Whey Protein VS. Soy Protein: Which One Is Best For Weight Loss?

Measuring protein synthesis (or breakdown) in muscle is the best biomarker of ongoing muscle catabolism or anabolism.

Whey protein stimulates muscle protein synthesis rates more than soy protein, but supplementing approximately 25 grams per day of either has similar effects on body composition over two weeks of dieting.

Protein synthesis during weight loss is important as maintaining muscle helps regulate blood sugar and boost metabolism.

Information on fat or protein turnover at any given time can tell us how well the body maintains lean mass during weight loss.

Who and what was studied?

This study aimed to examine the effects of whey and soy supplementation on protein synthesis and biomarkers of lipolysis during weight loss, with carbohydrate supplementation as a control.

Although body composition was measured during this study, the effect of whey or soy protein supplements on body composition was not a primary endpoint of the study.

This was likely due to the study’s relatively short duration, as small changes in body composition would not be detectable even with DXA (Dual X-ray Absorptiometry) measurements, which the study uses and is considered the gold standard for measurement of body fat.


Previous research has shown that the DXA error is around 1.2%, which would outshine the minor changes in DXA’s body composition.

Many studies have compared the effects of soy and whey protein in stimulating protein synthesis and affecting body composition in various conditions. However, few have specifically studied the effect on protein synthesis and lipolysis during weight loss.

Studies that previously compared whey to soy in its ability to stimulate protein synthesis and macroscopic muscle hypertrophy in other contexts have generally found that whey is superior to soy.

This is probably because whey has a complete amino acid profile (most soy is very low in two of the essential amino acids, methionine, and lysine) and mainly because of the higher leucine content of whey, the la ability of the significant factor protein to stimulate protein synthesis.

To examine the effects of soy or whey on protein synthesis during weight loss, 40 healthy, overweight, and obese participants (19 men and 21 women).

They were included and randomized to receive a whey supplement twice daily (27 grams per supplement), soy protein supplement (26 grams per supplement), or carbohydrate (CHO) controls (25 grams per supplement).

This resulted in a total daily protein intake of 1.3 grams per kilogram of body weight in the protein-supplemented groups and 0.7 grams per kilogram in the CHO group.

The research team provided all the food in prepackaged meals to increase compliance.

During the study, all participants were first put on a three-day maintenance diet (they were given maintenance calories and 1 gram per kilogram of body weight of protein), underwent the first day of testing, and then underwent 14 days of a hypocaloric diet, followed by the second test day.

The purpose of the maintenance diet was to balance the subjects and put them on flat ground, metabolically speaking.

The hypocaloric diet was calculated to provide an energy deficit of 750 kcals per day, which should result in a weight loss of one to three kilograms over 14 days.

On the two days of testing, subjects received stable isotope infusions with isotope-labeled phenylalanine and glycerol, analyzed with DXA, and blood and muscle samples were taken.

Body composition (total, fat, and lean mass) was obtained from DXA scans. Muscle protein synthesis was calculated using stable isotope-labeled phenylalanine tissue enrichment in muscle samples.

Lipolysis (fat burning) was calculated using the appearance of isotope-labeled glycerol in the blood. Glucose and insulin levels were obtained from blood samples.

Overweight and obese participants were randomized to receive soy or whey protein or a carbohydrate control and were given a hypocaloric diet. Measurements of body composition and muscle protein synthesis were measured and compared between groups.

What were the findings?

The researchers found no significant differences in body composition between the groups. This was the expectation of a study of this duration and diet composition.

Whey has been shown numerous times to be a so-called ‘fast’ protein, which means that the amino acids in it are absorbed faster than other proteins with similar amino acid content.

In this study, we see this again, as the release of leucine, essential amino acids, and total amino acids were significantly higher after ingestion of whey protein than after soy. Soy still outperformed the CHO control supplement.

The plasma leucine release (measured by the area under the AUC curve) from serum was almost three times that of soy, while the essential and total amino acid AUCs were about two times higher.

As expected, there were differences in insulin and glucose concentrations after ingestion of the supplement between the protein groups and the CHO control group, both before and after the dietary intervention.

Ingestion of the glucose control supplement resulted in higher glucose and insulin levels than ingestion of any protein supplement, with no difference between the soy and whey protein groups.

As amino acids are absorbed into the bloodstream more rapidly from whey protein than soy protein, it is plausible to expect higher levels of blood glucose and insulin after administration of serum.

There was also a significant difference between the amount of circulating glycerol between the protein groups and the CHO control group. Glycerol was lower in the CHO group than in any protein group.

This indicates that lipolysis is suppressed after ingestion of CHO relative to both groups of proteins. In the post-prandial condition, the amount of glycerol in the blood measures the rate of ongoing lipolysis.

Fat is stored as triglycerides, and when fat stores receive neuroendocrine signals to mobilize fat, triglycerides are hydrolyzed into fatty acids and glycerol, which are released into the circulation.

Fatty acids are metabolized in most of the body, while most glycerol is used for gluconeogenesis.

The most likely explanation for the lower glycerol levels observed after CHO ingestion is probably that insulin strongly inhibits lipolysis. Since the CHO supplement resulted in a higher amount of insulin, supporting this hypothesis further.

The study’s main finding was the effect of protein supplements on protein synthesis in the myofibrillar protein fraction of muscle (the so-called Physical Protein Synthesis, FPS) before and after the dietary intervention.

The researchers measured SPF before and after protein ingestion and before and after the dietary intervention. The reference SPF was around 0.02-0.03% / hour, but with the stimulation, it increased to 0.06-0.07% / hour for whey and 0.03-0.04% for soybeans.

When SPF was expressed as the change in SPF from before to after ingestion of a supplement, soy scored higher than the control supplement CHO and serum scored higher than both.

This means that whey stimulates muscle protein synthesis more efficiently than soy protein.

Whey protein was absorbed faster than soy protein and stimulated muscle protein synthesis by about twice the amount of soy supplementation. However, no differences in overall body composition were observed between the groups.

The panorama

The results regarding amino acid, glucose, insulin, and glycerol concentrations were consistent with numerous previous observations.

Concerning body composition findings, this study does not prove that whey is superior for maintaining lean mass during weight loss.

There is a good chance that whey is probably better, but that cannot be determined from these results because this study lacks the statistical power to find such differences.

A sufficient number of subjects must be enrolled for the differences between the groups to appear statistically significant.

If the difference between groups we see is slight about the variation in the observations, this requires more subjects.

The study duration was short (14 days); therefore, the changes in lean and fat mass are small compared to the margin of error using a DXA scanner.

This means that if there were a difference between the groups, it would have taken more participants or a longer study duration for significant changes to manifest.

If you know the variation in the measurement you are taking, you can calculate the number of subjects needed to detect a group difference of a given size.

The researchers describe that they knew they had inadequate power to detect differences between groups, which was acceptable, as it was not part of the study’s primary objective. Alternatively, this may be an accurate and valid finding.

It is possible that although one protein source may be more effective than another in a short period for measuring SPF, overall dietary intake may be more important in producing long-term adaptations in body composition.

However, the study showed that whey stimulates protein synthesis in myofibrillar proteins much more efficiently than soy protein supplementation.

How it translates into a reduction in lean mass during weight loss is very difficult to obtain for several reasons. SPF is typically a surrogate biomarker for a snapshot of hypertrophy processes.

However, hypertrophy or atrophy results from net protein synthesis or degradation, which is the product of macroscopic protein synthesis and net protein degradation.

Therefore, it should be apparent that modulation of protein degradation is as significant as modulation of protein synthesis.

This is supported by the results of some studies, which have shown that different protein sources can influence protein synthesis and breakdown.

Protein synthesis is reported more frequently because the technology to measure protein synthesis is much better than the technology available to measure protein degradation.

These factors make it difficult to conclusively establish the superiority of whey over soy based on this study. The measurement of protein synthesis used in this study is a surrogate for effect on muscle mass.

However, changes in muscle mass can be measured directly with something like DXA, although this requires a more challenging study setup, which means more subjects for longer and a higher study cost.

Of course, studies can always be better or more detailed, so perhaps this study is the springboard for the following more detailed study.

Frequent questions

How much daily protein synthesis and breakdown occurs commonly and under weight loss or muscle gain conditions?

The basal fasting FPS in this study was around 0.02-0.03% / hour, while the fed FPS was two to three times higher.

Based on these numbers and the MPS and FPS data in the literature, we can assume an average total protein synthesis of 1.0-1.5% / day.

This means that 1.0-1.5% of the protein from muscle protein is accumulated and broken down per day in the balance of muscle mass. The amount of muscle made from protein is constant, around 20% of the muscle’s wet weight.

As an average adult (60-80 kilograms / 130-175 pounds), you carry 30-40% of your body weight as muscle mass; this corresponds to 200-400 grams of muscle (or 40-80 grams of muscle protein) that you are built and decomposed, daily.

In the context of changes in muscle mass during sustained weight loss or resistance training, net changes in muscle mass are generally 10-50 grams per day.

These may be larger at the start of weight loss or weight gain, but it is still emphasized that more protein accumulates and breaks down daily than is ultimately gained or lost as hypertrophy or atrophy.

Has any research been connecting muscle protein synthesis to the result of muscle hypertrophy?

Another investigation compared protein synthesis measured after a series of resistance exercises in one part of a chronic resistance training study with gross hypertrophy measured after 16 weeks of the resistance exercise program.

The researchers found no correlation between the two measures. Although that was in a training study, the result underscores that perhaps the use of protein synthesis is not as good of a biomarker as we might sometimes like to think.

What about molecular biology markers of muscle gain and loss? Do they have findings similar to muscle protein synthesis?

It is also worth noting that there is a disconnect between measurements of protein synthesis and various biomarkers of molecular biologies, such as insulin signaling through the Akt / mTOR signaling cascade or expression of the FOXO transcription factor.

In this case, however, this is more likely due to the poor reliability of molecular biomarkers, as they can show anabolic signaling even in extremely catabolic cachectic subjects.

What should I know?

Ingesting whey protein stimulates myofibrillar protein synthesis during weight loss more efficiently than soy protein and naturally better than carbohydrates.

This indicates that proteins obtained from whey can save muscle mass more efficiently than proteins from soy, but the actual impact is outside the capabilities of this study to evaluate.