The short answer

In healthy adults, a high-protein diet can support fullness, muscle repair, and weight management. But consistently overshooting your needs—especially with animal-heavy, low-fiber diets—can increase kidney stone risk, strain fluid balance, alter your gut microbiome, and displace other important nutrients. Those with kidney or liver disease, kidney-stone history, or certain metabolic conditions should be particularly cautious.

How much protein is “too much”?

There’s no official “upper tolerable intake level” for protein, but there are well-established ranges:

• RDA (minimum to prevent deficiency): about 0.8 g/kg/day for most adults.
• Common optimal range for active adults and older adults: ~1.2–1.6 g/kg/day, spread across meals.
• Upper end used by athletes: up to ~2.0–2.2 g/kg/day appears safe for healthy people over the long term.
• Very high intakes: regularly exceeding ~2.5–3.0 g/kg/day provides little added benefit for most and may raise the likelihood of side effects for some individuals.

Context matters: the source of protein (plant vs. animal), overall diet quality (fiber, fruits/vegetables), hydration, and individual health status strongly influence risk.

What your body actually does with excess protein

1. Digestion and absorption: Dietary proteins are broken to amino acids and absorbed.
2. Use it or lose it: Amino acids repair tissues, build enzymes and hormones, and support immune function. Surplus amino acids can’t be stored as-is.
3. Deamination: The nitrogen is removed; your liver converts it to urea, which your kidneys excrete in urine.
4. Energy or fat: The remaining carbon skeletons are burned for energy or converted to glucose or fat, depending on your energy balance and carb/fat intake.

This processing raises metabolic work (the “thermic effect”) but also increases the load on your liver’s urea cycle and your kidneys’ excretion of urea, water, and certain acids.

Short-term effects of eating a lot of protein

• Increased thirst and peeing more often: Excreting urea pulls water with it; dehydration risk rises if you don’t drink enough.
• Digestive changes: High-protein diets that skimp on fiber can cause constipation; abrupt increases (especially from shakes/bars) can also trigger bloating or diarrhea in some people.
• Bad breath or “ammonia” smell: More common with very high protein and very low carb intake; poorly controlled liver disease can worsen this.
• Performance plateaus: Skewing protein too high can crowd out carbohydrates, which are crucial for high-intensity training and recovery.

Potential longer-term risks of chronically high protein

• Kidney stone risk (especially from animal protein): Higher acid load and increases in urinary calcium and uric acid, paired with lower citrate, can promote stones—particularly if you’re underhydrated and low on fruits/vegetables.
• Fluid and electrolyte stress: Persistently high urea excretion raises water needs; inadequate intake can compound fatigue, cramps, and headaches.
• Gut microbiome shifts: Diets heavy in animal protein and low in fiber can increase protein fermentation byproducts (e.g., p-cresol) and trimethylamine-N-oxide (TMAO), which are linked to adverse cardiometabolic profiles. Adequate fiber and plant foods temper these effects.
• Cardiometabolic risk depends on source: Replacing refined carbs with plant proteins (legumes, soy, nuts, seeds) generally improves blood lipids and outcomes; loading up on processed and red meats is associated with higher cardiovascular and all-cause mortality in observational studies.
• Nutrient displacement: Overemphasis on protein can push out fiber, phytonutrients, and essential fatty acids, raising long-term risk for GI and cardiometabolic issues.
• IGF-1 and life stage: In midlife, very high animal protein may raise IGF‑1 and correlate with higher cancer mortality in some cohorts; in older adults, higher protein helps preserve muscle and may lower mortality. Context and overall diet quality matter.

In people with existing kidney or liver disease, excess protein can accelerate complications; these individuals require personalized targets from a clinician or dietitian.

Myths versus nuance

• “High protein will destroy healthy kidneys.” In healthy adults, higher protein increases kidney filtration and urea excretion but hasn’t been shown to cause chronic kidney disease. Risk concentrates in susceptible groups and in the presence of other risk factors (e.g., dehydration, high animal protein, stone history).
• “Protein leaches calcium from bones.” With adequate calcium and fruits/vegetables, protein supports bone health. Problems arise when high animal protein is paired with low calcium and low produce intake.
• “More is always better for muscle.” Muscle protein synthesis plateaus per meal around 20–40 g for most adults; beyond total daily needs, extra protein offers diminishing returns.

Who should be cautious

• People with chronic kidney disease, kidney-stone history, or significant liver disease.
• Individuals on very low-carb diets who are not compensating with adequate hydration and electrolytes.
• Those with eating patterns low in fiber and produce or dominated by processed/charred meats.
• Anyone with urea cycle disorders or ammonia-handling issues (rare; requires medical supervision).

Practical guidelines for getting it right

• Set a sensible target: Most active adults do well at 1.2–1.6 g/kg/day; older adults and those in heavy training may benefit from the upper end. Rarely is >2.2 g/kg/day necessary.
• Distribute across the day: Aim for 20–40 g protein per meal and 10–20 g at snacks to support muscle protein synthesis.
• Prioritize quality and variety: Mix plant and animal sources—legumes, soy, dairy or fortified alternatives, eggs, fish, poultry, nuts, and seeds.
• Keep fiber high: At least 25–38 g/day from whole grains, fruits, vegetables, and pulses to support the microbiome and bowel regularity.
• Hydrate deliberately: Increase fluids as protein rises; a simple cue is pale-yellow urine. Add extra water during exercise or heat exposure.
• Mind the rest of your plate: Include healthy fats (e.g., olive oil, nuts, fatty fish) and adequate carbohydrates for training and hormonal balance.
• Watch for signs of imbalance: New-onset constipation, persistent thirst, dark urine, kidney-stone symptoms, or unexplained fatigue warrant reassessment—and medical advice if persistent.

FAQs

Can extra protein turn to fat? Yes. Once needs are met, excess amino acids can be converted and stored as fat if you’re in a calorie surplus.

Is plant protein safer? Plant-forward patterns are associated with lower cardiometabolic risk and fewer kidney stones, largely due to higher fiber, potassium, and lower acid load. Combining plant and animal proteins is fine; emphasize minimally processed sources.

What about protein shakes? They’re convenient but can crowd out whole foods. Choose products with minimal additives, and use them to fill gaps—not to exceed daily needs.