Well, a recent study ties this question to age. The study reports that middle-aged folk (ages 50-65) who had high-protein diets (20% or more of daily calorie intake is from protein) were four times more likely to die from cancer or diabetes than those with low-protein diets (less than 10% daily calorie intake is from protein). Even moderate protein diets (10-19% of daily calorie intake is from protein) increased mortality risk. These risks substantially decreased on plant-based protein diets as opposed to animal-based protein diets.
The opposite was true for folk over the age of 65: high-protein consumption significantly reduced cancer mortality rates. The only shared result between the two groups was that low-protein diets decreased diabetes risk.
So what this study says is that even though high-protein diets are good for weight loss – has certainly been for me – they may harm health in the long term. Consumption should be adjusted in various stages of life, and plant-based protein sources are best for overall health.
But the study is not definitive. There are many factors like lifestyle choices, red meat vs white meat, that could have influenced the results. I eat a high-protein and high plant-based diet. My main source of protein is oily fish which has tremendous health benefits. I also live a very healthy lifestyle and I am in good health. The associations found are important but more research is needed to prove causation between protein consumption and mortality.