Fasting diets that promote periods of calorie restriction have been show to improve health markers by normalising insulin and glucose to reduce the risk of diabetes.
Research also links them with improved metabolism, better body composition, enhanced performance, extended longevity, and enhanced brain health and function.
However, there is a worrying trend where the concept of fasting – prolonged periods of no food intake, followed by shorter periods (8 hours up to a few days) where normal calorie consumption resumes – is being conflated and used interchangeably with the concept of prolonged very low-calorie diets, or VLCDs.
Understand the differences
The fact is that fasting and diets that severely restrict calories over prolonged periods are fundamentally different.
When done correctly, individuals who fast will still meet their total calorie requirements, but these energy demands are met within specific time frames.
Those following a VLCD, which generally advocates prolonged daily calorie intakes of just 800-1000 in an effort to lose weight, will constantly remain in a negative calorie balance, which is unsustainable and can be downright unhealthy.
Known commonly as starvation diets, when followed indefinitely these extreme diets can lead to malnourishment, cognitive and/or emotional problems, and long-term health concerns that include endocrine system disruption, and liver and heart issues, with severe cases resulting in organ failure or a heart attack.
Metabolic havoc
And the long-term impact on the endocrine system can negatively affect your metabolism, making it easier to regain all the weight lost and also more difficult to lose weight during subsequent diets.
For instance, studies show that those who engage in severe calorie restriction are likely to have lower T3 and thyroxine levels, two important hormones required for a healthy metabolism. If T3 and/or thyroxine levels fall too low a person’s metabolism will be sluggish, and weight loss can slow.
They’ll also likely struggle to maintain a goal weight once achieved. In addition, the rebound weight gain that occurs under these conditions will more than likely result in fat gain rather than increases in lean body mass due to the metabolic damage caused.
And when VLCD eating is combined with macronutrient restriction, such as a VLCD diet and low-carb eating, the impact can be even more severe.
This type of diet can adversely affect the hypothalamus, pituitary and adrenal glands (known as the hypothalamic-pituitary-adrenal, or HPA axis), which interact in a complex way to keep hormones balanced.
Severely restricted energy intakes, when compounded by life stress and exercise, stress the axis, causing HPA dysfunction (often referred to as adrenal fatigue). This, in turn, can lead to general fatigue, a weakened immune system and weight gain.
The rebound
As counter-intuitive as that may sound – calorie restriction causes weight gain – the science is generally unequivocal.
For instance, a landmark study published in the Internet Journal of Advanced Nursing Practice in 2013 by Outland et al, looked at “promoting homeostasis (a state of equilibrium or balance) to avoid rebound weight gain in yo-yo dieters”.
The study identified numerous compensatory mechanisms that lead to rebound weight gain, including involuntary responses to hunger that lower metabolism, reduce thyroid function, and produce fat cells.
Researchers stated that “when calories are withheld to the point of hunger, the body becomes flooded with hunger hormones. The hungry individual is spurred to eat, and thus eats a lot. Eating in response to hunger is a voluntary way to replenish lost energy stores.”
What this basically means is that when energy stores are depleted through dieting, a type of overcompensation effect occurs to replenish them, which is characterised by overeating. It’s what we commonly refer to as rebound weight gain.
What’s more concerning is that hormone levels can remain altered for up to 12 months after a VLCD has ended, at least according to the findings of a 2011 study published in the New England Journal of Medicine.
This chronically altered hormonal state makes it extremely difficult to resist the urge to eat to maintain your post-diet weight and body composition.
The researchers in this study, who were studying the “long-term persistence of hormonal adaptations to weight loss” coached participants through eight weeks of an extreme diet that contain just 500 to 550 calories a day.
While dieters lost an average of 13.6kg during the study, they regained an average of 5kg, despite counselling support from researchers. The dieters also reported feeling far hungrier and more preoccupied with food than they did before losing the weight.
The researchers concluded: “One year after initial weight reduction, levels of the circulating mediators of appetite (hormones) that encourage weight regain after diet-induced weight loss do not revert to the levels recorded before weight loss.”
The participants’ ghrelin levels were about 20% higher than at the start of the study, while levels of their appetite-suppressing hormones, leptin and peptide YY, remained unusually, and unexpectedly low.
A new approach
However, an important point that is often missing from discussions around VLCDs is the quality and type of foods consumed on these diets.
And if new research is to be believed, this could hold the key to the effectiveness of VLCDs, at least when they are followed periodically and not for prolonged periods of time.
A 2017 study called the Droplet trial led by Oxford University obesity expert Prof. Susan Jebb showed that following an 800-calorie-a-day diet over eight weeks, which is then followed by a behavioural support programme, resulted in participants in the 800-calorie group losing an average of 9.5kg – they had also kept the weight off 12 months after the diet ended – compared to a loss of just 3kg in participants who followed standard diet advice.
The key to this success, other than the behavioural counselling, was also the source of those calories.
Rather than have participants follow extreme juice diets or have them cut out entire food groups, the researchers implemented modernised VLCDs that were “nutritionally complex”.
This includes nutrient-dense, low-calorie foods, which other studies have shown can help to increase feelings of satiety without causing any nutritional deficiencies.
Researchers in the Droplet trial recommend that dieters who want to lose weight on a VLCD should follow a well-planned approach that reduces food intake to 800 calories for just eight weeks, which is then followed by “a stepped return to eating less than you used to.”
Not for everyone
Despite the proven results, there are important considerations that need to be taken into account when it comes to the use of VLCDs.
Firstly, the results of research like the Droplet study are achieved when treating those who are severely overweight or those classified as morbidly obese. This type of dietary approach is not often suitable for otherwise healthy individuals who are looking to shed a few kilos or improve their body composition.
The other important point is that these diets are conducted under the strict supervision of health professionals, who closely monitor the health of the participants to mitigate any potentially life-threatening conditions from developing. It is therefore not recommended that anyone follow a VLCD without suitable medical or professional guidance and supervision.
Author: Pedro van Gaalen
When he’s not writing about sport or health and fitness, Pedro is probably out training for his next marathon or ultra-marathon. He’s worked as a fitness professional and as a marketing and comms expert. He now combines his passions in his role as managing editor at Fitness magazine.
