Types of Diets

Types of Diets

Countless diets have been described both for weight loss and promotion of good health. Expert opinion remains divided on the merits of various diets, with many yet to undergo rigorous scientific investigation. Much of the research in this field is plagued by the problems of assessment of dietary intake, confounding variables, follow up of insufficient duration, and bias due to industry funding. And to compound these challenges, the mainstream media is often responsible for incomplete and incorrect reporting of the science, misinterpreting studies and often publicising hyperbolic and distorted content that makes for good headlines.

After the introduction of the dietary guidelines, low-fat and reduced calorie eating was widely accepted and endorsed as the ideal dietary pattern. However, in recent years this has been challenged, and we have seen a proliferation of other diets claiming to be the one true way to achieve weight loss and optimal health. Diets can be broadly categorised based on macronutrient composition and quantity, although some diets have elements common to more than one category (e.g. low-fat, low-carbohydrate):

  • low-calorie – Weight Watchers, Jenny Craig, SlimFast
  • low-fat – Dr Oz, Zone
  • low-fat whole foods – Mediterranean, DASH, Pritikin, Sonoma
  • low-carbohydrate – Atkins, low-carb high-fat (LHCF/Banting), ketogenic, Protein Power, Sugar Busters
  • low-GI – South Beach
  • paleo – Paleo diet, Primal Blueprint, Paleo Solution, raw food
  • food-sensitivity – gluten-free, low-FODMAPs
  • vegetarian and vegan
  • intermittent fasting – 5:2, alternate day fasting
Do diets work?

Weight regain after weight loss is a common problem for all those who have had a recent weight loss. Not only do they regain all the weight they have lost, but as many as two thirds of people regain more weight than they lost over 4-5 years. For many peo­ple desperate to lose weight, dieting follows a cyclical pattern over many years: restrict, regain, gain extra, restrict again. The reasons for this are complex, including increased hunger and a slowing of metabolism in response to food deprivation.

The body adjusts to reduced calorie intake by lowering its resting energy expenditure (REE) – the energy that we use when at rest. When peo­ple find that they can no longer tolerate the deprivation/starvation involved in a calorie-deficit diet and return to eating as they did before, their total energy requirement (REE plus energy needed for activity) is lower and thus they regain weight.


The traditional approach to weight loss and management has been the prescription of diets that provide an energy intake below that of an individual’s normal energy expenditure. Reducing the total energy content of the diet can be achieved by restricting protein, carbohydrate, fat, or a combination of the three.

A low-calorie diet has been traditionally defined as a diet that provides an energy intake of 800-1500 kcal per day. A very low-calorie diet further limits energy intake to less than 800 kcal per day. However, these definitions are arbitrary. For example, a diet consisting of 700 kcal per day for a small, sedentary individual would induce only a modest energy deficit if the individual’s resting energy expenditure is 1200 kcal per day. In contrast, a diet consisting of 1200 kcal per day for a tall, active individual would result in a substantial energy deficit if their daily energy requirement is 3000 kcal.

Low-calorie diets were initially developed around the concept of creating a near-fasting metabolic response without nutrient depletion, and were designed to produce rapid weight loss while preserving lean body mass. They are often comprised solely of liquid formulations that are intended to completely replace other food intake for a specific period of time.

Although these diets may produce rapid weight loss and result in symptom improvement – such as reduced joint pain, improved sleep quality in obstructive sleep apnoea, reduced shortness of breath on exertion, reduced peripheral oedema and rapid improvement in metabolic control in diabetes-  there are concerns regarding subsequent weight maintenance as well as the diet’s sustainability.


Since the widespread introduction of the first dietary guidelines, low-fat diets have been promoted as a means of both improving cardiovascular health and losing weight. However, in recent years, there has been much critique of low-fat diets due to the failure of low-fat national dietary guidelines to prevent and now improve the obesity and diabetes pandemics.

As its name suggests, a low-fat diet is one that restricts fat, especially saturated fat and cholesterol. Typically, it promotes consumption of whole grains, lean meats, white fish, reduced fat dairy, legumes, vegetables and fruit, and discourages butter, whole eggs, and animal fat of any kind.

When compared with dietary interventions of similar intensity, evidence from randomised controlled trials does not support low-fat diets over other dietary interventions for long-term weight loss. And when comparing low-fat to high-fat, there is no clear evidence that low-fat eating is superior with regards to health outcomes over the life span.

One particular concern with low-fat diets is the heavy reliance on carbohydrates, which are often processed grains (e.g. breads, cereal, and pasta). When the diet is consistently high in carbohydrate, such as processed grains or added sugars, the level of insulin in the blood remains elevated. This can lead to storage of the excess carbohydrate as fat through de novo lipogenesis when glycogen stores are full. Additionally, high-carbohydrate diets can lead to an earlier return of appetite and hunger following meals, which may facilitate weight gain over the long-term.

Insulin resistance

Insulin is a peptide hormone produced by the beta islet cells of the pancreas that assists in the regulation of blood glucose levels by promoting the absorption of glucose into liver, fat and skeletal muscle cells and decreasing hepatic gluconeogenesis.

Glucose is stored in muscle as glycogen, in fat as triglycerides, and in the liver as both. The net effect of insulin is to remove glucose from the circulation by increasing tissue uptake and facilitating the conversion of glucose into glycogen or fat. Insulin resistance is a state in which liver, fat and skeletal muscle cells do not respond appropriately to insulin. Hyperinsulinemia is a surrogate marker for insulin resistance.

Insulin resistance is considered a key aetiological factor in the development of metabolic syndrome, a condition associated with increased risk of cardiovascular disease, diabetes, chronic kidney disease and increased all-cause mortality. Metabolic syndrome can be diagnosed in the presence of at least three of the five following medical conditions: abdominal obesity, high blood pressure, high blood sugar, high serum triglycerides and low high-density lipoprotein (HDL) levels.

The two dominant underlying causes of insulin resistance are thought to relate to hepatic production of diacylglycerol and inflammatory states. Hepatic diacylglycerol production occurs in the state of excess carbohydrate consumption, and thus reduced carbohydrate diets may be protective for the development of metabolic syndrome.


The Mediterranean diet has long been considered paradigm for healthy eating. There is no single definition of a Mediterranean diet, complicating research. The aim of the diet is to mimic the traditional dietary pattern that is prevalent throughout Mediterranean countries – including vegetables, fruits, whole grains, nuts, seeds, legumes, olive oil, selective dairy intake, moderate red wine intake during meals, moderate fish and limited meat consumption.

The Mediterranean diet is arguably the best-studied and most evidence-based diet to prevent cardiovascular disease and reduce the risk of other chronic diseases such as cancer, depression, diabetes, obesity, erectile dysfunction, Parkinson’s and Alzheimer’s disease. The diet is also known to improve features of cardiovascular disease, such markers of inflammation, as well as cardiovascular disease outcomes such as death and cardiovascular events.

Extra virgin olive oil, one representative component of this diet, is associated with reduced risks of cancer, cardiovascular disease and mortality – perhaps due to its anti-inflammatory, antioxidant and vasodilatory effects.


The ‘dietary approaches to stop hypertension’ (DASH) diet was originally tested for its effects on blood pressure, from which the name derives, but has since been applied to both weight loss and general health promotion.

The diet is largely based on two studies, DASH(253)and DASH-Sodium, which explored methods of reducing blood pressure through dietary changes. In the DASH study (1997), over 450 adults were fed a diet low in fruits, vegetables, and dairy products, with a fat content ‘typical of the average diet in the United States’. They were then randomly assigned to receive for eight weeks the control diet, a diet rich in fruits and vegetables, or a ‘combination’ diet rich in fruits, vegetables, and low-fat dairy products, and with reduced saturated and total fat. Sodium intake and body weight were maintained at constant levels. The researchers found that the combination diet significantly reduced systolic and diastolic blood pressure by 5.5 and 3.0 mm Hg more, respectively, than the control diet, and that the fruits-and-vegetables diet reduced systolic blood pressure by 2.8 mm Hg more and diastolic blood pressure by 1.1 mm Hg more than the control diet.

The DASH diet is now characterised by consumption of fruits, vegetables, and low-fat dairy products; includes whole grains, poultry, fish, and nuts; and attempts to reduce the intakes of red meat, sweets, sugar-containing beverages, total fat, saturated fat and cholesterol. Since the original trials, more recent data also suggest that the diet may be effective for improvements in blood pressure, as well as reducing total cholesterol and LDL concentrations. However, data suggest the diet does not significantly affect triglycerides, glucose or HDL. Notably, simple substitution of low-fat for full fat dairy foods in the DASH diet may lead to greater reduction of triglycerides and large and medium very-lowdensity lipoprotein (VLDL) particle concentrations, while still reducing blood pressure to the same extent as the low-fat dairy DASH diet.

In recent years, the U.S. News & World Report has consistently ranked DASH to be the best overall diet. This particular ranking system is based on the consensus opinion of a panel of expert judges who rate diets based on criteria including short- and long-term weight loss, diabetes and cardiovascular disease management, ease of compliance, ‘nutritional completeness’ and health risks. However, ignoring the fact that the ranking system is based on expert opinion, one could argue that data comparing DASH to other reasonable contenders is lacking.


There is no official agreed definition of a low-carbohydrate diet. As the name suggests, it is defined by restricting intake of total carbohydrate below a particular threshold. Given that the average Western diet consists of greater than 200g of carbohydrate per day, it has been suggested that an intake of less than 150g should be defined as low-carbohydrate. Many advocates of a low-carbohydrate diet would regard 50-100g of carbohydrate as low-carbohydrate, while the extreme form of carbohydrate restriction, known as the ketogenic diet (see below) is less than 30-50g per day.

All grains (whole or processed), legumes, starchy vegetables and most fruit are largely avoided, and are replaced with real food containing a higher percentage of healthy fats and protein (e.g. meat, fish, eggs, dairy, nuts and seeds, olive oil). The percentage of fats and protein varies; one could follow a low-carbohydrate high-fat diet (with low-moderate protein), or a low-carbohydrate high-protein diet (with low-moderate fat). When discussing macronutrients, it becomes a balancing act.

Interest in low-carbohydrate diets is not a new phenomenon; they have been explored for diabetes management for decades. However, interest in low-carbohydrate eating has resurged in recent years, seen as a possible solution to obesity and metabolic syndrome. In particular, low-carbohydrate proponents have emphasised the public health failures of low-fat recommendations for weight management and chronic disease prevention that were introduced with the dietary guidelines.

There is a substantial and rapidly growing body of evidence indicating that low-carbohydrate diets are effective for weight loss and improving metabolic risk factors. The authors of a 2009 systematic review of randomised controlled trials of low-carbohydrate vs. low-fat/low-calorie diets concluded that ‘there were significant differences between the groups for weight, high-density lipoprotein cholesterol, triacylglycerols and systolic blood pressure, favouring the low-carbohydrate diet’. Additionally, the authors found that there was a higher attrition rate in the low-fat compared with the low-carbohydrate groups.

Similarly, a2012 meta-analysis of 23 randomised controlled clinical trials found that both low-carbohydrate and low-fat diets lowered weight and improved metabolic risk factors. However, compared with individuals on low-fat diets, individuals on low-carbohydrate diets experienced a significantly lower reduction in total cholesterol and LDL, as well as a greater increase in HDL cholesterol and a greater decrease in triglycerides.


Since the boom of the Atkins diet in the early 2000s, different variations of low-carbohydrate high-protein diets have been increasingly popularised by the media as a promising strategy for weight loss and optimal health by improving satiety, decreasing fat mass and increasing lean body mass.

Long-term effects of high-protein diets depend on the population studied as well as the composition of their particular diet. In general, the diet has been shown to help weight loss and maintenance, as well as have beneficial effects on proposed metabolic risk factors. Lower triacylglycerol levels and fat mass loss, increased satiety (possibly mediated by increased leptin sensitivity), and fluid loss associated with reduced .

High fat – ketogenic

The ketogenic diet is a form of low-carbohydrate diet that reduces daily carbohydrate intake to around 20–30 g/day, although there is considerable vari­ation in the amount of carbohydrate restriction required. A low carbohydrate intake forces the body to rely on fat as its primary fuel source – a state known as nutritional ketosis. The fuel source produced by the breakdown of fat is called ‘ketone bodies’ or ‘simply ketones’.

In a ketogenic diet, the carbohydrate restriction is so severe that the only carbohydrates eaten come predominantly from green vegetables and nuts. It emphasises eating real foods high in fats and moderate protein, such as oily fish, eggs, butter, lard, liver and other organ meats, grass-fed beef, coconut and olive oils, and drinking bone broths, water, tea, or coffee with cream.

The ketogenic diet continues to gain momentum in the sporting world, particularly in endurance events given the possibility of body fat being used as a primary energy source without the need to rely on limited glycogen stores. However, it is also increasingly being explored for general health and wellbeing, and for its application in particular conditions, such as diabetes. According to data from a 2018 online survey of over 300 people with type 1 diabetes who follow a very low-carbohydrate diet (36 ± 15g of carbohydrates), a very low-carbohydrate diet can help people with type 1 diabetes maintain ‘exceptional glycaemic control’ with low rates of hypoglycaemia and other adverse events.

Some people confuse nutritional ketosis with ketoacidosis. The latter is a serious medical emergency seen in type 1 diabetics, and is associated with blood ketone levels three to five times higher than those seen in nutritional ketosis – so it is not a concern for those on a ketogenic diet.  Even in prolonged starvation ketosis, ketones do not typically rise beyond 6mmol/L, which is well below the threshold of symptomatic ketoacidosis. Most people on low-carbohydrate diets will not produce ketone levels greater than 1.5mmol/L.


The low-GI diet is similar to the above two diets, but perhaps not as severe as the ketogenic diet. In simple terms, the diet promotes switching from high-GI carbohydrates (e.g. white bread and pastas) to lower-GI carbohydrates (e.g. whole grains, certain fruits and vegetables).

A low-GI diet may induce favourable metabolic effects – such as rapid weight loss, decrease of fasting glucose and insulin levels, reduction of circulating triglyceride levels and improvement of blood pressure however there is a need for updated and better controlled studies testing these effects.


Palaeolithic diets attempt to emulate the dietary patterns of our Stone Age ancestors with an emphasis on avoiding processed foods, while promoting the intake of vegetables, fruits, nuts, seeds, eggs, and meats. Dairy and grains are avoided entirely.

A particular challenge in reaching conclusions about the Palaeolithic diet is the known variation in our ancestral dietary pattern and disagreement regarding its primary features. Many of the plant foods and nearly all of the animal foods consumed during the time of our hunter and gatherer ancestors are now extinct, and so replicating their diet may be easier said than done. Nonetheless, a number of trials using varying Palaeolithic diets have shown a range of health benefits, especially short-term improvements in metabolic syndrome components.

Gluten-free diet

The avoidance of wheat- and gluten-containing products is a worldwide phenomenon that has given rise to multi-billion dollar industry that grows annually. Gluten is a group of proteins found in wheat, barley, rye, oats and their derivatives. Gluten, which means ‘glue’ in Latin, gives elas­ticity to dough, enabling it to rise, and is responsible for the chewy texture of bread and other products.

Wheat contains hundreds of different proteins. Coeliac disease is characterised by an immune response to one specific protein and a specific enzyme, however it is not widely understood that individuals can and do react to several other components of wheat. There’s increasing evidence that many individuals are sensitive one of the wheat proteins, a condition referred to as ‘gluten sensi­tivity’.

Gluten sensitivity (and coeliac disease) does not necessarily present with gut symptoms, as it can affect nearly every tissue in the body. Because of this and the inability to make a diagnosis on the basis of a simple test, a helpful way of determining if an individual has the condition is to trial a wheat-free diet.

The incidence of coeliac disease has indeed increased over the past few decades, but it’s still relatively uncommon, affecting 1% of Western populations. Some individuals with coeliac-like symptoms, or sometimes non-bowel symp­toms such as behaviour disturbances, test negative to coeliac disease yet respond very well to a gluten-free diet. This is referred to as ‘non-coeliac gluten sensitivity’, which may be induced by the consumption of fructans (an oligosaccharide of the FODMAPS – see below).

Many individuals who are intolerant of gluten are also intolerant of other proteins found in foods such as dairy, eggs and coffee. Studies have shown that 50% of people with coeliac disease show intolerance to casein, a protein found in milk. This may explain many individuals with coeliac disease continue to experience symptoms after adopting a gluten-free diet. For this reason, some recommend a completely grain- and dairy-free diet during the trial period.


A proportion of people with irritable bowel syndrome (IBS), which causes symptoms such abdominal pain, bloating and diarrhoea, are sensitive to a group of carbohydrates known as FODMAPs- an acronym which stands for:

  • Fermentable– meaning the carbohydrates are broken down (fermented) by bacteria in the large intestine.
  • Oligosaccharides– ‘oligo’ means ‘few’ and ‘saccharide’ means sugar. These are sugars longer than disaccharides and shorter than polysaccharides.
  • Disaccharides– double sugars.
  • Monosaccharides– single sugars.
  • And Polyols– sugar alcohols.

Foods that contain FODMAPs include:

  • vegetables – asparagus, onions, garlic, legumes, beetroot, celery, corn
  • fruit – apples, pears, mangoes, watermelon, nectarines, peaches, plums
  • dairy – cow’s milk, yoghurt, soft cheese, cream, custard, ice cream
  • grains – rye and wheat-containing breads, wheat-based cereals with dried fruit, wheat pasta

As with gluten sensitivity, there is no one simple test to diagnose FODMAP sensitivity, so it may be recommended to trial removal of all or particular FODMAPs from one’s diet.

Although removal of all these foods from one’s diet is restrictive, a low-FODMAP diet is not designed to be permanent. It can be considered an elimination diet; it is highly restrictive for several weeks, and then foods are slowly reintroduced selectively to determine what in particular causes symptoms.

Practice Pearl

The long-term health effects of a adhering to a low-FODMAP diet, for relief of IBS symptoms or otherwise, are not known. Strict FODMAP avoidance is not recommended due to risks of inadequate nutrient intake and potential adverse effects from altered gut microbiota.

Vegetarian and Vegan

Vegetarian diets, also referred to as ‘plant-based’ diets, have exploded in popularity in recent years. As people continue to become more aware of the supposed health, ethical and environmental implications of their eating habits, they are increasingly turning away from animal products.

Vegetarian diets consist primarily of whole grains, vegetables, fruits, legumes, nuts and seeds. Variations include:

  • Lacto-ovo-vegetarian – includes dairy products and eggs
  • Lacto-vegetarian – includes dairy products but not eggs
  • Ovo-vegetarian – includes eggs but not dairy products
  • Vegan – does not include dairy products, eggs, or any other animal-derived products

Vegetarian diets have been associated with reductions in overall, cardiovascular and cancer mortality; long-term weight management;glycaemic control; decreased incidence and severity of high-risk conditions including obesity,hypertension,and hyperlipidaemia; and potential reversal of coronary artery disease and type 2 diabetes.

However, it is often difficult to separate the observational effects of long-term vegetarian diets with other healthy lifestyle behaviours such regular exercise, avoidance of tobacco products and moderation of alcohol intake. And importantly, eating only plant foods does not guarantee a nutritious diet, as sugar is of plant origin.

Although nutrient deficiency is a primary concern when considering a vegetarian diet, ‘well-planned vegetarian diets are appropriate for individuals during all stages of the life cycle, including pregnancy, lactation, infancy, childhood, and adolescence, and for athletes’.If ill-constructed however, vegetarian diets can combine the adverse effects of plant-based junk food with nutrient deficiencies.

Practice Pearl

Vegans must ensure they obtain sufficient B12 from fortified food or supplements, and should consider supplementing with algae-based omega-3 to obtain EPA and DHA, as the body is poor at converting ALA found in plant foods (e.g. flax seed, chia seeds, walnuts) to DHA.



The Blue Zones: dietary patterns of longevity

The ‘Blue Zones’, a term coined by National Geographic writer Dan Beuttner in 2005, are regions of the world where people live much longer than average. Buettner has identified 5 Blue Zones: Okinawa (Japan); Sardinia (Italy); Nicoya (Costa Rica); Icaria (Greece) and among the Seventh-day Adventists in Loma Linda, California (USA).

People living in these regions all share common lifestyle characteristics, such as social and family engagement, constant moderate physical activity throughout the day and not smoking(284). Although these factors as well as genetics may confound the impact of diet on their impressive longevity, their diets are remarkably similar and are based on common principles, including:

·       moderate caloric intake

·       moderate alcohol intake, mostly red wine; drink mostly water and tea

·       whole foods plant-based, semi-vegetarian diet with very limited meat, fish, and sheep or goat cheese

·       daily consumption of healthy fat sources, such as olives and nuts

·       daily consumption of legumes

·       No refined carbohydrates or added sugars

The Blue Zones demonstrate an established theme of healthy eating, relevant across generations and geographical borders that could, theoretically, have a profound influence on public nutrition and dietary guidelines.

Intermittent Fasting

Also known as ‘time-restricted feeding’,intermittent fasting alternates periods of normal food intake with prolonged periods (usually 16-48 hours) of restricted or no food intake. Different variants of fasting diets include:

  • 5:2 diet – calorie restriction for two non-consecutive days a week and unconstrained eating the other five days
  • Alternate-day fasting
  • Random meal skipping
  • Feeding window – only eat during a set ‘window of time’ every day (e.g. from 12 p.m. to 6 p.m.)

Although interest in fasting is increasing, due to reported benefits on metabolic health and physiological and molecular markers of health and longevity, clinical relevance remains low because of insufficient human data, lack of controlled trials and limited safety data.

One interesting aspect associated with fasting diets is its proposed benefits on longevity, or ‘delayed ageing’. Fasting catalyses regenerative processes in the body that reduce oxidative damage and inflammation, improve cellular protection and optimise energy metabolism.Caloric restriction has been shown to extend lifespan in multiple animal models, however it is not known whether fasting can extend human lifespan, and if it can, which variant of fasting is optimal.

Of course, the inability of most people to adhere to diets, especially restrictive diets such as fasting, somewhat limits fasting’s feasibility. However, emerging evidence regarding ‘fasting-mimicking’ diets, characterised by consumption of a low-calorie diet for five days straight each month instead of completely fasting, suggests that periodic reduction of calorie intake can also reduce markers and risk factors for aging and age-related diseases.


Practice Pearl

Further clinical research in humans is needed before the use of fasting as a health intervention can be recommended to all patients.


Different dietary approaches suit different people, and it is accepted that there is no one universal diet suitable for everyone.  However there is general agreement that a diet focussed on real foods with minimal sugars, starches and ultra-processed foods is most compatible with good health.