels are going to be way lower during the day, meaning that you won't be have the energy to effectively burn calories through exercise.
The relationship between very low calorie diets and binge eating
No matter how much will power they have, a very low calorie diet is impossible to maintain for a long period of time and will often be broken the person 'binge eating'. This is a common cause of the 'Yo-yo dieting' phenomenon that has surrounded diet culture for so many years. People attempt extremely restrictive diets which are promoted by thinner celebrities or influencers and will see short term results. However once these results plateau, the person become too physically weak to continue (due to them essentially starving themselves), or their cravings become too much as a result of putting caloric foods on a forbidden, high pedestal- bingeing is inevitable. This is when somebody eats a large amount of highly caloric/rich foods in a short amount of time, and is often followed by feelings of guilt, disappointment and sadness as they feel they have 'broken their diet'.
Once the diet is over, many people want to indulge in all of the foods that they have been restricting/not allowing themselves to enjoy at all, which will lead to rapid weight gain as our bodies try to hold onto as much energy as possible to avoid being 'starved' again. This results in more negative feelings to do with body image and food, which can cause the cycle of restricting (followed by bingeing) to repeat again and again.
In order to combat this, we recommend making small changes to your diet if you want to lose weight rather than such drastic ones, never forbidding yourself from eating any foods (as this is what builds bad relationships with food) and not thinking of foods as being either 'good' or 'bad'- simply enjoying everything in moderation! Many people don't want to hear this, but it is often slower weight loss that is more sustainable in the long term
In conclusion, I don't tend to recommend extremely low calorie diets to people unless their BMIs are extremely high and they are asked to undertake one by their doctor. Instead, I recommend that most people looking to lose weight combine a slight calorie deficit with exercise to see healthy results. It is important to recognise that before beginning any drastic diets you should always consult with a doctor, and remember that these very low calorie diets are only suitable for obese people with a lot of weight to lose in order to be healthy and feel their best.