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Does a long-term, high-protein diet have harmful effects?

The first one-year longitudinal study in resistance-trained males helps to fill the evidence gap for high-protein consumption, demonstrating no harmful effects to key health markers.

Jose Antonio
Jose Antonio Professor, PhD, Associate Professor and Director of the Exercise and Sports Science Program, Department of Health and Human Performance, Nova Southeastern University, USA

Is a high-protein diet harmful?
Around a century ago, a rat study explored the effects of high protein intake and reported effects ranging from at least some to very severe damage to kidneys. The study triggered concerns about whether, in fact, diets rich in protein could have negative effects on human health.

However, a more recent rat study, involving a 30-day period of consuming six human-equivalent 20-gram doses of protein daily, showed no adverse effect on blood results or histological markers of liver or kidney health. Instead, the results implied that high-protein intake may actually improve liver health.

Some of the work my colleagues and I have completed in the recent past have backed these results, indicating that consuming a high-protein diet in the short-term has no harmful effects on clinical blood lipids and metabolic measures.

Useful insights
Most investigations of the effects of high protein diets have been based around a relatively short time period – or they’ve involved a significant change in exercise patterns – neither of which reflect the realities of athletes or, for that matter, many sports hobbyists.

Now, however, the details of a one-year, high-protein study conducted in resistance-trained athletes by my own lab have been revealed. And the results are good news for those worried about the long-term impacts on their kidney or liver from high-protein diets.

How high?
But first, what amount of protein constitutes a high-protein diet? As might be expected with such an arbitrary description, opinions among the scientific community differ. Some argue that protein intakes exceeding the recommended dietary allowance (RDA) of 0.80g/kg/day fit the bill. Others argue that intakes greater than 15-16 percent of total energy intake – as high as 35 percent – are considered as ‘high-protein’. My own experience suggests that high-protein diets should exceed 2.0 g/kg/day. And we’ve used the latter as a broad guideline for the study in question.

One-year study
We took 14 males already engaging in resistance training through a randomized crossover trial for a year. We asked them to consume their habitual diet and a higher-protein diet for two months and four months, respectively, over a six-month period. The additional protein was obtained either from their own choice of protein source or a whey protein powder.

We asked them to continue their own strength and conditioning program and supported them in maintaining a training log. To track eating habits, subjects kept a food diary using a smartphone app, recording their intake three days a week for an entire year.

For each person, we conducted blood lipid analyses and comprehensive metabolic panel (CMP) tests at baseline and throughout the course of the year. Height and total body weight were measured, and each subject’s body composition was assessed.

Following the year-long investigation, the study revealed no significant differences between subjects consuming a normal diet versus a high-protein one (2.51–3.32 g/kg/day).

The blood lipid and CMP tests showed no negative effects and, in fact, the study showed little or no effect on body composition, despite consumption of more protein and total calories. Neither body weight, fat mass nor lean body mass changed. And that result is likely to surprise nutrition experts who, naturally enough, expect increased energy intake to lead to weight gain.

In fact, some of our other, shorter-term trials have produced similar findings. In one study, where subjects consumed protein in amounts three to four times higher than the RDA, we found a similar fat-free mass (FFM) increase for both the normal and high-protein groups. Yet the high-protein group lost more fat mass compared with the normal protein group even though they consumed approximately 400 kcals more per day. To put it in simple terms: Adding a lot more protein to the diet doesn’t seem to make one fatter!

Confidence booster
While this latest study uses a relatively small sample size of resistance-trained male participants, the results add to a growing body of evidence that consuming high-protein diets to meet nutritional needs isn’t harmful – and may even promote health for a significant part of the population.


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This blog is a medium for all industry experts to share knowledge about and viewpoints on whey protein and lactose - and, in particular, their documented or potential benefits to the world.

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