CAN YOU EAT TOO MUCH PROTEIN?

Yes, Short answer: YES. But we don’t yet know how much is too much. Not many people have heard of protein toxicity (1, 2). Even many dietitians and nutritionists are not aware of it. It was the cause of a condition given the name rabbit starvation suffered by unlucky pioneer Americans exploring the far west. At times, they found themselves having nothing to eat except very lean rabbits. Within a few days, they would feel nauseous. If they kept eating them, they would eventually find themselves comatose …and dead within 3 weeks.

At certain times of the year, wild rabbits had no fat on their bodies. They were starving too, and their fat stores had become severely depleted. There are no carbs worth mentioning in animal flesh, so all the Calories were in the form of protein. When humans burn protein as a source of energy, they must deaminate the amino acids (remove the amino group) and convert the ammonia into urea. The urea is excreted in urine as a waste product. But these metabolic pathways have a finite limit and become saturated. If we eat excessive protein, eventually the ammonia accumulates in the blood, and we hit the upper ceiling of what our bodies can handle.

We now know the upper limit on protein is about 40% of energy intake or about 200 g of pure protein per day. Most people never get anywhere near it. We’d need to eat 800 g of steak each day, although it’s not unheard of. In fact, pioneer Australians often ate mutton (sheep meat) for breakfast, lunch and dinner. They were lucky, however, because the sheep had plenty of fat on them too. Eating either fat and/or carbohydrate with protein keeps us under the ceiling. Most people are happy consuming 15-20% energy as protein.

There are advantages to eating higher amounts of protein. In fact, the CSIRO Total WellBeing Diet has about 35% energy from protein or 100 g protein on a 1200 Calorie (5000 kJ) weight loss diet (3). This diet is designed to make you feel very satisfied (satiated) after each meal and less likely to crave additional food. If the protein’s from good quality sources you’ll automatically consume micronutrients like iron, zinc and vitamin A in sufficient quantities too.

If you are a senior (like me!), a high protein diet will also help to maintain muscle mass as you age (4). One reason that men and women become frail as they age is because they aren’t eating sufficient protein of high enough quality each day. Humans need all essential amino acids each day to build and maintain new tissue.

But there’s another side to every story, especially when it comes to nutrition (moderation in all things). In a secondary analysis of the PREVIEW Study led by Ruixin Zhu in Beijing (5), participants randomised to the higher protein, lower carbohydrate arm were less likely to achieve remission from pre-diabetes compared to those consuming more typical amounts. PREVIEW was the world’s largest multinational type 2 diabetes prevention study, enrolling over 2300 individuals who were overweight or obese and met the criteria for ‘pre-diabetes’ (a high fasting glucose or high 2 hour reading after a glucose tolerance test). This is a surprising result but there were many strengths to PREVIEW because volunteers were followed for 3 years and participants received intensive instruction from dietitians and exercise physiologists. Protein intake was monitored by measuring nitrogen in urine.

Nonetheless we should be cautious in interpreting these findings because it is the first study of its kind. More research is needed, particularly of the mechanisms. We don’t know if there was a difference in the source of protein in those who achieved remission versus those who didn’t. Or whether there were other factors, such as saturated vs unsaturated fat intake. But we should also be aware that some amino acids called branched chain amino acids have been associated with insulin resistance, one of the underlying defects in many people who develop type 2 diabetes.

As in many other aspects of life, we need to weigh up the risk and the benefits. As Nicole states in this month’s Good Carbs Food Facts, you can’t go wrong with lentils as long as you cook them properly. I, for one, am very fond of total meal replacement meals for breakfast, especially when my weekend indulgences have been somewhat excessive (humph!). The brand I enjoy has a low GI and provides ~20 g protein and ~20 g carbohydrate along with all the vitamins and minerals needed for a 200 Calorie (850 kJ) meal.

Read more:

1. Cordain and colleagues. Origins and evolution of the Western diet: health implications for the 21st century. The American Journal of Clinical Nutrition,2005.
2. Noli and Avery. Protein poisoning and coastal subsistence. Journal of Archaeological Science,1988.
3. CSIRO. Total Wellbeing Diet.
4. Walrand and colleagues. Functional impact of high protein intake on healthy elderly people. Am J Physiol Endocrinol Metab. 2008.
5. Zhu and colleagues. Long-term effects of dietary protein and carbohydrate quality on prediabetes remission: a secondary analysis of the PREVIEW randomized multinational diabetes prevention trial. Under review. 2025.

Emeritus professor Jennie Brand-Miller held a Personal Chair in Human Nutrition in the Charles Perkins Centre and the School of Life and Environmental Sciences, at the University of Sydney until she retired in December 2022. She is recognised around the world for her work on carbohydrates and the glycemic index (or GI) of foods, with over 300 scientific publications. Her books about the glycemic index have been bestsellers and made the GI a household word.