In the eyes of the general/lay public, the world of weight-loss diets can appear to be a massive, insurmountable maze. It’s therefore crucial for professionals in the allied fields of health & fitness to understand the characteristics of each major type of diet in order to best educate patients, clients, students, friends, and family. As science marches on, we occasionally discover things about the diets that make them effective or worthy of consideration for practice. We also discover aspects of the diet types that refute their hype and lore. My focus here is weight & fat loss. Let’s dive in.
Very-low-calorie diets (VLEDs)
How it works: VLEDs are characterized by their provision of 400-800 kcal/day, while LEDs (low carbohydrates diet) provide 800-1200 kcal/day. Note that LEDs have also been given a more liberal definition, providing 800-1800 kcal. VLEDs typically are in liquid form and commercially prepared.
The aim of the diet is to induce rapid weight loss (1.0-2.5 kg/week) while preserving as much lean mass as possible. VLEDs are designed to replace all regular food intake, and therefore shouldn’t be confused with meal replacement products intended to replace one or two meals per day. As such, they’re fortified with the full spectrum of essential micronutrients. The protein content of VLEDs typically ranges ~70-100 g/day, fat can be up to 15 g, and carbohydrate ranging around 30-80 g. A protein-sparing modified fast (PSMF) can be considered the higher-protein variant of a VLED, with intakes ranging 1.2-1.5 g/kg of ideal body weight per day.
Benefits: Interestingly, even at protein intakes as low as 50 g/day, the proportion of lean mass loss from VLEDs has been reported to be 25% of total weight loss, with 75% as fat loss.3
Resistance training while consuming 800 kcal has been seen to result in the complete preservation of lean mass. Muscle hypertrophy has also been seen with an 800-kcal diet with resistance training. Keep in mind that this occurred in untrained, obese subjects, and their results can’t necessarily be extrapolated to lean trained subjects. In obese populations,VLEDs are a potentially powerful tool for treating obesity since a greater initial weight loss is associated with greater long-term success in weight loss maintenance.
Downside: However, a meta-analysis found that VLEDs did not result in greater long-term (1 year or more) weight loss than LEDs. There’s an ongoing debate over how long VLEDs can safely be sustained, but 8-12 weeks before transitioning to less severe caloric restriction is typical in clinical practice. As a caveat to folks with fabulous manes, hair loss was reported to be the most common complaint of prolonged VLED use.
How it works: Low-fat (LFDs) and very low-fat diets have been defined subjectively as having an upper limit of 30% and 15% of total energy from fat, respectively. The premise of dietary fat reduction for weight loss is to target the most energy-dense macronutrient in order to impose hypocaloric(eating fewer calories than you burn) conditions.
Benefits: Over the long-term, diets with lower energy density have not consistently yielded greater weight loss than energy restriction alone.
Downside: There is a substantial body of research examining the effects of low-fat, low-carbohydrate, and other diets, but weight loss differences, on the whole, are small (often less than 1-2 kg), thus lacking clinical or practical significance.
Similar to low-fat diets, low-carbohydrate diets (LCDs) as a broad category have a subjective definition. As such, there is no universal agreement on what concretely or quantitatively characterizes the diet. The Acceptable Macronutrient Distribution Ranges (AMDRs) set by the Institute of Medicine’s Food and Nutrition Board lists 45-65% of total energy as the appropriate carbohydrate intake for adults. Therefore, it can be contended that intakes below 45% fall short of the authoritative guidelines. LCDs have been repeatedly defined as containing as much as 40% of total energy from carbohydrate.
How it works: In absolute rather than proportional terms, LCDs have been defined as having less than 200 g of carbohydrate.
Benefits: Many studies indicate that low-carb diets can aid weight loss and may be more effective than conventional low-fat diets.
Downside: Meta-analyses comparing the weight loss effects of LFDs with LCDs have provided mixed results.
How it works: The proposed weight/fat loss advantage of carbohydrate reduction beyond a mere reduction in total energy is based largely on insulin-mediated inhibition of lipolysis, and presumably fat oxidation. It involves drastically reducing carbohydrate intake and replacing it with fat. This reduction in carbs puts your body into a metabolic state called ketosis. Despite a substantial number of weight loss studies comparing varying carbohydrate amounts, very few have controlled both protein and total energy intake. There thus has been long-standing uncertainty of whether the purported “metabolic advantage” of ketogenic diets is due to higher protein or lower carbohydrate content. After all, higher protein intake has inherent advantages in terms of thermic effect, satiety, and lean mass preservation.
Downside: The limited number of controlled, isocaloric comparisons where protein is equated does not show a weight/fat loss advantage of LCDs or ketogenic diets.
How it works: High-protein diets within the context of hypocaloric conditions and carbohydrate restriction have been defined as containing 30-60% of total energy from protein.
They have also been defined as intakes ranging from 1.2-1.6g/kg.
Benefits: One study showed that protein consumed at double the RDA (1.6 g/kg) repeatedly outperformed the RDA (0.8 g/kg) for preserving lean mass and reducing fat mass. However, another study found that triple the RDA (2.4 g/kg) did not better preserve lean mass than double the RDA.
More recently, a study found that in dieting conditions involving high-intensity interval sprints and resistance training, protein intake at 2.4 g/kg caused lean mass gain and fat loss while 1.2 g/kg resulted in the preservation of lean mass and less fat loss.
Of the macronutrients, protein has the highest thermic effect; it is the most energetically expensive in terms of processing within the body. 20-30% of its energy content is expended on its metabolism and/or storage, whereas 5-10% and 0-3% is spent on that of carbohydrate and fat, respectively. Given these qualities, it is unsurprising that higher protein intakes have been seen to preserve resting energy expenditure while dieting. In addition, protein is the most satiating macronutrient, followed by carbohydrates, and fat being the least. With just one exception, a succession of recent meta-analyses supports the benefit of higher protein intakes for reducing body weight, body fat, and waist circumference, and preserving lean mass in an energy deficit. A study found that protein consumed at 2.3-3.1 g/kg fat-free mass (FFM) was appropriate for lean, resistance-trained athletes. This is perhaps the first time in the literature that protein dosing requirements were reported on the basis of FFM rather than total body weight.
These findings suggest that the known thermic, satiating, and FFM-preserving effects of dietary protein might be amplified in trained subjects undergoing progressive resistance exercise
How it works: IF can be divided into three subclasses:
Alternate-day fasting (ADF)
Whole-day fasting (WDF)
Time-restricted feeding (TRF).
The most extensively studied IF variant is ADF, which typically involves a 24-hour fasting period alternated with a 24- hour feeding period. The complete compensatory intake on the feeding days does not occur, and thus the total weight and fat loss occur with ADF.
Benefits: Lean mass-retention has been reported in ADF, although lean mass loss has also been observed. WDF involves one to two 24-hour fasting periods through the week of otherwise maintenance intake in order to achieve the deficit.
Downside: Although WDF has been consistently effective for weight loss, a study finds no difference in body weight or body fat reduction between the WDF group and controls when the weekly energy deficit was equated over a 6-month period.
TRF typically involves a fasting period of 16-20 hours and a feeding period of 4-8 hours daily. The most studied form of TRF is Ramadan fasting, which involves approximately 1 month of complete fasting (both food and fluid) from sunrise to sunset.
Note: Significant weight loss occurs, but this typically includes a reduction in lean mass as well as fat mass. One recently conducted largest systematic review of IF research, comparing the effects of intermittent energy restriction (IER) to continuous energy restriction (CER). They found that overall, the two diet types resulted in “apparently equivalent outcomes” in terms of body weight reduction and body composition change.
And there they are, the major diet types. Their common mechanism of action is sustaining an energy deficit over time in order to achieve weight/fat loss—not via the almighty suppression of insulin, or other such fairy tales. And this is not to say that hormones are not involved. It’s a false dichotomy to pit hormones against calories when explaining weight/fat loss since they are inextricable. Hormones influence appetite & eating behavior (among other things), while caloric balance influences the production of said hormones. It’s an integral relationship, but it nevertheless boils down to the persistence of hypocaloric conditions if long-term fat loss is the goal. Of course, there are optimal and sub-optimal ways to set up macro nutrition for fat loss, and perhaps the most important element aside from the deficit itself is sufficient protein, next in line would be individualizing the proportion of carbohydrate and fat to suit the individual’s preference, tolerance, and athletic demands. All of these constituents must comprise a diet whose food choices are personalized to suit individual tastes. Ultimately, an unsustainable diet is a useless diet.
“Moreover, the difference in weight loss among these diets is only 1-2 kg or less, which appears to be of little clinical significance. Thus, overweight and obese people can choose many different weight-loss diets on the basis of their personal preferences.”
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