Protein and fibre: the balance that protects the liver, kidneys and bowel

In recent years, high-protein diets and protein-rich ‘fit’ products, including bars, yoghurts and supplements, have become widely associated with weight loss and muscle gain. At the same time, a quieter and less discussed trend has emerged: lower consumption of fruit, vegetables, pulses and whole grains, which are the principal dietary sources of fibre.

Although this combination may seem harmless, doctors are concerned for two closely linked reasons: the strain that excessive protein intake can place on the liver and kidneys, and the role that low fibre consumption may be playing in the rising number of colorectal cancer diagnoses among people in their 20s and 30s.

According to estimates from Brazil’s National Cancer Institute (INCA), the country is expected to record around 53,810 new cases of colorectal cancer each year between 2026 and 2028. Excluding non-melanoma skin tumours, this already makes it the third most common type of cancer in the country. More alarming than the number of cases is the shifting profile of the disease: once linked almost exclusively with ageing, it is now being diagnosed increasingly often in young adults.

What is the recommended amount of protein?

One of the most common mistakes is to equate ‘more protein’ with ‘better health’, without recognising that the body has a daily limit to the amount of this nutrient it can use. For healthy adults without underlying conditions, the World Health Organization (WHO) recommends roughly 0.8 to 1 g of protein per kilogram of body weight per day for those with a sedentary lifestyle. This may rise to 1.2 to 2 g per kilogram for physically active people, depending on the type and intensity of their activity.

“Protein intake is essential for building muscle, repairing tissue and producing hormones, but the body does not store any excess as a valuable reserve, as it does with glucose, which becomes an energy store. Anything beyond daily requirements must be metabolised by the liver and removed by the kidneys as urea, which can place strain on these organs when sustained for long periods,” explains Rita Simões, a nutritionist at Hospital Nove de Julho, part of Rede Américas.

Be cautious of high-protein diets without professional guidance

Cutting carbohydrates and basing meals around meat, eggs, whey protein and supplements may speed up results in the short term, but it often brings an overlooked side effect: a sharp reduction in fibre intake, as many of these diets remove whole grains, fruit and pulses from the menu.

“Highly restrictive high-protein diets tend to be low in fibre, which can encourage constipation, alter the gut microbiota and deprive the body of one of its main protective mechanisms against bowel diseases. For healthy people without pre-existing kidney or liver disease, there is no formal contraindication to higher protein consumption, but the effects of maintaining this eating pattern for years are still not fully understood. That is why personalised guidance is always the safest approach,” says Lisa Saud, a hepatologist at Hospital Nove de Julho.

Increase your fibre intake gradually

Introducing too much fibre too quickly can cause abdominal discomfort, so it is best to increase it gradually. Swap white bread for wholemeal bread, add pulses such as beans, lentils and chickpeas to meals, keep the skins on fruit such as apples and pears whenever possible, and include oats, chia seeds and linseed in meals such as breakfast.

“Gradually increasing fibre, always alongside good water intake, allows the bowel to adapt without discomfort and can bring noticeable benefits within a few weeks, including improved bowel transit and greater fullness throughout the day,” advises Rita Simões.

Watch for signs of liver and kidney strain

Persistent tiredness, swelling, changes in urine tests and difficulty staying hydrated may indicate that protein intake exceeds what the body can process in balance, particularly when it is combined with insufficient water consumption.

“Without proper hydration, the excess urea produced during protein metabolism can promote kidney stone formation and place strain on liver function over time. These are subtle signs that often only become apparent once there is already some degree of organ impairment,” Lisa Saud stresses.

Do not ignore bowel symptoms, even when you are young

Blood in the stool, a persistent change in bowel habit, frequent cramps and unexplained weight loss should not automatically be blamed on minor dietary causes, regardless of the patient’s age. Colorectal cancer is no longer considered a disease linked solely to ageing.

“It is a type of cancer with significant potential for prevention and early detection, particularly through healthy lifestyle habits, attention to warning signs and appropriate monitoring for people at higher risk. Maintaining a fibre-rich diet with fruit, vegetables, pulses and whole grains, reducing processed meat, and not dismissing persistent symptoms, even in younger people, are measures that make a real difference to prevention,” explains Luiz Araújo, Rede Américas’ regional oncology director in Rio de Janeiro.

Balancing protein and fibre intake

Achieving a balance between protein and fibre consumption does not require radical diets or cutting out any food group entirely. What matters most is paying attention to the source and quantity of foods eaten each day, maintaining good hydration and not treating digestive symptoms as normal simply because of age.

Maintained over time, this balance helps protect the liver, kidneys and bowel in the long term. It can be crucial both for people seeking physical performance and for those who simply want to prevent disease and maintain good health.

By Bárbara Penha