9 Things You Need To Know About Body Composition

The results are in…

The International Society of Sports Nutrition (ISSN) is a non-profit academic society dedicated to promoting the science and application of evidence-based sports nutrition and supplementation. Recently, the group released a position statement on diets and body composition.

You can read the 19-page paper on your own, or simply skim this summary for the takeaway points.

Here’s What You Need To Know

  1. There are a lot of different diet types and eating styles
  2. All body composition assessment methods have strengths and weaknesses
  3. Diets focused on fat loss do so through a caloric deficit and the higher the starting body fat the more aggressive the caloric deficit may be used while slower rates of weight loss can help preserve muscle mass in leaner individuals
  4. Diets focused on muscle gain do so through a caloric surplus with diet composition and amount of the surplus as well as training status of the athlete influencing the nature of the gains
  5. Most dietary approaches are similarly effective for improving body composition
  6. Dietary protein intake above current recommendations may result in improved body composition and may be needed to preserve muscle mass when in a calorie deficit
  7. Intermittent caloric restriction has been shown to have no significant advantage over a daily calorie restriction in terms of body composition changes
  8. The long-term success of a diet depends on compliance and adjusting for adaptive thermogenesis
  9. We need more research on women and older adults as well as more scientific studies on variations of feeding frequency and macronutrient distribution at various energy balances combined with training as well as behavioral and lifestyle modification strategies related to weight management


A “diet” is the sum of energy and nutrients obtained from foods and beverages consumed regularly by a person.

The paper looked at very-low- and low-energy diets (VLED and LED), low-fat diets (LFD), low-carbohydrate diets (LCD), ketogenic diets (KD), high-protein diets (HPD), and intermittent fasting (IF). Most branded diets (Atkins, Paleo, Zone, Ornish, etc.) fall under one of these categories.

For reference, the macronutrient ratio of the typical American diet is roughly 15% protein, 50% carbohydrate, 35% fat.

Macronutrient ratios for an athletic population, such as bodybuilding where body composition is a key factor, is typically closer to 30% protein, 40% carbohydrate, 30% fat but can vary greatly due to nature of the chosen sport and individual preference.

The paper focused on body composition, not health or disease treatment. Although, improved body composition often leads to better health outcomes.


Methods common in clinical practice and sports/fitness settings include underwater weighing, air displacement (BOD POD), skinfold thickness (calipers), and bioelectrical impedance analysis (BIA). These are the most commonly used methods for adults due to their relatively low cost, ease of operation, and non-invasiveness. Dual energy X-ray (DXA or DEXA) absorptiometry is a more accurate method, but is still subject to error.

It has been determined that there is no single method which is best for assessment of the athlete population, but rather whichever method is the most practical for the needs of the assessment.

diet types



Commonly defined as 800-1200 kcal/day (LED – can be up to 1800 kcal/day) or 400-800 kcal/day (VLED) for rapid weight loss (2-5.5 lbs/wk) while preserving lean mass as much as possible. VLED diets are often liquid and commercially prepared, fortified with all needed vitamins and minerals and a typical macronutrient distribution of 70-100g protein, 15g fat, and 30-80g carbohydrates. Resistance training improves results. Most often used in obese subjects rather than healthy and athletic populations. Safe duration of this type of diet is debated but is commonly used for 8-12 weeks in clinical practice. Adverse reactions: hair loss, cold intolerance, fatigue, constipation, muscle cramps, headache, lightheadedness, death.


Defined as providing 20-35% of total calories from fat, with 10-35% protein and 45-65% carbohydrate – why it is also referred to as a high-carbohydrate diet. Systematic review has shown reducing the proportion of calories from dietary fat modestly but consistently reduced body weight, body fat, and waist circumference – maybe due to de facto reduction of total energy intake inducing body fat reduction over time. Based on the fact that fat is the most energy-dense macronutrient. Higher energy density of higher-fat diets results in greater weight gain and/or less weight loss, possibly due to increased palatability of energy-dense foods.

Very-low-fat diets (VLFD) are defined as providing 10-20% of total calories from fat. Research on these diets is limited, mainly made up of examining the health effects of vegetarian and vegan diets that aggressively minimize fat intake. Research shows these diets are effective for weight loss but body composition data is lacking.


There is no set amount which characterizes what is considered “low-carb”. Any diet containing less than 45% of total energy from carbohydrates or diets having less than 200g carbohydrates may both be considered LCD. It can also be hard to distinguish between KD and LCD. LCD generally allow more carbohydrates than KD.

Meta-analyses comparing LCD with LFD have yielded mixed results due to the lack of universal agreement on what is considered “low-carb” – a diet with 45% of total energy from carbohydrates may not show significant differences while a diet with <20% does.. Again, most studies have been done on obese subjects. LCD may be successful due to higher protein intake compared to control diets rather than the diets themselves.


Technically a subtype of LCD, KD is a beast all of its own. While “low-carb” is a subjective gray area, KD is objectively defined by its ability to measurably increase ketone bodies in the blood (“nutritional ketosis”). This state can be reached by fasting or by restricting carbohydrate intake to ~50g per day or 10% of total energy intake. Protein is kept moderate (1.2-1.5 g/kg) with the rest coming from fat (60-80%+).

Ketosis is not the same as KETOACIDOSIS – usually seen in type-1 diabetics who are not receiving enough exogenous insulin. The primary circulating ketone is b-hydroxybutyrate and is what is commonly measured when testing for ketosis in the blood.

In terms of KD for body composition (vs. medical necessity, such as in seizure disorders), the proposed advantage for fat loss is based largely on insulin-mediated inhibition of lipolysis and (presumably) enhanced fat oxidation – the rate of which plateaus within the first week of a KD. Some research is proposing that it is actually the higher protein amount of LCD rather than the lower carbohydrate intake itself, that is the crucial factor in promoting greater weight loss under dieting conditions. Studies have shown that, when protein and total energy intake is equal, there is no advantage of KD over non-KD conditions in terms of fat loss.

Any advantage of a KD over non-KD diet for fat loss would be the potential of appetite regulation due to increased satiety through suppression of ghrelin hormone production (the “fullness” hormone). Fat is satiating and therefore has the potential to suppress hunger independent of protein intake – also a hunger-satiating macronutrient. Hunger suppression may also be due to the presence of ketone bodies in the blood.

KD may be more ergolytic (enhancing athletic performance and/or exercise capacity) for endurance athletes than for strength/power athletes, as shown by impairment in high-intensity work output in high-fat/carbohydrate-restricted diets.


Generally defined as intakes reaching or exceeding 25% of total energy intake or 1.2-1.6 g/kg. Protein intakes above the Recommended Dietary Allowance (RDA) of 0.8 g/kg repeatedly reign supreme for preserving muscle mass and reducing fat mass.

How much is optimal? In one study, protein intake triple the RDA (2.4 g/kg) did not outperform double the RDA (1.6 g/kg) in terms of preserving lean mass – yet, in another study, triple the RDA (combined with dieting conditions, HIIT sprints, and resistance training) resulted in lean mass gains and fat loss while double the RDA resulted in preservation of lean mass and fat loss to a lesser extent. A recent systematic review suggests that intakes of 2.3-3.1 g/kg (1-1.4 g/lb) lean body mass (rather than total body weight) is an appropriate intake for lean, trained individuals under dieting conditions.

“Protein pacing” – spreading protein intake throughout the day – has been shown to be superior for improving body composition under dieting conditions.

Protein has the highest thermic effect of food – meaning your body has to burn more energy to digest it than it does either fat or carbohydrates. It is also metabolically expensive – which is why lean mass tends to be lost under hypocaloric conditions due to the body trying to reserve energy for more vital life processes. Protein is also very satiating, more so than either fat or carbohydrates, which may make dieting more bearable in terms of hunger.

No adverse effects have been found with long-term high protein intakes in healthy individuals in terms of measured clinical markers (blood lipids and metabolic panel).


The three main types of IF are alternate-day fasting (ADF), whole-day fasting (WDF), and time-restricted feeding (TRF).

The most extensively studied is ADF which typically involves fasting for 24 hours alternated with feeding for 24 hours. Subjects do not completely compensate for the fasting days’ calorie deficit on feeding days, thus weight and fat loss does occur. A recent study showed that ADF showed similar results to daily caloric restriction on body composition.

WDF involves fasting for one or two days throughout the week of otherwise maintenance level intake in order to achieve a calorie deficit, although not all WDF studies completely abstain from energy during their fasting days – perhaps limiting to 600-700 kcal/day.

TRF involves eating only within a designated feeding window during the day, typically 4-8 hours of feeding following 16-20 hours of fasting. Most of the research done on this type of IF is on subjects who fast for Ramadan – religious fasting in which food and fluid is avoided between sunrise and sunset for about 1 month.

A recent systematic review found that, overall, both IF and daily caloric restriction resulted in the same outcomes in terms of weight loss and body composition change, but that IF was found to be better at suppressing hunger – possibly due to ketone production during the fasting periods.




This is the idea that weight loss or gain is determined by a caloric deficit or surplus, regardless of diet composition or macronutrient distribution. While true, it does not account for composition of the weight loss (fat vs. lean mass) nor the other factors that are involves in eating behavior.

Thermic effect of food (TEF) is just one component of energy expenditure – the “out” side of the energy balance equation. TEF makes up about 8-15% of total daily energy expenditure (TDEE), the largest component of which is resting energy expenditure (REE) – which is sometimes referred to as basal or resting metabolic rate (BMR or RMR – although RMR is technically 3-10% higher than BMR due to testing conditions). This is the energy cost to sustain basic life processes at rest. BMR account for 60-70% of TDEE.

The rest of one’s TDEE is made up of non-exercise activity thermogenesis (NEAT) and exercise activity thermogenesis (EAT). NEAT is composed of expenditure from one’s occupation, leisure activities, activities of daily living, and spontaneous activity such as fidgeting. NEAT and EAT can vary greatly within and across individuals.

The concept of “calories in vs. calories out” for weight loss has led to the “eat less, move more” solution to our national/international obesity problem. However, the solution has not proven to be so simple.


The body likes homeostasis (staying the same). This is why, when dieting, the body up-regulates hunger and down-regulates energy expenditure in an attempt not to lose weight.

Adapative thermogenesis is a term used to describe the gray area where losses in metabolic tissue cannot simply explain reduced energy expenditure – for example, losing > 10% total body weight results in a ~20-25% decrease in TDEE, 10-15% higher than would be expected. This has been found mostly due to decreased non-resting energy expenditure (NEAT and EAT) and possible due to increased sympathetic drive and decreased thyroid activity.

This reduced energy expenditure continues for quite some time – TDEE below expected levels may persist for over a year after dieting, although higher protein intake and resistance training do help.


There is substantial interindividual variability in energetic response to overfeeding. Some appear to resist weight/fat gain – suggesting a concurrent increase in TDEE with increased intake. Others show a greater efficiency for energy storage. NEAT usually increases in overfeeding situations, resulting in a greater TDEE and a difficulty gaining weight for some – oftentimes unbeknownst to the subject themselves due to the spontaneous/unconscious nature of NEAT.

Partitioning of this excess energy (fat vs. lean mass) and what determines it is an understudied area.

It can be argued that a calorie surplus is not needed for muscle gain since lean mass gains have been reported under dieting conditions, although possibly at a slower rate. Therefore, if lean mass gain is the goal, results will be optimized by being in a sustained caloric surplus to aid anabolic processes and support increased training demands.


People are bad at reporting accurately. There can be huge gaps between perceived compliance and actual compliance and not every study can be done under perfectly controlled laboratory conditions. People tend to under-report their intake and over-report their physical activity. We need more studies – especially in trained individuals, women, and older adults as most diet research focuses on those who are overweight or obese.


In terms of order of importance or impact on body composition:

total daily macronutrition (and micronutrition) > nutrient timing > supplementation

  • Want to lose weight? It does not matter the method, as long as you are in a calorie deficit.
  • Want to improve body composition (read: look lean, toned, shredded, etc.)? Combine a calorie deficit with a higher protein intake, preferably spread throughout the day, and make sure resistance training is included for best results.
Aragon et al. Journal of the International Society of Sports Nutrition (2017) 14:16
DOI 10.1186/s12970-0174-y

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