A high protein diet can strain your kidneys, increase your risk of kidney stones, raise calcium loss through urine, and shift hormonal signals linked to cancer risk in middle-aged adults. The recommended daily protein intake is 0.8 grams per kilogram of body weight, so a 150-pound person needs roughly 55 grams per day. Many popular diets push well beyond that, sometimes doubling or tripling the recommendation, and that’s where problems begin to surface.
What Counts as “High Protein”
The international Recommended Dietary Allowance sets protein at 0.8 grams per kilogram of body weight for adults. For a 180-pound person, that works out to about 65 grams a day. Adults over 65 have slightly higher needs, around 1.0 to 1.2 grams per kilogram, because aging muscle requires more protein to maintain itself. People recovering from serious illness or injury may need up to 2.0 grams per kilogram temporarily.
When people follow ketogenic, carnivore, or aggressive bodybuilding diets, they often consume 1.5 to 2.5 grams per kilogram daily as a baseline, not because of illness. That sustained excess is what drives most of the adverse effects below.
Kidney Strain and Filtration Pressure
Every time you eat protein, your kidneys ramp up their filtration rate to clear the byproducts. This is called hyperfiltration, and it happens within hours of a protein-heavy meal. The amino acid alanine, common in meat, directly increases both the filtration rate and blood flow through the kidneys. In healthy kidneys, this temporary surge isn’t dangerous. But when it happens meal after meal, day after day, it places chronic pressure on the kidney’s filtering units.
Protein metabolism produces urea as its primary waste product. A high protein diet increases urea production substantially, and the kidneys must work harder to excrete it. Under normal conditions, about 10 grams of urea nitrogen pass through the kidneys daily. Higher protein loads push that number up, and when fluid intake doesn’t keep pace, urea concentrates in the blood. This is one reason people on high protein diets often feel thirsty and need significantly more water than usual. For people with already reduced kidney function, even mild reductions they may not know about, the added workload can accelerate decline.
Kidney Stone Risk
High protein intake creates a chemical environment in your urine that favors stone formation through several overlapping pathways. Protein metabolism generates acid, which lowers urine pH. That acidity reduces citrate in the urine (citrate normally prevents crystals from clumping together) while simultaneously increasing both calcium and uric acid excretion. The result is a higher concentration of stone-forming minerals in more acidic, less protected urine. Calcium oxalate stones, the most common type, are particularly sensitive to this combination of low pH and high calcium.
Calcium Loss and Bone Health
One of the more counterintuitive effects of eating a lot of protein is that it pulls calcium out of your body. In controlled studies, participants on a high protein diet excreted roughly 182 milligrams of calcium per day in their urine, compared to 134 milligrams on a normal diet. That’s about a 36% increase.
Researchers initially assumed this happened because the acid load from protein forced the body to pull calcium from bone to neutralize it. But when scientists gave participants a compound that fully neutralized the dietary acid, calcium losses barely changed. This means the calcium drain from high protein diets operates through a separate mechanism that isn’t fully understood yet, and it isn’t easily fixed by simply balancing your body’s pH. Over months or years, consistently elevated calcium loss could weaken bones, particularly in postmenopausal women or others already at risk for osteoporosis.
Ammonia Buildup and Liver Load
When your body breaks down amino acids from protein, the process releases ammonia, a compound that is toxic to cells, especially brain cells. Normally your liver converts ammonia into urea, a harmless waste product, through a process called the urea cycle. A healthy liver handles this efficiently at normal protein intakes.
But the system has limits. Modeling studies show that increasing protein intake by 72% above normal raises blood ammonia levels by 59%. For most healthy people, the liver compensates. The real danger emerges when liver function is compromised. In simulated liver cirrhosis, blood ammonia levels rose by 41 to 130% depending on how much protein was consumed. Even people who carry certain genetic variants affecting ammonia processing (roughly one copy of a less-functional gene for a key enzyme) can see ammonia levels more than triple on a high protein diet. At high enough concentrations, ammonia impairs brain cell function and viability. Lab experiments showed that elevated ammonia reduced the survival of nerve cells by 14%.
Most people don’t know whether they have subtle liver issues or carry these genetic variants. That’s what makes very high protein intake a gamble for some individuals who feel perfectly healthy.
Heart Disease Risk Depends on the Source
Not all protein carries the same cardiovascular risk. The adverse effects on heart health come primarily from where the protein originates, not just how much you eat. Replacing one serving of red meat (about 3 ounces) with plant protein sources reduces coronary heart disease risk by 13 to 30%, depending on the substitution. Swapping in nuts cuts risk by 30%, fish by 24%, and poultry by 19%.
Red meat and high-fat dairy, which are staples in many high protein diets, raise total cholesterol and LDL cholesterol. By contrast, plant-heavy dietary patterns can lower LDL cholesterol dramatically. Diets built around plant proteins like soy, nuts, and legumes have achieved LDL reductions of 28 to 35% in short-term studies, comparable to the effect of first-generation cholesterol-lowering medications. Even replacing animal protein almost entirely with soy protein significantly reduces cholesterol in both healthy people and those with elevated levels.
So a high protein diet built around chicken breast and lentils looks very different, from a heart disease perspective, than one built around bacon and steak.
Cancer-Related Hormonal Shifts
High protein intake raises levels of a growth hormone called IGF-1, and that increase appears to have serious consequences for people between 50 and 65. A large study tracking over 6,300 adults for 18 years found that those in this age range who ate a high protein diet had a 75% increase in overall mortality and a four-fold increase in cancer and diabetes mortality compared to those eating less protein.
The connection runs through IGF-1, which promotes cell growth, including the growth of tumors. For every 10 ng/ml increase in IGF-1, cancer mortality risk rose an additional 9% among high protein consumers aged 50 to 65. Animal studies confirmed this: high protein intake accelerated the development of both breast and skin cancers through the same IGF-1 pathway.
Interestingly, this relationship flips after age 65. In older adults, higher protein intake and moderate IGF-1 levels appear protective, reducing frailty and potentially lowering mortality. IGF-1 naturally declines with age, and too little of it in older adults may signal malnourishment. This age-dependent reversal means that protein recommendations aren’t one-size-fits-all across a lifetime.
Digestive Problems on Low-Carb, High-Protein Diets
High protein diets don’t inherently cause constipation, but the way most people follow them does. Analysis of national nutrition survey data found that protein intake increased constipation risk only in people who also ate very few carbohydrates. For every 10-gram increase in protein, constipation risk rose by 8% in the low-carb group. In people eating moderate carbohydrates alongside their protein, the same increase in protein actually lowered constipation risk by 6%.
The explanation is straightforward: carbohydrate-containing foods are the primary source of dietary fiber. When you cut carbs aggressively to make room for more protein, you lose the fruits, whole grains, and legumes that keep your digestive system moving. The protein itself isn’t the problem. The displacement of fiber-rich foods is.
Increased Fluid Needs
Processing protein waste requires water. Your kidneys need adequate fluid to dissolve and flush urea, and at low urine flow rates (below about 2 milliliters per minute), they reabsorb up to 60% of filtered urea back into the blood instead of excreting it. Higher urine flow, above 2 milliliters per minute, drops reabsorption to 40%, allowing more waste to leave the body. Dehydration or low fluid intake on a high protein diet means your kidneys recirculate more waste, blood urea nitrogen climbs, and you may experience headaches, fatigue, and dark urine. People on high protein diets need to deliberately increase their water intake to keep waste clearance efficient.