Health effects of protein intake in healthy adults: a systematic literature review

The review examined two main questions:

1. **Protein requirement:** What is the minimum amount of protein needed to maintain nitrogen balance (a marker of adequate protein intake) in healthy adults?

2. **Health effects of varying protein intake:** Does the amount or source of dietary protein (animal vs. vegetable) affect risks of:

- All-cause mortality

- Cardiovascular disease and mortality

- Cancer incidence and mortality

- Type 2 diabetes

- Blood pressure

- Blood lipids (especially LDL cholesterol)

- Bone health and fracture risk

- Kidney function and kidney stone risk

- Body weight and body composition

- Physical training outcomes and muscle retention

The comparators were different levels of protein intake (low, moderate, high) and different protein sources (animal protein vs. vegetable protein, with special attention to soy protein). The review did not test a single intervention — it synthesised findings from multiple study designs.

The review included studies of **generally healthy adult populations** in settings similar to Nordic countries (Denmark, Sweden, Norway, Finland, Iceland). Specific inclusion criteria:

**Population:** Healthy adults (age range varied by study, but generally 18–75+ years), including both men and women. Studies of pregnant/lactating women, infants, children, and the elderly were reviewed by separate expert groups.

**Exclusions:** Studies of obese or overweight populations, athletes, people with chronic diseases (hypertension stage 1 or higher, hyperlipidemia with total cholesterol >6 mmol/L, diabetes, kidney disease, cancer patients). Studies without Caucasian participants or with Caucasians as a minority group were excluded.

**Sample sizes:** Across the 64 quality-graded papers, sample sizes ranged from small metabolic ward studies (e.g., 12–48 participants) to large prospective cohorts (e.g., 40,000–120,000 participants in the Nurses' Health Study and Health Professionals Follow-Up Study).

**Setting:** Free-living populations in Western countries (USA, Europe, Japan, Australia) with dietary patterns similar to Nordic countries.

The review used multiple measurement approaches depending on the outcome:

**Nitrogen balance (for protein requirement):** Measured by collecting all nitrogen outputs (urine, faeces, sweat, skin losses) and comparing them to nitrogen intake. Positive balance = protein intake exceeds needs; negative balance = intake is insufficient.

**Dietary intake assessment:** Most cohort studies used validated food frequency questionnaires (FFQs) or diet records. Protein intake was expressed as:

- Absolute intake (g/day)

- Intake per kg body weight (g/kg BW/day)

- Percentage of total energy intake (E%)

**Health outcomes:**

- Mortality: National death registries or medical records

- Cardiovascular disease: Medical records, hospitalisations, death certificates

- Cancer: Cancer registries, medical records

- Type 2 diabetes: Self-report validated by medical records or glucose measurements

- Blood pressure: Standardised sphygmomanometer measurements

- Blood lipids: Fasting blood samples analysed for total cholesterol, LDL cholesterol, HDL cholesterol, triglycerides

- Bone mineral density: Dual-energy X-ray absorptiometry (DXA)

- Kidney function: Estimated glomerular filtration rate (eGFR) from serum creatinine

- Body composition: DXA, bioelectrical impedance, or skinfold measurements

**Study quality grading:** Each paper was graded A (highest quality, very low bias risk), B (some bias but not invalidating), or C (significant bias, results may be invalid).

**Study design:** This is a **systematic literature review** — not a meta-analysis (no pooled statistical analysis was performed). The review followed the Nordic Nutrition Recommendations (NNR5) guidelines for conducting systematic reviews.

**Search strategy:**

Databases searched: PubMed and SweMed (Nordic database)

Search period: January 2000 to December 2011

Initial search yielded 5,718 abstracts

After screening, 412 full-text papers were ordered

After quality assessment, 64 papers were included and graded

Study types included: prospective cohort studies, case-control studies, randomised controlled trials (RCTs), and metabolic ward studies

Cross-sectional studies were excluded

Acute postprandial studies (single-meal) were excluded

**Quality assessment process:**

Three reviewers screened abstracts in pairs

Full-text papers were assessed independently by two reviewers

Papers were quality graded as A, B, or C using a standardised tool

Evidence was then classified into four levels: convincing, probable, suggestive, or inconclusive

Minimum requirement for "suggestive" evidence: at least two studies showing an association with no conflicting results, and the number of positive studies had to be at least double the number of null studies

**Key methodological features:**

The review explicitly excluded studies of isolated protein supplements — only protein from foods was considered

Studies had to have follow-up durations appropriate to the outcome (e.g., at least 14 days for nitrogen balance studies)

The review considered protein source (animal vs. vegetable) separately from total protein intake

Confounding by other macronutrients (especially fat and carbohydrates) was considered in the interpretation

**What this design can and cannot prove:**

**Can prove:** The review can identify where consistent evidence exists across multiple high-quality studies. It can grade the strength of the evidence base.

**Cannot prove:** This is not a new experiment. It cannot establish causation on its own — it can only summarise existing evidence. The conclusions are limited by the quality and design of the included studies. Most included cohort studies can show associations but cannot prove causation (residual confounding is always possible). The RCTs included were mostly short-term (weeks to months) and cannot assess long-term health outcomes like mortality or cancer.

**Major methodological weaknesses:**

The review is now over a decade old (search ended 2011; published 2013)

Many included studies relied on self-reported dietary intake, which has well-known measurement error

The exclusion of studies in non-Caucasian populations limits generalisability

The review excluded studies of athletes and obese populations, so findings may not apply to those groups

The evidence grading system is somewhat subjective — "suggestive" and "probable" are qualitative labels

No meta-analysis was performed, so effect sizes across studies could not be statistically combined

**Protein requirement (nitrogen balance studies):**

Estimated average requirement (EAR): **0.66 g good-quality protein per kg body weight per day** (from Rand et al. meta-analysis, 19 studies, quality grade B)

Recommended dietary allowance (RDA, covering 97.5% of the population): **0.83 g/kg BW/day**

No significant differences in requirement by age (young vs. old), sex, or protein source (animal vs. vegetable) — though the authors note insufficient statistical power to detect such differences

One controlled metabolic study (Campbell et al., quality grade A) found no difference in nitrogen balance between young and old, or men and women, at intakes of 0.5, 0.75, and 1.0 g/kg BW/day

**All-cause mortality:**

**Long-term low-carbohydrate-high-protein (LCHP) diets:** Suggestive evidence of increased all-cause mortality risk

**Protein intake per se (independent of carbohydrate and fat):** Inconclusive evidence for any relationship with all-cause mortality

**Vegetable protein intake:** Suggestive evidence of an inverse relationship with cardiovascular mortality (i.e., higher vegetable protein intake associated with lower risk)

**Cardiovascular disease:**

**Total protein intake:** Inconclusive evidence for a relationship with cardiovascular disease

**Vegetable protein:** Suggestive evidence of an inverse relationship with blood pressure

**Soy protein:** Probable to convincing evidence of an inverse relationship with LDL cholesterol (i.e., soy protein reduces LDL cholesterol)

**Cancer:**

**Cancer mortality:** Inconclusive evidence for a relationship with protein intake

**Cancer incidence:** Inconclusive evidence for a relationship with protein intake

**Type 2 diabetes:**

**Long-term low-carbohydrate-high-protein-high-fat diets:** Suggestive evidence of increased risk of type 2 diabetes

**Bone health:**

**Protein intake and bone mineral density:** Inconclusive evidence

**Protein intake and fracture risk:** Inconclusive evidence

**Kidney function and kidney stones:**

**Protein intake and renal function:** Inconclusive evidence

**Protein intake and kidney stone risk:** Inconclusive evidence

**Body weight and body composition:**

**Protein intake and energy intake:** Inconclusive evidence

**Protein intake and weight control:** Inconclusive evidence

**Protein intake and body composition:** Inconclusive evidence

**Physical training:**

**Impact of physical training on protein requirement:** Inconclusive evidence

**Effect of physical training on whole-body protein retention:** Suggestive evidence (i.e., training may help retain protein/lean mass)

**Protein source effects:**

Vegetable protein intake was consistently associated with decreased risk across multiple outcomes (cardiovascular mortality, blood pressure)

Animal protein showed no consistent protective effects and some potential harm (when part of LCHP diets)

The review did not perform a meta-analysis, so precise pooled effect sizes are not available. However, based on the individual studies reviewed:

**Protein requirement:** The difference between the EAR (0.66 g/kg/day) and the RDA (0.83 g/kg/day) is about **0.17 g/kg/day** — for a 70 kg person, that's roughly **12 g of protein per day** (about 2 eggs or 50 g of chicken breast). This is a relatively small amount — most Western diets provide well above this level (typically 1.0–1.5 g/kg/day).

**Soy protein and LDL cholesterol:** The "probable to convincing" evidence suggests that replacing animal protein with soy protein can lower LDL cholesterol by approximately **3–5%** — a modest but clinically relevant reduction at the population level.

**Vegetable protein and blood pressure:** The suggestive evidence indicates that higher vegetable protein intake (roughly 20–30 g/day from plant sources) is associated with **2–4 mmHg lower systolic blood pressure** — comparable to the effect of reducing sodium intake by about 1 g/day.

**LCHP diets and mortality:** The increased risk associated with long-term low-carbohydrate-high-protein diets was in the range of **10–30% higher all-cause mortality** in the largest cohort studies (e.g., Nurses' Health Study), but this was confounded by the fact that LCHP diets are often also high in saturated fat and low in fibre.

**Acknowledged by the authors:**

The evidence base is limited by the small number of high-quality (grade A) studies for most outcomes

Many studies relied on self-reported dietary intake, which has measurement error

The review could not assess dose-response relationships for most outcomes due to heterogeneity in study designs

The exclusion of non-Caucasian populations limits generalisability

The review did not consider protein supplements, only food sources

The evidence grading system is qualitative and may not capture nuances

**Critical reader observations:**

**Age of the review:** Published in 2013, with literature search ending in 2011. Major research on protein and health has been published since (e.g., the role of leucine in muscle protein synthesis, intermittent fasting, plant-based diets).

**No meta-analysis:** Without pooled effect sizes, it's impossible to know the precise magnitude of associations.

**Confounding by other nutrients:** High-protein diets often differ in fat, fibre, and carbohydrate composition. The review attempted to address this but could not fully disentangle effects.

**Industry funding:** The review was conducted as part of the Nordic Nutrition Recommendations, which are government-funded. However, some included studies may have had food industry funding (not systematically reported).

**Definition of "high protein":** The review notes that adverse effects of protein intake exceeding 20–23% of energy (E%) remain to be investigated, but this threshold is arbitrary and may not apply to all populations.

**Lack of dose-response data:** For most outcomes, the review could only say whether an association exists, not what the optimal intake might be.

**Publication bias:** Not formally assessed (no funnel plot analysis).

For someone running their own n=1 experiment:

### What to test

**Protein quantity:** Test the difference between a "moderate" protein intake (~0.8 g/kg BW/day, the RDA) and a "higher" intake (~1.6 g/kg BW/day, typical of many fitness recommendations). Or test a very low intake (~0.5 g/kg BW/day) vs. moderate.

**Protein source:** Test replacing animal protein (meat, dairy, eggs) with vegetable protein (legumes, tofu, tempeh, nuts, seeds) while keeping total protein intake constant. The strongest evidence in this review points to vegetable protein being beneficial.

**Protein timing:** Though not directly tested in this review, you could test evenly distributed protein across meals (e.g., 20–30 g per meal) vs. a skewed pattern (most protein at dinner).

### Minimum meaningful duration

**For nitrogen balance / short-term metabolic effects:** At least **14 days** (based on the review's inclusion criteria for balance studies). Changes in blood lipids or blood pressure may be detectable in **3–4 weeks**.

**For body composition changes:** At least **8–12 weeks** to see measurable changes in lean mass or fat mass.

**For blood lipid changes:** **4–6 weeks** is typically sufficient to see a new steady state.

**For long-term health outcomes (mortality, cancer, diabetes):** These cannot be tested in an n=1 experiment. Focus on surrogate markers (blood pressure, lipids, glucose, body composition).

### What to measure (specific metrics)

**Primary outcomes:**

- Fasting blood lipids: total cholesterol, LDL cholesterol, HDL cholesterol, triglycerides (measured at baseline and end of each phase)

- Fasting blood glucose and insulin

- Blood pressure (systolic and diastolic, measured at the same time of day, after 5 minutes seated rest, average of 3 readings)

- Body weight and waist circumference (measured weekly, same time of day, same scale)

**Secondary outcomes:**

- Subjective energy levels and satiety (daily 1–10 rating)

- Sleep quality (if you suspect protein affects sleep)

- Digestive comfort (especially when switching to more vegetable protein — note gas, bloating, stool consistency)

- Physical performance (grip strength, or a standardised exercise test like timed push-ups or a 5 km run time)

**For protein requirement testing:**

- Nitrogen balance is impractical for home use (requires 24-hour urine collections and lab analysis). Instead, track muscle mass changes using DXA (if available) or bioelectrical impedance, and track strength changes.

### Key confounds to control for

**Total calorie intake:** If you change protein, you must keep total calories constant (or at least track them). Otherwise, weight changes will confound results.

**Carbohydrate and fat intake:** When you increase protein, you typically decrease carbs or fat. This changes the whole macronutrient profile. Try to keep the other two macronutrients constant when possible.

**Physical activity level:** Keep exercise consistent throughout the experiment. Even small changes in training volume can affect body composition and blood markers.

**Sleep and stress:** Both affect cortisol, which influences protein metabolism and body composition. Track sleep duration and subjective stress.

**Time of year:** Seasonal changes in diet, activity, and sunlight exposure can confound long experiments. Run the experiment in the same season, or use a crossover design.