Nutrition 19 MIN READ

The Long term effects of Fad Diets on Metabolic Health

A fad diet is usually categorized as a type of ‘crash diet’, a diet that doesn’t take into consideration the body’s nutritional needs or metabolic health, but allows for weight to be dropped rapidly.

Written by Team Ultrahuman

Oct 15, 2021
Diets Metabolic Health

A fad diet is usually categorized as a type of ‘crash diet’, a diet that doesn’t take into consideration the body’s nutritional needs or metabolic health, but allows for weight to be dropped rapidly.

It’s important to know that fad diets build their credibility through celebrity endorsements or promotional content and not science. Some people do lose weight through these diets, albeit temporarily, while for most they don’t work at all. The defining factor of a fad diet is that they are not sustainable, meaning one can’t truly integrate them into their everyday lifestyle for more than a few weeks without giving up because of the many restrictions these diets impose, or without putting themselves at risk of caloric and nutrient deficiencies. 

The effects of fad diets on our body

Fad diets lure people with the promise of fast and easy results. But the effects of these diets on our bodies are detrimental to our health, and the lasting impact they can have on our metabolism makes them especially dangerous. But first, let’s understand exactly what is happening to us when we begin a crash diet. According to nutritionist Filip Koidis, the following changes start to come into effect:

  1. Your body reduces its ‘thermic effect’ as you are eating less. The thermic effect of food is a reference to the increase in metabolic rate (the rate at which your body burns calories) that occurs after ingestion of food. When you eat food, your body must expend some energy (calories) to digest, absorb, and store the nutrients in the food you’ve eaten.
  2. Your resting metabolic rate decreases as you weigh less. Resting metabolic rate (RMR) is the rate at which your body burns energy when it is at complete rest.
  3. Non-exercise activity thermogenesis decreases. NEAT describes the calories we burn by the movements we make when we go about our daily activities; this includes the physical movement in our lives that isn’t planned exercise or sports (or sleeping, breathing, and eating).
  4. You absorb the full spectrum of the calories you consume, as opposed to your normal state when you wouldn’t absorb as many calories.
  5. Your hunger signals start to accumulate and push food cravings through the roof, making you feel more hungry and overwhelmed. 

1. Slower Metabolism

When we put ourselves on restrictive diets with calorie intakes that are too low (in a bid to lose weight more quickly), our bodies go into starvation mode. This means our body slows our metabolism in order to save energy. Although our body won’t go into this mode just because we eat a couple of restricted meals or skip a few of them, long term restricted diets—especially where you are eating 1000 or fewer calories—can slow down our metabolism, making it much more difficult for us when we eat a normal diet because our body will still be metabolising it at a slower rate. Research suggests that an individual’s metabolism may begin to decline as early as three days into a very low-calorie diet.

2. Nutrient Deficiency

Another defining characteristic of a fad diet is that it asks you to eat a certain type of food, or features one main ingredient or a type of soup. Fad diets generally lack the nutrients we need to be healthy. They usually ask you to avoid all fats and carbs, both of which are required for a healthy body. All fats are not bad; for instance, good fats found in avocado have omega-3. Omega-3 is a very important nutrient, and also a part of cell membranes. Omega-3 creates the foundation for hormone production, regulation of blood clotting, and artery health. They have also been linked to regulating genetic function. Similarly, all carbs are not bad in the right portions. For example, oats have been linked to many health benefits like lowering blood sugar and keeping our cholesterol in check. They are high in fibre and in beta-glucans (sugars that attach to cell walls), absorb cholesterol and therefore lower it in the body.

3. Cognitive function and performance

The brain needs glucose, which is the end product of digested carbohydrates, to function well. When a person lowers their carb intake to less than 100 grams in a day, there is a shortage of glucose for the brain. The body copes with this shortage by breaking down fatty acids into ketones (ketones are substances that your body makes if your cells don’t get enough glucose). However, the brain cannot function at its optimum level on ketones, leading to impaired memory, concentration and attention levels. Additionally, most fad diets are low in salt content, and sodium plays a key role in vital brain functions such as maintaining alertness and concentration.

Crash diets also make the brain more susceptible to stress because they increase the stress hormone (corticosterone) levels, which leads to a higher risk of depression. If you jump from one fad diet to another, over time, it causes your blood vessels to shrink, leading to a condition called atherosclerosis. This reduces the blood supply to the brain, putting you at risk of a brain stroke. 

4. Digestive issues

If you’ve tried any of the trending fad diets, you may have noticed your digestion getting affected. You may either feel bloated or experience indigestion or other issues: 


Bloating is when your stomach feels swollen after eating. You could experience this if you’re not eating enough fibre. Fibre is generally found in fresh fruit, vegetables and whole grains; so if you’ve been desperately avoiding carbs because of your latest diet, that could cause problems. There are two types of dietary fibre—soluble and insoluble—and both work in slightly different ways to keep our bowel moving along. If you aren’t going to the loo as regularly, you’re much more likely to feel constipated and gassier than normal.

You could also feel bloated if you’re feeling hungrier than usual. If you’ve reduced your calorie intake quite significantly, then you might overindulge at your next meal. Eating when you’re really hungry means that you may be at a greater risk of eating more quickly, not chewing your food properly and swallowing excess air, all of which can easily make your stomach bloat. We need to eat slowly and mindfully in order to give our stomachs the best chance of digesting properly.


Depending on the diet you’ve embarked on, your system may have fallen out of balance. The problem with most fad diets is the requirement to really lower the intake of carbs. Most people mistake it to mean the elimination of carbs which poses a problem because something needs to replace those lost carbs. There are 3 main macronutrients; carbs, protein and fat, so a low intake of carbs must mean higher protein or more fat, both of which can put more stress on your system, especially if your body is not used to higher concentrations of one macronutrient over the other. Fat and protein take more effort to break down, and if your gastric secretions are lacking, to begin with, then you’re much more likely to suffer from symptoms such as acid reflux.


Constipation occurs when bowel movements become less frequent and stools become difficult to pass. It happens most often due to changes in diet or routine, or due to inadequate intake of fibre. Many diet plans often suggest including pre-packed items into our daily lives, for instance, oats or yoghurt. While both of these food products are considered healthy, instant oatmeal or sweetened yoghurt can have negative effects on your body. As research shows, processed foods lack fibre and could cause a bit of a backlog in our system. Fibre is also essential for supporting the good bacteria in our gut. 

Keeping yourself hydrated every day is crucial, even more so when indulging in a fad diet that is likely to leave you dehydrated. Dehydration is one of the key underlying factors when it comes to constipation. The food you consume travels from your stomach to the large intestine or colon. In the absence of enough water in your body, the large intestine soaks up water from your food waste. This results in hard stools that are difficult to pass. It’s easy to forget about the basics when it comes to prescriptive regimes, and not having enough water can have significant effects.


Diarrhoea—loose, watery and possibly more frequent bowel movements—is a common problem. Making any change to your diet, good or bad, will take your gut a little while to adjust because our gut bacteria help break down the starches and sugars that make up the indigestible carbohydrate portion of our diet. Hence, if your diet changes, particularly the proportions of carbohydrates or sugar, the balance of bacteria may get altered, resulting in diarrhoea. Even a sudden high content of fruit or vegetable juice, for example, or an experiment with artificial sweeteners could have similar results. While the general notion around artificial sweeteners is that they are healthy and help reduce your calorie content, they can wreak havoc on your gut.

Although having too much fibre isn’t a common modern-day issue, if you’re embarking on a more extreme regime, such as juicing or a cabbage soup diet, then the increase in fibre content could result in your system flushing things out rather quickly. It would be good to strive for a balanced diet because everything needs to be consumed in moderation, even the good stuff.

5. Gut Microbiota

Gut microbiota, which also goes by the name of gut flora or microbiome, refer to microorganisms including bacteria, archaea, eukaryotes and fungi that reside in the digestive tracts of humans. Their myriad functions include fortifying the gut, offering a defence mechanism against pathogens, modulating immunity, nourishing the host with nutrients and moulding the gut epithelium. 

Effects of (digestible) carbohydrates and fibre

Sugars and starch are digestible carbohydrates, while fibre includes non-digestible carbohydrates. The presence of the former can augment the gut microbiota and insufficiency can lessen the heterogeneity of the gut microbiota. 

A study suggests that when people consumed dates with a mix of sugar there was a rise in the volume of Bifidobacteria, a species that has the ability to synthesize folate. Folate is a vitamin required for DNA synthesis and repair. This was also complemented by a reduction in Bacteroidetes. The phylum “Bacteroidetes” is composed of three large classes of gram-negative, non-spore-forming, anaerobic or aerobic, and rod-shaped bacteria that are widely distributed in the environment, including in soil, sediments, and seawater, as well as in the guts and on the skin of animals. 

This was also observed in another study that used lactose along with a decline in Clostridia species, specifically those in the Clostridium cluster XIVa, that have been connected with irritable bowel syndrome. Clostridia are spore-forming, gram-positive, anaerobic bacilli present widely in dust, soil, and vegetation and as normal flora in mammalian gastrointestinal tracts. Pathogenic species produce tissue-destructive and neural exotoxins that contribute to disease manifestations.

While most carbohydrates can be broken down into sugar molecules, fibre can’t, and instead have to pass through the body undigested, making it available to the gut microbiota. It is therefore known as a “microbiota accessible carbohydrate.” Research suggests that a diet that is deficient in fibre can reduce diversity in gut microbiota. Increasing fibre consumption can correct this deficit. 

The increment of Bifidobacterium and lactic acid bacteria such as Lactobacilli was associated with a rise in fibre intake. Further, it has been observed that a low fibre diet translates into a lower generation of short-chain fatty acids by the gut microbiota, in effect reducing their anti-inflammatory properties. 

Effects of fats

There is evidence of a high-fat diet leading to a spike in anaerobic microbes (any organism that does not require molecular oxygen for growth) and Bacteroides in the gut microbiota. Many anaerobic bacteria are known to generate enzymes that annihilate tissue or, at times, release potent toxins. A study by Fava et al found that a low-fat diet evoked an increase in Bifidobacterium, and this elevation resulted in a decrease in fasting glucose and total cholesterol levels.

The ketogenic diet was originally conceived to treat epilepsy, but it has gained favour among the non-epileptic population as well. Research suggests that this high-fat and low-carbohydrate diet impacts gut microbiota in the same way as other carbohydrate cancelling diets. 

Another important study by Olson and co. showed that the ketogenic diet amplifies the extent of Akkermansia (a bacterial species that may be both beneficial and detrimental) and Parabacteroides, which were found to be tied to the anti-seizure effects of the ketogenic diet. 

Akkermansia was shown to aid in the rebuilding of the integrity of the gut epithelium in initial studies. The latest findings show connections between Akkermansia and the onset of Parkinson’s disease, multiple sclerosis, and Alzheimer’s.

Fad diets and weight gain

Most of the weight that people tend to lose in a brief period of time due to fad diets is mere water weight. Once the individual resumes their regular eating habits, they regain weight promptly. Deprivation also leads to binge-eating and resultant weight gain. Continually swaying between different fad diets can make it more difficult for an individual to lose weight in the long run. After completing yo-yo diets you regain fat and lose muscle. It becomes more arduous for the person to reacquire the lost muscle mass, which helps to burn more calories. 

Some studies have also discovered that up to 55% of women experience weight variation due to fad diets. Weight fluctuations can also cause high visceral fat (fat around internal organs) accumulation that can put people at risk of heart diseases, irrespective of weight.

Katherine Kimber, a registered dietician and spokesperson for the British Dietetic Association, warns against the adverse effects of crash diets. Resorting to an extreme dietary plan and then withdrawing from it after losing weight can lead you back to your old ineffective eating habits that caused the weight gain in the first place. Gradual changes, on the other hand, give you the space to reassess your relationship with unhealthy foods and adopt a healthier lifestyle for a longer, sustained duration. 

A limitation of calories also robs the body of vital nutrients. This dearth of essential minerals and vitamins can exert stress on your immune system. Nutrition professor Linda Bacon says that extreme weight loss leads to fatigue and dehydration and calorie-cutting leads to the loss of heart muscle.

There is another theory at play here too—the set point theory, which posits that every individual’s body has a particular weight range, hardwired into their DNA.

Faddiets weight gain

What is metabolism?

Metabolism is a word used to describe all chemical reactions involved in maintaining the living state of the cells and the organism. It can be further divided into two categories:

Catabolism – the breakdown of molecules to obtain energy

Anabolism – the synthesis of all compounds needed by the cells

Metabolism is closely connected to nutrition and the availability of nutrients

What is metabolic health

Metabolic health is described as having ideal levels of blood sugar, triglycerides, high-density lipoprotein (HDL) cholesterol, blood pressure, and waist circumference, without using medications.

Fad Diets and Metabolic Health 

Research has shown that extreme weight loss in a short amount of time will cause your metabolism to slow down, which will only result in more weight gain in the future. This is because of the increased hunger that accompanied the weight loss. 

Fad diets are not only slowing down your metabolism but also hurting your resting metabolic rate by slowing down your body’s ability to burn calories, sabotaging your efforts towards weight loss. 

It is shown that people tend to regain the weight they lost quickly because they are hungrier and more deprived of energy. This is because each individual’s resting metabolic rate is different. Resting metabolic rate (RMR) is a measurement of how much energy (calories) the body burns while performing basic functions when you are at complete rest—for instance, while you’re laying in bed or watching television. Some of these basic bodily functions include breathing, circulating blood, brain function and organ function. Resting metabolic rate is an important framework to determine energy balance, which accounts for approximately 60% of total energy expended. It estimates the number of calories burned at rest to determine baseline caloric needs. It also generally offers a measure of fat and carbohydrate utilization. For instance, even a set calorie diet would not work if the diet was being executed by a group of people belonging to the same gender, age, height and weight and following a strict 1,500 calorie per day diet; they will each yield very different results because all the members of that group could have varying RMRs. 

One of the reasons why fad diets don’t have a high success rate and negatively impact our bodies is that they alter our metabolism. Our body follows a natural pattern of telling us when to eat and when to stop. However, when following a diet, we are required to stick to the dictated food restrictions, which goes against the body’s way to function and hence leaves a lasting effect. Leptin is a regulator of long-term regulation of energy balance, suppressing food intake and inducing weight loss. Ghrelin, on the other hand, is a fast-acting hunger hormone, signalling hunger and playing a role in meal initiation. Research suggests that fad diets can cause leptin resistance. According to ­­­­Nutriformance, “When dieting consists of a drastic decrease in caloric intake, your body will compensate for this “starvation mode”, which will slow the rate of your metabolism.” This is the opposite of what a person trying to lose weight wants because a fast metabolism is what makes weight loss easier.

How to Incorporate Healthy Eating Habits

The choice to eat right is fraught with indecision. Nutritional needs are personalised based on a plethora of factors including age, genetic variation, pre-existing health conditions and fitness goals.

Nutrition Plan

Carbohydrates enhance the secretion of the happy hormone, serotonin, making us veer towards chips and sweets when overwhelmed with stress or unpleasant emotions. A nutrition plan can help you stay on track and induce accountability. Goals like reducing blood pressure or gaining muscle mass can help you stay on track. Set a long-term health goal and then make a daily and weekly nutrition plan to get you there. Log your progress and stay on track. 

Knowing your macros & micros

Nutrients can be segmented into two categories––macronutrients and micronutrients. Our bodies need large amounts of macronutrients, which provide our body with energy in the form of carbohydrates, proteins, and fats. Macros are the building blocks of our muscles and tissue. Most people consume far more micronutrients than they need––leading to adverse side effects such as being overweight or obese. 

Your body requires small amounts of vitamins and minerals that are collectively known as micronutrients. Many people forget about micronutrients and end up being nutrient-deficient in several categories. While supplements and multivitamins can help prevent major deficiencies, it’s best to get your nutrients from your food.

Whole Food 

Whole foods are foods that remain close to their state in nature. They do not have added sugars, starches, flavourings, or other manufactured ingredients. They are not produced or manufactured in a factory. Because they are not manufactured, whole foods are not manipulated to be addictive like many foods containing added sugar. Choosing mostly whole foods will result in a nutritious diet that is naturally higher in fibre, vitamins, and minerals.

Hydration is key

Experts recommend that men should drink roughly 16 cups of water each day (3.8 litres), while women should drink at least 12 cups daily (2.8 litres). There are many health benefits to staying hydrated including regulating body temperature, lubricating joints, preventing infections, increasing organ function, and also better quality sleep, cognition, and mood. 

Listen to your body

Everybody has a unique genetic makeup that determines our metabolism, vitamin absorption, risk of obesity, taste, and more. For example, one person may love Brussels sprouts while someone else can’t stand them––all because of those two individuals’ differing genetic compositions. The same concept applies to our body types and can explain why certain diets work for some people, but not for others. The best source of information for finding the optimal diet for you is your own body. Track your health metrics and biomarkers. Maintain a food journal. Paying attention to your body and eating mindfully can go a long way in helping you eat right. 

Control Portion Sizes

When whipping up meals at home, choose smaller plates. Smaller portions and turning half your plate into a melange of colours by choosing red, orange, and dark-green vegetables is one of the ways of eating healthy. Studies suggest that there is substantial evidence surrounding the effects of portion size on energy intake. Offering large portions of high-energy-dense (HED) foods increases overall intake in children and adults. This is the enduring portion size effect (PSE). 

Long term Weight Management 

Weight maintenance strategies help you to build insight for long-term management, anticipate struggles and prepare contingency plans, moderate behavioural fatigue and put into perspective the inevitable lapses and relapses of any long-term engagement. Frequent self-monitoring, reduced calorie intake, smaller and more frequent meals/snacks throughout the day, increased physical activity, and more frequent at-home meals are all great practices to help you manage your weight. 


To fad diet or not to fad diet has been the question that’s been debated for quite some time, and even though the praise for and the effectiveness of fad diets have been vastly heard of and read, the fact is that the science behind these fad diets is seldom concrete and more research has uncovered the many negative effects of fad diets on our bodies. 

Crash diets and the philosophy that accompany them often lead to distorted eating behaviours (binge-eating, overeating), unpleasant emotional states (guilt and eating-related stress), as well as poor metabolic and body composition effects. Opting for the right methods to help you reach your goal weight and then adopting weight management practices is a far more sustainable option than a quick-fix diet. As this would lead to a better healthier lifestyle. 

Disclaimer: The contents of this article are for general information and educational purposes only. It neither provides any medical advice nor intends to substitute professional medical opinion on the treatment, diagnosis, prevention or alleviation of any disease, disorder or disability. Always consult with your doctor or qualified healthcare professional about your health condition and/or concerns and before undertaking a new health care regimen including making any dietary or lifestyle changes.



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