Are High Protein Diets Useful for Weight Loss?

(I would like to credit information in this week’s blog post to Professor Manohar Garg of the University of Newcastle)

There is much debate around this hot topic and justifiably so. To the common person with little nutritional knowledge, carbs and fats are what make you put on weight right? Protein takes longer to break down and keeps you fuller for longer and therefore it must be the answer. Unfortunately, it is not that simple. Before we delve into this discussion lets take a few steps backwards and define the key components.

What are Proteins

Amino acids are the building blocks of protein and are made up of carbon, hydrogen, oxygen, nitrogen and sulphur. Percentage wise, 43% of our body’s protein is in our muscle, 15% in our skin and 16% in our blood. Amino acids transform from primary to secondary to tertiary and then finally to its quaternary structure, where it forms a protein. Proteins have many functions in our bodies. They form bodily structures and frameworks, part of our immune system, digestive enzymes, part of our skin hair and nails, blood clotting and more.

What are the recommended dietary intakes (RDIs) for protein for males and females?

Important to note as well that all animal based proteins e.g. meat, eggs and dairy contain all amino acids, whereas with plant based proteins one needs to eat the correct combination of foods in order to obtain all amino acids. According to the Handbook of Clinical Nutrition and Dietetics women aged 19-70 should be consuming 46g/day, and males aged 19-70 should be consuming 64g/day. For children the recommendations are lower, and for women who are pregnant or lactating recommendations are higher. The RDI for fibre is 30g for males and 25g for females. Overall, it is recommended that one’s protein consumption should comprise 15-25% of one’s daily diet, fat should comprise 20-35% and carbohydrate should comprise 45-65%. Clinically, a diet becomes ‘high protein’ when one is consuming >1.2g/kg/day but particularly >1.5g/kg/day. s

How much protein is in the food I eat?

The table below illustrates how many grams of protein is contained in 100g of high protein foods.

Food SourceAmount of Protein (g)/ 100gFood SourceAmount of Protein (g)/100g
Bacon, middle, fried, fat removed30.5Lamb18.8
Barramundi, grilled or baked27.3Lentil24.2
Bean, Legume6.4Milk7.6
Bean, haricot, dry21.9Mixed nut and seed mix21.6
Bean, soy, dry, cooked13.5Mussels19.2
Cheddar cheese25.4Rice6.6
Brie cheese19.3Salmon, baked or grilled24.3
Cottage cheese15.2Seeds, Sesame22.2
Feta cheese17.8Seeds, Pumpkin24.4
Chicken fillet21.3Seeds, Sunflower22.7
Eggs fried13.9Tuna24.4

Branched chain amino acids

Are essential amino acids that make up 30% of our muscles. Essential amino acids are those that the body is unable to make itself and therefore needs to be obtained from the diet. Branched chain amino acids are not metabolised in the liver, rather they go straight to the muscle where they are either metabolised and used as fuel for energy or are built into new protein.

Protein and weight loss

There are three main mechanisms which could simplistically satisfy the argument that a high protein diet is conducive to weight loss.
1. Thermogenesis: More energy is expended metabolising protein than is spent metabolising carbohydrates or fats. 20-30% of the energy derived from protein foods is actually used in the metabolism of them, compared to carbohydrates and fats which use approximately 5% and 15% respectively.
2. Satiety: Because protein takes longer to break down in the gut, it stays in your stomach for longer which inturn delays gastric emptying and thus increases satiety.
3. Decreased energy intake: As a result of increased satiety one experiences a decrease in energy intake. In other words, because you stay fuller for longer, you don’t feel the need to eat as often and therefore your total energy (caloric) intake decreases.

What happens when protein becomes my main energy source?

1. High protein, low carbohydrate diets cause the body to go into ketosis
2. During these diets, the body’s primary energy source, glucose, is unavailable
3. The body turns to triglycerides (fat) as the new energy source. This causes ketones
4. Ketosis promotes the burning of fat and a lowered appetite
5. After a few weeks, the body adapts and ketone levels return to normal, and the body burns fat more efficiently.

What are the negatives of a high protein diet?

Renal dysfunction in the Chronic Kidney Disease population

• The following symptoms occur mostly in those with preceding Chronic Kidney Disease (CKD). Renal dysfunction takes years to develop, particularly in those with perfectly healthy kidneys.
• With an increase in protein metabolism comes an increase in uric acid production. This can result in gout and uric acid kidney stones.
• Higher amounts of ammonia are produced which places increased pressure on the kidneys to excrete higher levels of urea.
• An increased glomerular filtration rate (the rate at which kidneys filter substrates), kidney volume and weight, may cause chronic glomerular injury and even fibrosis (scarring of tissue).

Calcium excretion

• If in ketosis, the body is in an acidic state meaning the pH of the blood decreases. Once in ketosis, calcium from the bones is leached out and used as a buffer.
• Therefore, the calcium levels in our bones decreases putting us at increased risk of fractures due to reduced bone mineral density. This is particularly problematic in the elderly population, so be cautious if you are in the >65 age group and you are in ketosis.
• An increased glomerular filtration rate also reduces calcium resorption in the kidneys


• By restricting the amount of food you eat you will inevitably go into a caloric deficit.
• Sticking to a high protein diet means cutting out other nutritional foods which in turn results in a nutritional deficit. On this diet carbohydrates are avoided and so are fruits and vegetables, at least in the required amounts. Thus, vitamins and minerals as well as fibre (also derived from many carbohydrate containing foods) which are mostly found in fruits and vegetables may not be obtained.
• The Australian Dietary Guidelines can help guide you in forming a healthy balanced diet, making sure you are receiving all the nutrients you need from all food groups.
• Unless adequate fruit and vegetable intake is maintained or a fibre supplement is taken, lack of fibre can become an issue causing constipation.

The evidence

Three randomised control trial studies were conducted and found the following results:
1. Due A, Toubroi S, Skov AR, Astrup A. 2004. Effect of normal diets, either medium or high protein, on body weight in overweight subjects: a randomised 1-year trial.
– High protein diet provided weight loss in the short term (6months) but these benefits were not maintained in the long term
– After 12 months 28% of participants withdrew from the experiment
2. Clifton PM, Keogh JB and Noakes M. 2008. Long-term effects of a high-protein weight loss diet.
This study trialled both a high protein and a high carbohydrate diet. Results showed that the average weight loss was 4.6kg and 4.4kg respectively after the 64-week period.
– A significant increase in HDL (good) cholesterol in the high protein group, insulin secretion reduced in both groups and glucose decreased in both groups. There was also a decrease in triglycerides and LDL (bad) cholesterol in both groups. Iron and B12 significantly increased in the high protein group.
3. Sacks et al. 2009. Comparison of weight-loss diets with different compositions of fat, protein, and carbohydrates.
– Weight loss of 6kg at 6months, weight regained after 12months. Only 23% of participants continued to lose weight post 12months


This goes to show that whilst weight loss may be achievable in the short term, it is more often than not unable to be maintained in the long term. These statistics, together with the above information on what occurs on a physiological level, should be enough to convince you that a high protein diet particularly in the long term is risky, hard to maintain and may not sustain weight loss.

Share this post: