Let’s start from the point that if excess protein gets converted into body fat, then most of the power and strength athletes like weightlifters, bodybuilders etc. would be obese. In fact, it is the other way around. They are among the most ripped and muscular people in the world.
However, the question becomes even more complicated when you hear statements in certain books and articles, which says that, in case of sufficient intake of carbohydrate and energy, excess protein intake will convert into body fat.
Let’s try and get things straight. The answer is relatively simple but reasoning may be a bit complicated. Actually NO, it is extremely difficult for the body to convert protein to fats, even if it would then the process is undesirable by the body.
Our basic source of energy is carbohydrate and we all know this since ages. What happens if I start starving my body off the carbohydrates which is the primary source for energy? The simple answer is that my body will start using stored fat as energy.
Fats are stored in the form of triglycerides in the body. As the name suggests triglycerides means a glycerol molecule attached to 3 fatty acid molecules. When we use fat for energy the glycerol molecule breaks away from fatty acids molecule and is converted to glucose by the liver.
However, the body needs to fuel the brain and raise the blood sugar which is done by breaking down the proteins into amino acids and sending them to the liver to convert to sugar by a process called Gluconeogenesis. But that’s only when you are going to be very low on carbs. So, Protein is first metabolized into amino acids and ammonia. The leftover carbon compound is converted into glucose, which your body uses for energy. If your cells have enough glucose, and there is no space left to store it as glycogen in your muscles or liver, the excess glucose is converted into fat and stored.
The general statement I have been hearing is that an average person can absorb only a limited amount of protein in one sitting (app. 30gm) and the rest is converted to fats. Firstly, the doubt of how much protein human beings absorb, and the myth of 30gm at a time, has already been cleared (later in the book). In short, the answer is that no one knows, as there are multiple factors governing how much protein a body absorbs in a sitting. A marathon runner, and a heavy weight bodybuilder cannot have the same protein absorption capacity.
The second question which comes out is that if you cannot absorb proteins then how the body converts it into fats as for conversion into fats the body needs to absorb the protein first.
In actual case the protein gets absorbed in the small intestines. If we take a large protein meal your stomach simply keeps the food for a longer time, slowly releasing the amino acids into the small intestines form where it goes to the blood circulation.
Acc. to a 2007 study in the International Journal of Sports Nutrition & Exercise Metabolism, by a research team led by A.M. Gabriella , with a balanced meal approximately 90% of the dietary amino acids are absorbed by the gut.
Examine.com owner & researcher Kamal Patel suggests that , the small intestines are able to absorb and hold onto a large amount of amino acids; waiting to release them until the body needs them, and can recycle some amino acids. Due to the aforementioned ability of the small intestines to ‘hold’ onto protein, they are considered a ‘free amino acid pool’ that the body can draw amino acids from on an as-needed basis.
Now over half of the amino acids go to the liver from the blood stream. The liver acts as the central processing unit and is responsible for the synthesis and metabolism of amino acids according to the needs of the body.
In the body, the pathways do exist which are responsible for the conversion of protein to fats but it is highly unlikely that the process occurs because it would take the body huge amount of protein for a long time which is unlikely to be taken even by pro athletes. Even if the weight increases it will be due to excess of calories from other sources apart from the calories from protein. In fact, most of the new diet gurus prefer a low carbohydrate high protein diet for faster fat loss.
The problem is generally indirect. It’s very rare that people have protein in its purest form. Protein foods are always with some accompanying carbs or fats. That’s the reason you can gain weight, if you consume excess protein. So, it means can you eat pure protein and be fit. The answer is no. You would then suffer from a problem called protein starvation.
In a 2012 study in the Journal of American Medical Association, Dr. George Bray & team, evaluated the effects of overconsumption of low, normal, and high protein diets on weight gain, energy expenditure, and body composition, in 25 male subjects, which were given diets containing 5% of energy from protein (low protein), 15% (normal protein), or 25% (high protein). Then, the subjects were force fed 140% (1000kcal/day) of their maintenance for calorie needs for 8 weeks.
Subjects saw that, after 8-weeks all the subjects gained weight, however, overeating produced significantly less weight gain in the low protein diet group compared with the normal protein diet group or the high protein diet group. However, the weight gain shown in the high protein groups was increase in lean muscle mass, not body fat.
Researchers suggested that, weight gain when eating a low protein diet (5% of energy from protein) was blunted compared with weight gain when eating a normal protein diet (15% of energy from protein) with the same number of extra calories. Calories alone, however, contributed to the increase in body fat. In contrast, protein contributed to the changes in energy expenditure and lean body mass, but not to the increase in body fat.
This means, eating an extra 1000kcal/day for 8 weeks, in combination with high protein intakes didn’t lead to any additional body fat gain, rather it led to an increase in lean body mass.
In a 2014 study in the journal Nutrition & Metabolism, researchers Dominik Pesta & Varman Samuel , “people on high-protein diets are advised to choose their source of protein very carefully (i.e. emphasize the use of high-quality protein sources from plant origin). Many protein-rich foods of animal origin (e.g. red meats, eggs and dairy products) also contain high levels of saturated fats and cholesterol. This may put consumers of high-protein diets at higher risk for heart disease, hyperlipidaemia and hypercholesterolemia… Finally, all excess protein will eventually be converted to glucose (via gluconeogenesis) or ketone bodies… When energy demand is low, these metabolites will be stored as glycogen and fat, which is undesirable if weight loss is the goal. Along these lines, weight loss can only be achieved by establishing a negative calorie balance, though this may be more tenable on a high-protein diet.”
In a 2017 study in the International Journal of Exercise Science, researchers Jose Antonio & Alex Leaf , examined the current literature on overfeeding that reports changes in body composition and formulate recommendations for athletes looking to increase lean body mass. Researchers concluded by saying that, “that overfeeding on carbohydrate and/or fat results in body composition alterations that are different than overfeeding on protein. It is commonly believed that 3,500 kcal is equivalent to 0.45 kg (1 pound) of fat and that changing energy balance in accordance with this will produce predictable changes in body weight. However, the overfeeding literature to date does not support this assertion. Dietary protein appears to have a protective effect against fat gain during times of energy surplus, especially when combined with resistance training. Therefore, the evidence suggests that dietary protein may be the key macronutrient in terms of promoting positive changes in body composition.”
However, this does not mean, you start consuming insanely high amount of protein, and forget the nutrition balance altogether. In a 2014 study in the Journal of the International Society of Sports Nutrition, researcher Jose Antonio & team, determined the effects of a very high protein diet (4.4g/kg/d) on body composition in thirty resistance-trained men and women. They found that, consuming 5.5 times the recommended daily allowance of protein saw, no significant change over time on body weight, fat mass, fat free mass, or percent body fat.
A 2016 study in the Journal of Nutrition & Metabolism, by Dr. Jose Antonio & team, determined the effects of a high protein diet over a one-year period. Fourteen healthy resistance-trained men completed the study. Subjects consumed their habitual or normal diet for 2 months and 4 months and alternated that with a higher protein diet (>3g/kg/d) for 2 months and 4 months. Thus, on average, each subject was on their normal diet for 6 months and a higher protein diet for 6 months. Researchers found that, in resistance-trained men that consumed a high protein diet for one year, there were no harmful effects on measures of blood lipids as well as liver and kidney function. In addition, despite the total increase in energy intake during the high protein phase, subjects did not experience an increase in fat mass.