Protein is an essential part of the diet, and something that our bodies need for growth and repair.
Over the past decade, there has been a large shift in the way we eat, and the foods we consider healthy. It used to be considered best practice to minimise fat intake, carbohydrates weren't a dirty word, and protein wasn't given much thought outside bodybuilding circles.
These days, protein is the most celebrated of the macronutrients. High protein foods are associated with health and weight loss, and many of the more popular diets advocate carbohydrate restriction, but almost unlimited access to protein.
It seems like we can't get enough protein these days. Or can we?
Can you Overdose on Protein?
It would be very hard to overdose on protein in the traditional sense of having to be rushed to hospital to have your stomach pumped, but you can definitely overdo things and bring about some adverse and serious effects, particularly when protein is consumed excessively, long-term.
There are two factors which influence the rate at which your body can use protein. The first is the rate at which protein is digested and absorbed by the body, which is estimated to be between 1.3 and 10g protein/hour. This rate is higher for fast digesting proteins like whey, and lower for slow digesting proteins like casein, and is influenced by a number of other factors like bodyweight and the amount of digestive enzymes the body is producing (1). Research has found that the body is able to slow down the rate at which it moves food through your small intestine in response to high levels of protein (2).
It is generally accepted that the maximum amount of protein that can be absorbed from a single shake or meal is about 30g, although, for the reasons mentioned above, this can vary quite significantly.
The second factor is in the rate at which the body can use protein is the rate at which it is able to detoxify and excrete the extra nitrogen that is a by-product of protein metabolism. The body firstly, and primarily uses amino acids to form new protein, including lean muscle. Excess amino acids above and beyond the body's requirement to build proteins are used to produce energy. The liver strips these amino acids of their amino groups, which it then makes into a non-toxic waste product called urea that is excreted by the kidneys. Problems can occur when the body becomes overloaded with amino groups and can't form them into urea quickly enough, meaning that they linger in the bloodstream in the form of toxic ammonium ions. Evidence suggests that this can occur at dosages greater than about 3.5g protein/kg bodyweight, although this estimate may vary considerably due to factors including rate of digestion and liver function (1).
There has been a lot of research into protein intake and kidney disease. It has been found that a high protein intake is safe for people who do not have kidney disease, but the strain that a high dietary protein intake has the potential to cause further damage in those people who do have problems with their kidneys (3).
Protein Overdose Symptoms
The most common symptoms experienced as a result of too much protein affect the digestion. Even though the body can slow down the transit of protein through the digestive system to maximise absorption, eating a lot of protein can mean that some protein still passes through undigested. This extra protein makes its way to the large intestine, where it can be acted upon by the naturally occurring microbes, causing gas and stomach cramps. The slowed rate of digestive transit has the potential to cause constipation in some people, while the general disturbance to the overall gut microbe population can cause diarrhoea in others.
When someone develops high levels of waste products in the blood, including toxic ammonium ions, as the result of protein overconsumption, this is known as protein toxicity. The symptoms include loss of appetite, vomiting, diarrhoea, nausea, headache and lethargy. Sometimes excess ammonia can be smelled on the breath. This is a dangerous situation, as ammonia is toxic, and the ionic imbalance in the blood can interfere with the reactions of metabolism and the blood's oxygen carrying capacity.
There is a reasonable gap between the amount of protein the body actually uses, and the amount that is likely to produce toxic effects. The amount of protein that is needed to overload the system varies from person to person, so it is easiest to express these recommendations in the form of a ratio per unit bodyweight. 2-2.5g/kg/day is generally recommended as the upper healthy limit for protein intake, but this is a maximum, and is quite a bit higher than the amount recommended for muscular growth. The Australian Institute of Sport recommends up to 1.7g/kg/day for power athletes like sprinters and footballers, and for people in the early stages of resistance training, while resistance athletes who are looking to maintain, rather than grow their body mass, are advised to consume 1-1.2g/kg/day, which is significantly less than many people believe is required (4). Somebody who is sedentary, or engages in recreational exercise generally has a lower protein requirement than this, and figures as low as 0.5g/kg/day are accepted as standard.
Another common way to express protein requirement is as a percentage of total calorie intake. Anything up to 25% of total calorie intake from protein is considered safe, while protein intake that exceeds 35% of total calories has been associated with negative effects.
Like most things, protein is best consumed in moderation. A once-off protein blow out is a lot less likely to cause damage than long term overconsumption of this macronutrient. People who have, or suspect they have liver or kidney protblems, or who are diabetic, are advised to consult their doctor before commencing protein supplementation, or a high protein diet.
(1)Bilsborough S, Mann N. A review of issues of dietary protein intake in humans. Int J Sport Nutr Exerc Metab. 2006 Apr;16(2):129-52.
(2) Storr M, Sattler D, Hahn A, Schusdziarra V, Allescher HD. Endogenous CCK depresses contractile activity within the ascending myenteric reflex pathway of rat ileum. Neuropharmacology. 2003 Mar;44(4):524-32.
(3) Martin, W; Armstrong, Rodriquez (December 2005). "Dietary protein intake and renal function". Nutrition and Metabolism 2 (25).
(4) Protein - Fact Sheet. Australian Institute of Sport, June 2009. http://www.ausport.gov.au/ais/nutrition/factsheets/basics/protein_-_how_much. Accessed 12 May 2015.