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Does excess protein turn into fat?

Robbie Puddick
Written by

Robbie Puddick

Medically reviewed by

Fiona Moncrieff

7 min read
Last updated June 2024
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Jump to: Controlled feeding trials show excess protein doesn’t increase body fat | What does protein do to your body? | Can you lose weight by eating protein? | Take home message

High-quality human evidence shows that excess protein isn’t converted into fat and stored when excess calories are consumed.

In-patient studies – also known as metabolic ward feeding trials – have shown that when higher protein levels are consumed alongside excess calories, the protein increases lean mass levels (muscle, bone, other non-fat tissue) and energy expenditure.

Consuming higher protein alongside excess calories does lead to more total weight gain compared to lower protein intake, but this is due to the increase in lean mass that protein contributes to, not because protein is being converted into fat and stored.

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How do you convert protein to fat?

Converting protein into fat is a last resort for the body if we eat a very high-protein diet. Protein must be converted into glucose (sugar) in the liver before the glucose can then be converted into fat if the body doesn’t burn or store the glucose.

The primary sources of body fat are dietary fat and carbohydrates. When we eat fat in food, it travels throughout the body in small boats that we call chylomicrons.

Chylomicrons carry fat around the body to be burnt for energy or stored in our fat tissue.

The body can also convert carbohydrates into fat if needed. If we eat excess carbohydrates and the body can’t burn them or store them as glycogen, it can convert them into fat in the liver or our fat tissue. Specifically in our subcutaneous fat.

However, this only occurs if we eat excess calories alongside a very high intake of carbohydrates. In most circumstances, the body stores excess fat, burns carbohydrates for energy, and uses protein to build lean tissue.

Protein has many essential functions in the body that it prioritises before it converts protein into fat and will only do so as a last resort if it has to.

At Second Nature, we use the highest quality evidence to develop our programme and support our members to eat the right amount of protein, lose weight, and keep it off for good.

If you’d like to join over 150,000 others who’ve mastered their protein intake, lost weight, and kept it off for good without counting macros or calories, click here to take your health quiz.

Otherwise, keep reading as we look at three lines of evidence to explore further the question: Does excess protein turn into fat?

1) Controlled feeding trials show protein doesn’t increase body fat

In nutrition, controlled feeding trials are considered high-quality evidence when investigating biological mechanisms in the body – in this example, whether excess protein intake is stored as fat.

The reason for this is the researchers have a very high level of control over what study participants are eating and doing, so there’s little risk of poor adherence to the type of diet participants have been assigned to.

Controlled feeding trials have consistently shown that consuming excess protein, up to a threshold of 25% of calories, doesn’t lead to excess fat storage from protein.

In an in-patient controlled overfeeding trial with three intervention groups, a low (5%), medium (15%), and high protein (25%) diet.

​​The three groups were overfed by approximately 40% in terms of calories. The additional calories were derived from fat, while carbohydrate intake remained constant.

All three groups gained weight, and the higher protein groups gained more than double the weight of the low protein diet. But (and this is the significant bit), all three groups gained the same amount of fat mass.

The medium and high protein groups gained fat-free mass alongside the fat gain, while the low protein diet lost lean mass.

This means the fat gain can be attributed to the added calories from fat, not protein. Protein led to an increase in lean mass, contributing to higher weight, but not an increase in fat mass.

Another randomised controlled feeding trial investigated the impact of overfeeding protein on weight gain and energy expenditure.

This study was very similar in design, with three intervention diets of low, medium, and high protein. Again, the participants were overfed by around 40% of calories from added fat.

The results were very similar to the study discussed above. To quote the conclusions from the authors (emphasis mine):

“Among persons living in a controlled setting, *calories alone* account for the increase in fat; protein affected energy expenditure and storage of lean body mass, but not body fat storage.”

The highest quality evidence suggests that eating a hypercaloric (eating calories beyond energy balance) diet, where up to 25% of calories come from protein, doesn’t lead to excess fat storage.

But what happens when we eat high-protein diets if it’s not converted to fat? Keep reading as we discuss this in the next section.

Key points:

  • In nutrition, controlled feeding trials are considered the highest quality evidence when investigating biological mechanisms, like whether overfeeding protein leads to excess fat storage
  • Several controlled feeding trials have shown that a high protein diet where 25% of calories come from protein, protein doesn’t lead to excess fat gain
  • High protein leads to an increase in weight as it increases lean mass, but excess fat storage is attributed to the excess intake of calories from either fat or carbohydrate

2) What does protein do to your body?

An important thing to remember is that the human body doesn’t view protein as ‘calories’ in the same way it does fat or carbohydrates.

Protein, specifically amino acids (the body’s building blocks), has several essential functions, including building and repairing tissues, acting as enzymes and hormones, and transporting nutrients. Protein and amino acids also play a crucial role in our immune system.

The body first wants to use protein and amino acids for these essential functions. Only in times of excess or deficiency of energy will the body resort to using amino acids for energy use and storage by converting them into glucose or fat.

High protein intake increases energy expenditure and lean mass development

When consuming a higher-protein diet, the body increases its development of lean tissues. This could be in the form of organ tissue, muscle, or bone.

This increase in lean tissue is related to increased energy expenditure as lean tissues need more energy for production and maintenance than fat tissue. So, you burn more calories on a higher-protein diet.

This was demonstrated in a randomised controlled feeding trial with three intervention groups, a low (5%), medium (15%), and high protein (25%) diet.

​​The three groups were overfed by approximately 40% in terms of calories. The additional calories were derived from fat, while carbohydrate intake remained constant.

The study showed that the low protein diet didn’t see an increase in lean mass or energy expenditure.

In comparison, the medium and high protein diets both saw an increase in lean mass (2.87 kg and 3.18 kg) and energy expenditure (160 kcal/day and 227 kcal/day).

The low protein diet group didn’t experience an increase in energy expenditure, despite consuming the same number of calories as the other groups. This indicates that the protein explicitly caused the increase in energy expenditure.

Key points:

  • Protein isn’t considered as ‘calories’ in the same way that carbohydrates and fats are in the body
  • Protein has many essential functions that the body needs to function, and using protein for energy or energy storage is often a last resort due to excess or deficiency of other energy sources
  • Higher protein intake leads to increases in lean body mass and energy expenditure

3) Can you lose weight by eating protein?

Human clinical trials have consistently shown that eating protein can support weight loss by increasing energy expenditure, supporting lean mass development, and lowering appetite.

A group of researchers reviewed nine studies with over 2,500 participants and showed that higher protein intake was positively associated with weight loss.

They concluded (emphasis mine):

“In conclusion, many clinical trials have shown that consuming more protein than the recommended dietary allowance induces weight loss and improves body composition regardless of total energy intake.

HPD [high-protein diet] was also observed to have long-term weight-loss effects and to prevent weight regain following initial weight loss.”

There are three key ways in which protein supports weight loss:

  1. Increases energy expenditure, so you burn more calories
  2. Increases lean body mass, so more energy is used to support the lean tissue rather than being stored as fat or glycogen
  3. Increases satiety (fullness) hormones and lowers hunger

How much protein should I eat a day to lose weight?

Consuming at least 1.6g/kg of body weight per day is recommended to support weight loss. For someone who’s 100 kg, this would mean eating 160g of protein per day, or ~50g of protein in each main meal (breakfast, lunch, and dinner).

Eating this much protein can be challenging, and it may not be possible for everyone, but here’s a list of the best protein sources to support weight loss:

  • Yoghurt (look for yoghurts that are >7.5g/100g of protein)
  • Meat, fish, and seafood
  • Eggs
  • Milk and dairy products
  • Tofu and tempeh
  • Lentils, beans, and legumes are also good sources of protein
  • Protein powders can be used if you’re struggling to eat enough protein in your diet

Key points:

  • Human clinical trials have shown a positive effect of higher protein intake on weight loss
  • It supports weight loss by increasing energy expenditure, lean mass, and lowering hunger
  • Consuming around 1.6g/kg of body weight per day of protein is recommended to support weight loss. For someone who’s 100 kg, this would mean eating ~160g of protein per day, or ~50g in each meal.

Take home message

Protein is an essential component of our diet. It contributes to our health in many ways. Human clinical trials have consistently shown that consuming higher protein than typically recommended benefits our health and supports weight loss.

Consuming more protein in your diet can be challenging as protein is typically the most expensive part of our food shop.

However, research shows that by basing your meals around a source of good quality protein, you’re more likely to retain higher levels of lean mass, have a higher energy expenditure, and maintain a healthier weight in the long run.

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