The evidence behind Replenish.

Our approach to evidence

Replenish was built around a simple idea: nutrition advice should be evidence-informed, but still practical enough for real life.

Nutrition science can feel confusing because different studies, diets and headlines often appear to point in different directions. One person says calories are all that matter. Another says carbohydrates are the problem. Someone else recommends fasting, high protein, low fat, keto, or a completely different approach.

Replenish does not try to turn that complexity into a single rigid rule. Instead, we use the strongest available evidence to guide how the app sets targets, tracks patterns and creates personalised insights, while recognising that people are not formulas.

Where the literature allows, Replenish grounds its recommendations in meta-analyses, systematic reviews, randomised controlled trials and trusted public health guidance. These are stronger forms of evidence because they either bring together findings from multiple studies or test interventions in a structured way.

Where evidence is mixed, limited, or based on smaller studies, we say so clearly. We also flag conflicts of interest, small sample sizes and methodological limitations where they matter.

The aim is not to make nutrition feel more technical. The aim is to explain why Replenish makes the choices it does, and to help you understand that our approach is based on evidence, flexibility and sustainability rather than diet trends.

How to read this page

Each section follows the same structure.

• TL;DRThe practical takeaway in plain English.
• How Replenish uses thisHow the evidence appears in the app.
• What the evidence saysA more detailed explanation of the research.
• Limitations and conflictsImportant caveats, stated openly.
• ReferencesThe studies and guidance we have used.

Calories

A formula can estimate your energy needs, but it cannot perfectly know your body, routine, sleep, activity, stress, appetite or real-life eating pattern. A range is more realistic. It allows for normal day-to-day variation while still giving you a useful structure.

How Replenish uses this

When you create an account, Replenish sets an initial calorie range using your profile information, such as age, height, weight, biological sex, activity level and goal. This gives you a starting point, but it is not treated as a final answer.

Energy prediction formulas are useful, but they are still estimates. Your actual intake, appetite, routine and energy needs may be different from what a formula predicts. Replenish recognises this from the beginning.

That is why the first 7 days are used as a calibration period. During these first 7 days, you are not expected to achieve anything. You do not need to hit your calorie range perfectly, and you do not need to change how you normally eat. The purpose is to help Replenish learn about you.

By comparing your initial estimated range with your actual logged intake, Replenish can help you understand whether the starting target feels realistic. After the 7-day calibration period, you can review the difference between your estimated target and your real intake pattern, then decide whether your targets need adjusting.

What the evidence says

Energy prediction equations have limits

The Mifflin–St Jeor equation remains one of the most widely used formulas for estimating resting metabolic rate in adults. In its original validation study of 498 healthy adults, it performed better than the previously dominant Harris–Benedict equation.1

However, even good equations have meaningful individual error. In plain English, two people with the same age, height, weight and biological sex may still have different energy needs.

The UK Scientific Advisory Committee on Nutrition used doubly-labelled water, considered the gold standard for measuring free-living energy expenditure, to establish population-level Dietary Reference Values for energy.2 SACN was clear that these values are population averages, not exact targets for individuals. For any one person, formula-based estimates can be meaningfully above or below true energy needs.

This is why Replenish treats calorie formulas as a starting point, not a personalised answer.

Calorie reduction can affect nutrient intake

A 2024 study by Fulgoni et al. modelled the nutritional impact of calorie reduction in more than 6,000 US adults with overweight or obesity using NHANES dietary data.3

The study found that simulated calorie reductions increased the proportion of adults falling below estimated average requirements for several vitamins and minerals, including vitamins A, C, D, E and B6, as well as calcium. At larger calorie reductions, shortfalls became more common.

This is one reason Replenish does not treat calorie reduction as the only goal. A lower calorie intake should still support nutritional quality. Replenish therefore tracks micronutrients alongside calories, so you can see whether your eating pattern is still nourishing you while you work towards a goal.

A calorie surplus and muscle building

For muscle gain, the evidence is more nuanced than "eat as much as possible".

A 2019 narrative review by Slater et al. concluded that resistance-trained individuals with adequate protein may be able to gain lean mass in energy balance or even a modest deficit, although a small surplus is often thought to support faster or more reliable gains.4

A trial by Helms et al. compared small and large calorie surpluses in 21 resistance-trained individuals.5 The larger surplus group gained more fat, without a clear additional advantage for muscle outcomes. Because this study was small, it should not be over-interpreted, but it supports Replenish's cautious approach: a modest surplus is usually more sensible than a large one.

Protein

For people losing fat, higher protein can help preserve lean mass and support fullness. For people building muscle, protein supports training adaptation and muscle repair. However, protein targets should still be realistic, personalised and safe.

How Replenish uses this

After calories are set, Replenish prioritises protein before splitting the remaining calories between carbohydrates and fats. This is deliberate. Protein has a clearer evidence base for body composition goals than the exact carbohydrate-to-fat ratio.

Replenish calculates protein using grams per kilogram of body weight, then turns this into a daily gram target so you can understand what it means practically.

What the evidence says

The UK reference intake and its limits

Public Health England's Government Dietary Recommendations set the adult Reference Nutrient Intake for protein at 0.75 g/kg/day, commonly rounded to 0.8 g/kg/day.1

This figure is designed to meet the basic needs of most sedentary adults. It is useful as a baseline, but it is not necessarily the optimal intake for people who are physically active, trying to lose fat, or aiming to build muscle. The International Society of Sports Nutrition recommends higher protein intakes for exercising adults, typically around 1.4–2.0 g/kg/day.2

Protein during fat loss

When calories are restricted, protein has two important roles: helping preserve lean mass and supporting satiety.

Longland et al. assigned 40 young men to either a lower-protein diet of 1.2 g/kg/day or a higher-protein diet of 2.4 g/kg/day during a calorie deficit, with both groups following the same exercise programme.3 The higher-protein group gained 1.2 kg of lean mass while losing 4.8 kg of fat. The lower-protein group lost 0.1 kg of lean mass while losing 3.5 kg of fat.

Ogilvie et al. also found that higher protein intake during calorie restriction helped reduce lean mass loss and improved overall dietary quality.4 The higher-protein group consumed more vegetables, fibre and micronutrients, showing that protein targets can also encourage better overall food choices when implemented well.

The protein ceiling for muscle building

A 2018 meta-analysis by Morton et al. reviewed 49 studies with 1,863 participants and examined how protein intake affected resistance-training gains.5 The analysis found that gains in fat-free mass plateaued at approximately 1.6 g/kg/day.

This does not mean higher intakes are harmful for healthy people. It means that beyond a certain point, extra protein is less likely to produce meaningful additional muscle gain. A dose-response meta-analysis by Tagawa et al. across 74 randomised controlled trials supported a similar conclusion.6

Plant and animal protein

A 2025 systematic review and meta-analysis by Reid-McCann et al. examined whether protein source affected muscle mass, strength and physical performance outcomes.7 Across 39 randomised controlled trials, no significant difference was found between plant and animal protein when total protein intake was matched.

Long-term health and kidney safety

Large prospective analyses by Naghshi et al. and Huang et al., covering more than 700,000 participants collectively, found no consistent association between higher protein intake and increased all-cause, cardiovascular or cancer mortality in healthy adults.89

Ko et al. explain that high dietary protein intake can increase pressure within the filtering units of the kidney.10 In technical terms, this is described as intraglomerular hypertension and kidney hyperfiltration. Over time, this may contribute to glomerular injury and proteinuria in susceptible individuals.

This does not mean that higher protein intake automatically damages healthy kidneys. The concern is strongest for people who already have impaired kidney function or are at higher renal risk. In those users, a high-protein target may be inappropriate unless advised by a healthcare professional.

Ko et al. also note that protein source may matter. Diets high in animal protein, especially when combined with other dietary risk factors, may have different renal implications from diets with more plant-based protein sources.10 This area is still debated, but it supports Replenish's cautious approach.

Mantzouranis et al. add wider long-term safety context. Their systematic review and meta-analysis examined high-protein diets and cardiovascular outcomes in adults without established cardiovascular disease.11 The evidence was mixed rather than definitive, but it reinforces the need to avoid presenting very high protein intake as universally beneficial for everyone.

Carbohydrates and fats

Low-fat and ketogenic diets can produce short-term weight loss for some people. However, the longer-term evidence does not show a clear advantage over balanced approaches for most people.

Instead, the app focuses on food quality: fibre-rich carbohydrates, fewer free sugars, more unsaturated fats, less saturated fat, and enough protein to support your goal.

How Replenish uses this

Once calories and protein are set, Replenish divides the remaining calories between carbohydrates and fats. This order matters. Protein is prioritised first because it has clearer evidence for lean mass, satiety and muscle-building goals.

Replenish uses a balanced default aligned with UK dietary guidance, but you can adjust your carbohydrate and fat split if you prefer. The app does not treat either carbohydrate or fat as "bad". Instead, it looks at the quality and pattern of intake.

For carbohydrates, this means fibre-rich foods, whole grains, fruit, vegetables, beans, lentils and minimally processed sources where possible. For fats, this means encouraging unsaturated fat sources and limiting excessive saturated fat, in line with UK guidance.

What the evidence says

Short-term versus long-term low-carbohydrate diets

Low-carbohydrate and ketogenic diets often produce faster weight loss in the first weeks or months. Some of this early change is related to glycogen and water loss, alongside appetite effects that may make some people naturally eat less.

The 2022 Cochrane review by Naude et al. included 61 randomised controlled trials and 6,925 participants.1 It found little to no difference in weight loss between low-carbohydrate and balanced-carbohydrate diets when studies were examined beyond the initial phase. In trials lasting 12 months or longer, the apparent advantage of low-carbohydrate diets largely disappeared.

This suggests that long-term outcomes are more likely driven by adherence, calorie balance, protein intake and overall food quality than by carbohydrate restriction alone.

Ketogenic diets specifically

A 2013 meta-analysis by Bueno et al. found that very-low-carbohydrate ketogenic diets produced slightly greater weight loss than low-fat diets over 12 months or more, but the absolute difference was modest.2

Silverii et al. found that low-carbohydrate diets could be effective for long-term weight loss in people with obesity, but also noted that adherence declined significantly beyond 12 months.3

An umbrella review by Patikorn et al. covering 34 meta-analyses found that ketogenic diets reduced body weight and waist circumference, but did not show a consistent advantage over other dietary patterns for long-term weight maintenance.4 Some studies also reported elevated LDL cholesterol in certain subgroups.

Carbohydrates and muscle building

For users aiming to build muscle, very low carbohydrate intake introduces a specific trade-off. Henselmans et al. reviewed 24 crossover and parallel-group trials and found that higher carbohydrate availability improved resistance-training performance.5

This matters because muscle and liver glycogen are important fuel sources for higher-intensity training. If carbohydrate availability is too low, training output may suffer. Reduced training output can reduce the stimulus that drives hypertrophy.

Vargas-Molina et al. found in a 2022 meta-analysis that ketogenic diets produced significantly less muscle mass gain compared with non-ketogenic diets in resistance-trained individuals.6

Figueiredo and Cameron-Smith examined whether carbohydrates directly stimulate muscle protein synthesis after resistance exercise and concluded they do not.7 The implication is that the carbohydrate effect on hypertrophy operates through training performance capacity, not anabolic signalling directly.

Dietary fat and long-term health

A large prospective cohort study by Shan et al. (37,233 US adults, 15-year follow-up) found that both very low carbohydrate diets and very low fat diets were associated with modestly elevated all-cause mortality compared with moderate, balanced dietary patterns.8 This was observational research, so it shows association rather than causation.

UK government guidance does not recommend extreme fat restriction. Instead, it focuses on replacing saturated fats with unsaturated fats, limiting free sugars and increasing fibre.9 This is a quality-focused approach rather than a strict macro-ratio approach.

Exercise

Exercise supports health, body composition, energy levels and long-term wellbeing. Cardio helps with heart health, fitness and energy expenditure. Resistance training helps preserve and build muscle.

How Replenish uses this

Replenish uses WHO physical activity guidance as the foundation for exercise targets. After that, the app adjusts the emphasis depending on your goal.

• For fat loss, Replenish gives more weight to aerobic activity while still encouraging resistance training to protect lean mass.
• For muscle gain, Replenish prioritises resistance training volume while keeping cardio in place for cardiovascular health and recovery.
• For general health, Replenish encourages a balanced approach: regular aerobic activity plus muscle-strengthening activity.

The app also allows you to set an exercise split, because the best plan is one you can actually maintain.

What the evidence says

WHO physical activity guidance

The WHO 2020 guidelines recommend that adults accumulate 150–300 minutes of moderate-intensity aerobic activity per week, or 75–150 minutes of vigorous-intensity activity.1 They also recommend muscle-strengthening activities involving major muscle groups on two or more days per week.

Importantly, WHO also states that any amount of physical activity is better than none. This matters because many people feel discouraged if they cannot meet the full target immediately. Replenish uses the guidance as a flexible reference point, not a pass/fail standard.

Aerobic exercise and fat loss

Jayedi et al. conducted a 2024 dose-response meta-analysis of 116 randomised controlled trials and 6,880 participants.2 The review found that body weight, waist circumference and body fat percentage decreased in a dose-dependent way up to approximately 300 minutes of moderate-to-vigorous aerobic exercise per week.

More clinically meaningful fat-loss effects tended to appear from around 150 minutes per week and increased further up to around 300 minutes. Notably, the gains from each additional 30-minute increment diminished progressively. The greatest return on time invested occurred within the WHO-recommended range.

Comparing exercise types for body composition

Lafontant et al. compared resistance training, aerobic training and concurrent training, meaning aerobic and resistance training combined.3 Concurrent training produced the strongest overall improvements in body composition. It outperformed resistance training alone for fat loss and aerobic training alone for lean mass preservation.

Resistance training volume and hypertrophy

Schoenfeld, Ogborn and Krieger conducted a dose-response meta-analysis examining the relationship between weekly resistance-training sets per muscle group and muscle hypertrophy.4

Across 15 studies, the analysis found a clear and statistically significant dose-response relationship. Performing 10 or more sets per muscle group per week produced significantly greater hypertrophy than fewer sets, while fewer than 5 sets per week represented a lower-stimulus threshold.

However, the response is not linear indefinitely. The evidence supports meaningful returns up to around 10–20 sets per muscle group per week for many individuals, after which marginal benefit may decrease and recovery demands increase.

Muscle-strengthening activity and long-term health

Momma et al. reviewed 16 prospective cohort studies involving nearly 1.5 million adults.5 Muscle-strengthening activity was independently associated with lower risk of all-cause mortality, cardiovascular disease, total cancer, diabetes and lung cancer, even after adjustment for aerobic activity.

The association with mortality reduction appeared strongest around 30–60 minutes of resistance exercise per week, although this comes from observational data and should be interpreted accordingly.

Meal timing

When you eat can matter, but it usually matters less than overall calorie intake, protein intake, food quality and consistency. Intermittent fasting can help some people because it creates structure and may make it easier to manage intake. However, fasting is not required for fat loss or good health.

How Replenish uses this

Replenish allows you to set up intermittent fasting if it fits your routine, but it is optional. The app can track eating windows, meal timing and patterns such as late eating, long eating windows, skipped meals, or inconsistent routines.

This information helps Replenish provide gentle insights. For example, if you regularly eat most of your calories late in the evening and also report poorer sleep or higher cravings, the app may highlight that pattern.

Replenish does not tell you that you must fast. It simply helps you understand whether your current routine is supporting your goals.

What the evidence says

Intermittent fasting versus continuous calorie restriction

A 2024 network meta-analysis by Huang et al. and a 2025 systematic review by Semnani-Azad et al. found that intermittent fasting strategies and continuous energy restriction both reduced body weight compared with unrestricted eating.12 However, only alternate-day fasting showed meaningful benefit over continuous restriction in some analyses.

In practical terms, intermittent fasting may work well for some people because it simplifies eating structure. For others, it may feel restrictive or increase hunger. Replenish therefore presents fasting as an option, not a requirement.

Front-loading calories

Young et al. found in a systematic review and meta-analysis that consuming more calories earlier in the day was associated with modestly better weight-loss outcomes in energy-restricted settings.3

This may relate to circadian rhythm, appetite regulation or behavioural consistency. However, the effect appears modest, and meal timing should not be treated as more important than total intake, protein, fibre and overall food quality.

Meal regularity

Lopez-Minguez et al. found that later meal timing was associated with higher BMI and less favourable cardiometabolic risk markers.4 Horikawa et al. found that regularly skipping breakfast was associated with higher prevalence of overweight and obesity in Asian and Pacific populations.5 Davis et al. reviewed broader meal-timing evidence and concluded that irregular patterns and later eating are consistently associated with weight-gain risk.6

However, many of these findings are observational or based on varied definitions of timing, fasting and meal regularity.

Guidelines and institutional sources

Alongside individual studies, Replenish draws on guidance from four institutions. Each is used for a specific purpose.