The importance of protein for everyone, not just gym bunnies and elite athletes, has largely been the most dominant nutrition trend of the past two decades. Not enough of it in your diet not only reduces lean body mass, muscle strength, and function but can also cause muscle cramping, weakness, and soreness. The risks of protein deficiency apply to everyone, even those who’ve never stepped into a gym in their life.
The question of how much protein you need is a well-discussed topic (between 1.2 to 2.0g of protein, per kg of body weight, per day based on systematic reviews). But what about the other end of the spectrum? As with most nutrition questions, the answer to whether you can consume too much protein is a complex one when you start to dig deep.
Basic Upper Limits For Protein Consumption
The simple answer is that in a single dose about 0.5g per kg body weight is probably a sensible upper limit. Long term, there is no evidence of kidney or other organ damage or effect on mortality with high protein intakes. That doesn’t mean to say an excessively high protein diet isn’t bad for you. In fact, no studies have been conducted to prove it is or isn’t.
As ever, strive for a balanced diet, with sensible use of supplements if required, and you’re not going to be pushing at the boundaries of what’s good/bad for you.
The Complex Answer
The more complex answer is you need to consider protein over-consumption on two levels: the acute effect, and the chronic, or long term effect.
The Acute Effect
Taking too much protein in the short term or via a single dose can result in the protein being oxidised. Metabolically this means it is turned to glucose and/or fatty acids and contributes to your body’s fuel. Hence too much protein (contributing to excess fuel overall) is still potentially going to make you, or keep you, fatter if you’re in a calorie surplus.
The deamination of the amino acids as part of this protein catabolism also means dealing with the amino groups (ammonium ions) NH4 which may lead to the secondary effect (more on that below). An excess amount of protein is likely to be around >0.5g per kg in one go, if in a meal, but if taking supplements it is likely lower and more like anything over 0.3g per kg will be oxidised.
The Chronic Effect
Or what we might call the long term effect. In this case, the effect of too much protein is an increased demand on processing the nitrogen (NH4 ammonium) produced through protein deamination. This will need to be excreted through the kidneys via urea production. In addition, there may be alterations in your acid-base balance as a byproduct, which could impact bone resorption, in order to buffer this shift in homeostasis (i.e by liberating calcium from the bones).
However, the evidence for this is not entirely clear. Protein consumption may actually be slightly protective to bones1. There is no strong or consistent evidence of kidney damage or morbidity/mortality with high protein intakes.
But then again, it is difficult to longitudinally prove, as no one has performed longitudinal studies (over several years) where you can satisfactorily attribute effects directly to protein alone. Nor are there any studies looking at long term protein supplementation over a number of years.
1 ‘Dietary protein and bone health: a systematic review and meta-analysis‘ Darling et al, 2009