There is no universally accepted definition of a low-carbohydrate diet (LCD) and so it can be very confusing for the public (and researchers) to know exactly what is being spoken about when people use terms like ‘LCHF’ and low-carb.
However for ‘fat-adaptation’ to occur optimally Westman and colleagues suggest that an LCD contains between 50 and 150g of carbohydrate (but that this level may still not result in a level of ‘nutritional ketosis’) (2).
A meta-analysis of low-carbohydrate diets by Hu and colleagues (3) in the American Journal of Epidemiology included a broader range of studies; including as ‘low-carbohydrate’ diets any that contained less than 45% of daily calories from carbohydrate. This suggests that any diet containing less than the commonly recommended carbohydrate intake is by nature a low carbohydrate one. A more thorough definition may be provided within a systematic review by Wheeler and colleagues in Diabetes Care which categorises LCDs as such:
- very-low-carbohydrate diet: 21–70 g/day of carbohydrate
- moderately low–carbohydrate diet: 30 to <40% of kcal as carbohydrate
- moderate-carbohydrate diet: 40–65% of kcal as carbohydrate
- high-carbohydrate diet: >65% of kcal as carbohydrate
The lack of a definitive, universally accepted categorisation of what are very low carbohydrate and low carbohydrate diets provides significant challenges to researchers seeking to synthesise and analyse the data, due to a lack of homogeneity between and within studies (5). This may cause confusion when evaluating the evidence for-and-against the use of low-carbohydrate diets when compared to standard best-practice diet guidelines.
High protein, low carbohydrate diets
Due to simple calorie displacement, isocaloric low-carbohydrate diets require an increase in one or both of the other macronutrients (protein or fat) to fulfil caloric requirements, although calorie restricted lower-carbohydrate diets may simply reduce carbohydrate and thereby increase the relative proportions of fat and protein.
High-protein, low-carbohydrate (HPLC) diets appear to enhance weight-loss with greater loss of body-fat and reduced loss of lean body mass due to a number of factors that are yet to be fully elucidated including (but perhaps not limited to): increased satiety, increased thermogenesis, muscle sparing and enhanced glycaemic control (6).
High-protein, low-carbohydrate diets have been studied for weight-loss and body composition with superior results demonstrated versus high-carbohydrate diets. Layman et al. compared two diets with similar fat content (~50g), one containing 68g protein and the other 125g (with the balance in both cases from carbohydrate). Participants in the higher protein group lost significantly more fat, retained more lean tissue, and reduced triacylglycerol (TAG) and increased satiety more than the lower protein group.
Piatti and colleagues (7) investigated the effects of two hypocaloric diets (800-kcal) in normal, glucose tolerant women (n=25), one containing 45% protein (35% CHO and 20% fat) and one containing 20% protein (60% CHO, 20% fat) demonstrating similar weight loss effects in both groups, but retention of fat-free mass only in the higher protein diet.
Similarly in obese and hyperinsulinaemic women a higher protein intake (27% vs. 16%) and similar fat intakes encouraged similar weight and fat loss with retention of lean mass only observed in the higher protein group, along with reduced TAG and improved glycaemic control versus the lower protein (higher carbohydrate) group (8).
Noakes and others have demonstrated that there may be further nutritional benefits (aside from weight-loss which was demonstrated) resulting from higher protein diets—with greater fat loss in the obese exhibiting high triacylglycerol TAG) counts, reduced TAG and improved B12 status compared to higher carbohydrate diets (9).
The positive effects of higher protein intake on anthropometry in particular can be explained due to several factors, including increased satiety and thermogenesis when compared to equivalent amounts of either carbohydrates or fat (10). Increased energy expenditure (EE) is required for the formation of additional glucose from amino acids (and other substrates such as lactate and glycerol) (11), and there is a higher thermic effect of feeding (TEF) associated with protein ingestion as compared to either carbohydrate or fat (12; 13; 14) which is not solely explained by the metabolic demands of increased gluconeogenesis, and may also be accounted for by increased protein accretion in tissue, which requires greater energy expenditure than for example storage of lipids within adipose tissue, which has been demonstrated in infants (15) and within the voluminous data showing increased protein accretion and retention with higher protein intakes, and other factors.
Higher protein intake is also considered to be more satiating than the carbohydrate it is displacing. A 2004 review by Halton and Hu (16) found there to be convincing evidence that a higher protein intake increases thermogenesis and satiety compared to diets of a lower protein content.
Low carbohydrate, high fat (LCHF) diets
Low carbohydrate, high fat diets (often with low-to-moderate protein) have now demonstrated sufficient evidence to be considered a therapeutic option for the adjunctive treatment of: fatty liver disease (17); type 1 diabetes (18); type 2 diabetes (19); cancer (20); and cognitive impairment (21).
LCHF diets are also likely to be superior to low fat diets for improving several markers of cardiovascular health (22; 23; 24) with the possible exception of low density lipoprotein (LDL). However the HDL:triglyceride ratio appears to be more favourably impacted with an LCHF diet in comparison with a higher carbohydrate diet (25), and lipid sub-fractions (including large particle LDL) may be increased favourably with an LCHF diet (26).
LCHF diets may provide ‘metabolic advantage’ of greater retention of lean mass, with greater fat-loss when compared to higher carbohydrate diets providing the same amount of calories. This has been demonstrated in short term studies since the 1960s (27). This does not contravene the laws of energy conservation (first law of thermodynamics) as there are inefficiencies and efficiencies within systems based on macronutrient inputs and the thermic effect of foods of varying macronutrient contents may provide for a net calorie loss (28; 29).
Short term studies suggest that carbohydrate restriction—irrespective of what is substituted, has the greatest effect on weight-loss. For example a 2005 randomised controlled trial by Luscombe-Marsh et al. compared a low carbohydrate high protein diet to a low carbohydrate, high fat (standard protein) diet—both yielding similar results for weight loss with little difference in other parameters (bone turnover, inflammation and calcium excretion) (30) and lower-carbohydrate and higher-fat diets (not ‘LCHF’ per se) have demonstrated improved post-prandial glycaemic responses and reduced insulin when compared to higher-protein, isocaloric diets (31).
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