Though definitions of LCHF diets differ, the following three-tiered definition will be used in this paper.[2]

  • Moderate carbohydrate diet (26–45% of daily kcal)
  • LCHF diet (<26% of total energy intake or <130 g CHO/day)
  • Very LCHF (ketogenic) diet (20–50 g CHO/day or <10% of daily kcal of 2000 kcal/day diet)

Reduced carbohydrate diets are those that have carbohydrate intakes below the Dietary Guidelines for Americans (DGA) recommendations (of 45–65% of total energy intake). However, we define LCHF diets as those that restrict carbohydrate intake to 130 g/day or less. Very LCHF (ketogenic) diets may induce ketosis in some people. Though individual responses vary, ketosis usually occurs in people who restrict their carbohydrate intake to below 20–50 g/day with some degree of protein restriction.

Since the carbohydrate content of the diet is significantly reduced, the relative proportion of energy derived from protein and fat will increase. In practice, however, LCHF diets typically produce a reduction in hunger,[18] with the result that the individual’s total caloric consumption will usually decrease on the LCHF diet, sometimes significantly. Therefore, even though the relative contribution of fat to dietary energy intake may increase, the absolute fat intake may not. As a result, the term ‘high fat’ diet may be misleading. Hence, the term low-carbohydrate healthy fat is probably more appropriate.

LCHF diets are defined by what is ‘not’ eaten, instead of what is eaten. Although the details may vary depending on the specific type of LCHF diets (Atkins, Banting, Paleo, South Beach, etc), in each of these examples, there is a consistent focus on eating unprocessed food, consisting primarily of cruciferous and green leafy vegetables, raw nuts and seeds, eggs, fish, unprocessed animal meats, dairy products and natural plant oils and fats from avocados, coconuts and olives.

LCHF diets, even if ketogenic, are not ‘no’ carbohydrate diets. Rather, all encourage a high intake of green leafy vegetables, cruciferous vegetables and other non-starchy vegetables with moderate intakes of berries. Table 1 provides a list of foods recommended on a ‘Banting’ diet,[19] a popular LCHF eating plan. LCHF eating plans promote meals such as omelettes, salads and animal protein such as steak, salmon or chicken with vegetables.

Obesity is the fifth leading risk factor for premature mortality.[20]The prescription of the LCHF diet as one strategy for weight loss has been known since at least 1860,[21] as this particular eating plan was promoted as the preferred treatment for obesity in Sir William Osler’s textbook from the early 1900s.[22] The publication of Dr Atkin’s Diet Revolution in 1972[1] later resurrected interest in the LCHF diet in the USA and elsewhere. Four decades later, numerous randomised clinical trials (RCTs) and systematic reviews now allow a critical evaluation of the safety and efficacy of LCHF diets for weight reduction. This information was not available to either Osler or Atkins, making a review of this new evidence particularly opportune now.

Repeatedly, LCHF diets have performed as well or better than LFHC diets for weight loss in overweight or obese adults.[9,14–17]Bazzano et al‘s[23] recent 1-year trial randomised 148 obese men and women without T2DM or cardiovascular disease to an ad libitum LFHC (<30% fat, <7% saturated fat, 55% carbohydrates) or an LCHF (<40 g carbohydrates/day) diet. After 12 months, the LCHF diet group had lost significantly (p=0.002) more weight (−5.3 kg) than the LFHC group (−1.8 kg), and experienced a 1.3% decrease in % body fat compared with a 0.3% gain in the LFHC group. This equivalent or superior performance of LCHF diets over LFHC diets for weight loss has also been established in randomised trials in adolescents,[24–26] and in adults with,[27–29] or without[30–35] T2DM. A recent reanalysis (‘The universities of Stellenbosch/Cape Town low-carbohydrate diet review: Mistake or mischief?’35a) shows that lower carbohydrate diets outperform isoenergetic high carbohydrate ‘balanced’ diets for weight loss of the systematic analysis comparing weight loss and the response of other health markers with isocaloric high and moderate (35%) carbohydrate diets[7] found that weight loss was slightly but significantly greater on the moderate carbohydrate diet, even though the diets were isoenergetic.

The efficacy of LCHF diets extends beyond weight loss. In a 24-week trial of 84 patients with T2DM, Westman et al[36] found that a ketogenic LCHF diet produced significantly greater weight loss than an energy-restricted (500 kcal/day below RMR), low glycaemic (GI) diet (−11.1 vs −6.9 kg). The ketogenic LCHF diet also significantly increased blood HDL-C concentrations and reduced blood HbA1c values. Additionally, more patients on the LCHF diet were able to reduce or cease their use of diabetic medications.

The 1-year A to Z study[37] randomised 311 overweight/obese premenopausal women to the Atkins (<20 g carbohydrates/day induction, <50 g carbohydrates/day maintenance, ad libitum energy intake), Zone (40% carbohydrate, 30% protein, 30% fat, energy restricted), Ornish (<10% fat, ad libitum) or LEARN (55–60% carbohydrate, 10% saturated fat, energy restricted) diets. After 12 months, the mean weight loss in the LCHF Atkins group was −4.7 kg, compared with −1.6 kg on the Zone Diet, −2.2 kg on the Ornish and −2.6 kg on the LEARN groups. Further, blood HDL-C and TG concentrations were significantly improved in the Atkins group compared with all other diet groups, at least initially.

Shai et al[38] randomised 322 moderately obese participants to an energy restricted Mediterranean diet (MED), energy-restricted LFHC diet or an ad libitum LCHF diet. The most significant weight loss occurred in the LCHF group at 6 months, even though this was the only diet eaten ad libitum—that is, it was not energy-restricted. At the end of the 24-month trial, weight loss was −4.7 kg for LCHF, −4.4 for MED and −2.9 kg for LFHC participants. Weight loss on the LCHF diet is greatest early in these trials when participants comply most rigorously to the carbohydrate restrictions, as also occurred at 6 months in the A to Z trial.[37]Subsequent weight gain occurs as participants begin to ingest more carbohydrates daily, so-called ‘carbohydrate creep’, seen in the 6–12-month period in that trial and between months 6 and 24 in the study of Shai et al.[38] Obviously, this reintroduction of greater carbohydrate ingestion increases energy intake with consequent weight regain. Thus, weight regain is not necessarily the fault of the LCHF diet; rather it is the consequence of its discontinuation.

This applies to all dietary interventions, where weight regain occurs with decreased adherence. Conversely, motivated free-living individuals who maintain diet adherence, sometimes to address serious personal medical conditions, self-report weight losses an order of magnitude greater than the rather modest weight losses measured in clinical trials. LCHF diets are no exception, where some have reported effortless weight losses of greater than −80 kg following adoption of the LCHF diet.[39] Indeed, a recently reported low-cost lifestyle intervention study[40] based on a high-fat diet recorded weight losses in excess of 12% in 372 participants; exceeding typical losses of 3–7% in other well-funded studies in which the chosen diets were higher in carbohydrate.

In summary, while some studies show no difference in weight losses between LFHC and LCHF diets,[7,8,11]others report that LCHF diets are more effective.[9,14–17,35a] Notably, all the tested diets are effective at inducing at least short-term weight loss, usually followed by some weight regain as adherence diminishes. However, LCHF diets clearly perform at least as well as do any other dietary approach, even in trials in which energy intake on the LCHF diet is unrestricted (ad libitum).

Mechanisms for Successful Weight Loss on the LCHF Eating Plan

Two main mechanisms have been proposed to explain how LCHF diets produce weight loss, despite an increased consumption of energy-dense ‘fatty’ foods:

  1. increased satiety, allowing a lower energy intake without hunger and
  2. a specific metabolic advantage.

Increased Satiety Allowing a Lower Energy Intake Without Hunger

A recent systematic review compared weight loss between participants on ‘LCHF diets’ and ‘low fat balanced diets’[7] but excluded all trials that were not isoenergetic. However, in doing so, they excluded trials that demonstrate the advantage of LCHF diets in producing greater satiety and a subsequently reduced energy intake. Indeed, this was the unique biological advantage that Banting,[21] Ebstein[41] and Atkins[1] all originally ascribed to the LCHF diet on the basis of their clinical observations. Although the original study did not find any differences in weight loss between the different diets, a reanalysis35a of the same data found a small but significantly great weight loss on the lower carbohydrate diet.

As an illustration, Table 2 lists a collection of studies which show that participants on LCHF diets given unrestricted access to eating foods ad libitum do not necessarily consume more calories than participants assigned to LFHC diets, even when the latter are required consciously to ‘energy restrict’ their caloric intake according to their experimental design.

These results are supported by lower measures of perceived hunger in some participants eating LCHF diets.[34] Greater satiety on LCHF diets in persons responding to the diet may result from a number of mechanisms: (1) some LCHF diets may have increased protein intake, which promotes satiety;[43] (2) ketogenic LCHF diets may also suppress appetite,[44] though the exact mechanisms remain uncertain;[45]and (3) participants may experience fewer instances of rebound hypoglycaemia, a common cause of hunger in those eating high-carbohydrate foods,[46] especially if they are IR. Regardless of the exact mechanism, it is notable that LCHF diets can achieve an energy deficit and subsequent weight loss with little hunger and without conscious energy restriction, as originally noted by Stock and Yudkin.[18]

Postulated ‘Metabolic Advantage’ of LCHF Diets

The superior satiating effects of LCHF diets may not fully explain weight losses from LCHF diets. For example, some trials have shown greater weight loss for LCHF diets, despite higher energy intakes than prescribed LFHC diets.[3,25,31,47] Similarly, although some trials find no differences,[48] some isoenergetic trials still find greater weight losses on LCHF diets.[33,35a,49] Meta-analyses report similarly variable outcomes.[7,50]

Although contentious, it has been suggested that LCHF diets may provide a metabolic ‘advantage’ favouring greater weight loss, despite the ingestion of an equal number of calories. This metabolic advantage could be attributed to a number of mechanisms, including: (1) increased thermogenic effects of protein intake, (2) greater protein turnover for gluconeogenesis and (3) loss of energy through excretion of ketones in sweat or urine.[51,52]

LCHF diets increase reliance on fat oxidation for energy production, especially during exercise,[53,54] as shown by increased blood ketone concentrations and with reductions in respiratory quotient and blood insulin concentrations.[53–55] This state of increased lipolysis with reduced lipogenesis contributes to a metabolic milieu theoretically favouring fat loss. This effect is dependent on reduced blood insulin concentrations, uniquely produced by the LCHF diet. However, this remains a contentious idea, with recent metabolic ward evidence suggesting that, at least in the short term (5 days), there is not a preferential fat-loss effect of LCHF diets