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High Protein and Fat Oxidation

What the study found was that fat oxidation wasn’t suppressed in obese people who ate meals high in protein.

Most fat-loss diets recommend a higher protein intake, and for good reason. More protein yields a greater thermogenic effect, meaning calories are converted into heat and not stored as bodyfat. Protein boosts calorie burn-off by 20 to 30 percent, compared to 5 to 10 percent for carbs and 0 to 3 percent for fat. In addition, despite the fact that some of the amino acids in dietary protein can be converted into glucose, protein intake is not associated with the large release of insulin that occurs when you eat high-glycemic, or rapidly absorbed, carbohydrate. Because a high insulin count is linked to both retention and synthesis of bodyfat, the control of insulin also aids fat loss.

Fat oxidation, or burning, is impaired in people who have a lot of bodyfat for a number of reasons. For one thing, they often have a blunted sympathetic hormonal response that affects hormones involved in fat mobilization, such as epinephrine and norepinephrine. Most of the supplements touted to help reduce bodyfat modulate the blunted sympathetic response and may improve the blunted calorie burn.

A recent study examined the effect on fat oxidation in obese people who ate high-protein meals that included carbohydrates of various glycemic-index contents.1 The diets consisted of the following:

1) Control diet—14 percent protein, glycemic index 65 percent

2) High protein, high GI—33 percent protein, GI 74

3) High protein, low GI—35 percent protein, 45 GI

The glycemic index is a measure of how rapidly the body absorbs glucose. The highest GI is 100, often represented by white bread or glucose itself. It was originally developed for diabetics, since rapid carbohydrate absorption can influence their insulin requirements, but it’s also relevant to nondiabetics. The GI system applies only to single foods and is less applicable to mixed meals. For example, a meal containing carbohydrate and protein tends to result in a lower GI number, since protein and fat considerably blunt carb uptake.

What the study found was that fat oxidation wasn’t suppressed in obese people who ate meals high in protein. Surprisingly, the GI number of the meal didn’t have any effect on the boost in fat oxidation they got from eating protein. The effect appears specific to those with higher bodyfat, however. When people who have low bodyfat eat a high protein meal, fat use is spared.

The study underscores the importance of a high protein intake for anyone who wants to lose excess bodyfat. Not only does it aid fat oxidation, but it also spares lean mass at the expense of bodyfat. That, in turn, maintains the resting metabolic rate, which would otherwise drop with a loss of lean mass—something that occurs in those who eat low-calorie diets with insufficient or poor protein, such as plant foods, and who don’t engage in resistance exercise. In those instances, the loss of vital lean mass guarantees a regain of lost bodyfat. That explains the 95 percent recidivism of most diets, particularly the ones that feature both insufficient protein intake and no call for weight-training exercise.

Another way that a high-protein diet aids fat loss is by inducing increased satiety. Eating a meal high in protein provides the greatest feeling of fullness, followed by carbs, then fat. A recent study compared two different amounts of casein as the sole source of protein in a diet that also contained 55 percent carbs.2

Twelve men and 12 women stayed for 36 hours in a respiration chamber. Their diets contained either 25 percent or 10 percent of energy derived from casein. That was the only difference between the two diets; total calorie intake was identical because the fat content of the diets was manipulated.

A slowly absorbed milk protein, casein releases amino acids over a seven-hour period. The 25 percent casein diet resulted in a 2.6 percent higher 24-hour total energy expenditure than the 10 percent casein diet. Those in the higher casein group also showed a higher sleeping metabolic rate and a more positive protein balance than the 10 percent group, and satiety was 33 percent higher. That resulted in a whopping 41 percent less hunger than in the 10 percent group, which explains why it’s far easier to stick with a higher-protein diet than a lower-protein, higher-carb, lowfat diet.

A previous study, which used the same measures of casein during breakfast, found that the 25 percent casein meal was more satiating than the 10 percent version, an outcome attributed to prolonged elevation of plasma amino acids. The lessened hunger in the later study was also attributed to the prolonged release of amino acids from casein.

Although both diets contained the same level of carbohydrate (55 percent), the plasma glucose levels were lower in the 25 percent diet, likely related to slower gastric emptying. Those getting the 25 percent casein diet had lower insulin counts, which aids in fat reduction. The 25 percent diet produced lower amounts of intestinal hormones that modify appetite, again thought to occur because casein delayed gastric emptying.


1 Batterham, M., et al. (2008). High protein meals may benefit fat oxidation and energy expenditure in individuals with higher bodyfat. Nut Dietetics. 65:246-52.

2 Hochstenbach-Waelen, A., et al. (2009). Comparison of two diets with either 25 percent or 10 percent of energy as casein on energy expenditure, substrate balance and appetite profile. Am J Clin Nutr.89(3): 831-838.

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