High-protein, low-carb diets have gained in popularity as they achieve the best weight loss results. Initally, such diets were rejected by nutrition experts and dieticians in favour of the conventional low-fat, high-starch approach.
If you do the maths, low-fat diets should have a lower calorie content as fat has more calories than carbohydrate, and high-protein diets usually contain more fat. However, the principle of high-protein, low-carb diets has now gained some acceptance among academics, and sparked a huge debate. Could it be that calories from particular types of food affect the body differently?
A calorie, by its simplest definition, is a unit of energy. In the popular press and in the labelling of food products, a food calorie actually refers to a kilocalorie, or 1,000 calories. That is, 1 food cal equals 1 kcal. According to thermodynamics, energy can neither be created nor destroyed, so a calorie is of course a calorie. How is it possible, then, that calories from different nutrients produce different results?
Metabolisable energy
There are a number of factors that influence the impact of different types of food on the energy available to the body. When energy from the combustion of food is measured in experimental conditions in a test tube, it is known as absolute energy, but the energy actually available to the body after the food has been eaten is known as the metabolisable energy. The two amounts are unlikely to be the same, because not all of that food will be digested and absorbed - some will be lost from the body in urine and faeces.
Of the three energy-providing nutrients - namely protein, carbohydrate and fat - it is protein that has the highest absolute energy compared with metabolisiable energy. This is because a number of the products of protein breakdown are used by the body for the excretion of excess nitrogen; these substances can account for up to 23% of the total energy of the food. The energy available from different protein foods depends on the chemical structure of the protein; more energy is lost from the consumption of meat protein compared with vegetable protein.
Atwater Factors
At the end of the nineteenth century, the scientist Wilbur Atwater devised "coefficients of availability" for protein, carbohydrate and fat. These are derived from the heats of combustion of protein, carbohydrate and fat, corrected for losses in digestion, absorption and excretion of nitrogenous waste products. For his experiments Atwater used foods that were typical of the American diet at that time, such as beef, butter, biscuits and baked beans, and calculated a single value for each of the energy-providing nutrients regardless of which food it was contained within.
These values are 4 calories per gram for protein, 9 calories per gram for fat and 4 calories per gram for carbohydrates (25g = 1oz). These so-called Atwater factors are still used today to asses the energy value of a single food.
The Fibre Factor
In the early 1970s it was recognised that separate factors were needed to take into account the division of total carbohydrate into available carbohydrate and fibre. Available carbohydrate was given a new value of 3.75 calories per gram. More recent research has shown large differences in the digestibility of foods, which are not accounted for by the Atwater factros. A classic example of this is almonds: studies have shown that about 20% of the calories in almonds are not available for digestion or absorption because of the nut's fibrous cell walls. Nuts are often rejected as too calorific by people wishing to lower their caloric intake, yet their low digestibility is a significant reason (among others) that the daily inclusion of a handful of nuts can actually help with weight loss. Consuming more dietary fibre results in the excretion not just of the fibre but also of fat and nitrogenous substances, which have an energy value, and it decreases the amount of time it takes for foods to pass through the digestive system. This "decreased transit time" gives food less of an opportunity to be fully digested and absorbed, so more is lost in the faeces. This means that applying the Atwater factors to a high-fibre diet could overestimate the metabolisable energy by 5-10% (approximately 100-200 calories day).
The bottom line: Differences in the energy lost during metabolism and fermentation can also influence the weight-loss effects of our diets. A greater amount of energy is used to fuel metabolism of protein, for example, while some of the energy in fermentable carbohydrate is lost through the production of gas. This may ex[lain why weight loss is greater on a high-protein, high-fibre diet compared with a different diet of equal calories.
Of course, the increased energy expenditure associated with increased protein intake still does not violate the laws of thermodynamics. It does, however, suggest that diets that induce a difference in energy expenditure can introduce a difference in energy balance and thus a difference in weight loss.
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