Soy Protein for Weight Loss

Last updated August 31st 2015

Soy protein, taken from soybeans, is an inexpensive and animal-free source of protein. Although soy protein may not be as effective as whey protein, there is some evidence supporting its use for weight loss. 

Soy protein is a protein product, which as the name suggests, is produced from soybeans. It was originally produced and used in paper coating as a pigment binder, which still remains the main use soy protein. Soy protein is also often used in foods.

Soy protein has been used as a source of protein for muscle growth, however there is some debate over its effectiveness or possible negative side effects. Although of course there are both positives and negatives, there is some good evidence supporting its use for weight loss and ability to lower levels of cholesterol in the blood.

What is Soy Protein?

Soybeans are legumes, which as well as being eaten whole, are also used to make soymilk and tofu. They are unique amongst vegetables in that they contain all eight essential amino acids, as well as supposedly containing no cholesterol and several vitamins and minerals. Soy protein is used as a supplement for those looking to gain muscle, but also for children with a lactose intolerance, vegetarians and women who are pregnant as a good source of vitamin D.

Soy protein is made from the soybeans that have had their husks and fat removed, where they are then processed into either soy protein isolates, flour or concentrates, with the isolates having the highest protein content of around 90%. This particular form of protein is also considered to be one of the cheapest form of protein. Unlike whey protein that is obtained from milk, soy protein is produced from a plant source making it popular amongst vegans or vegetarians. 

How Soy Protein Benefits Weight Loss

There are several purported health benefits associated with soy protein, including some for weight loss. One such benefit is that it may be able to help lower cholesterol levels in the blood, particularly the ‘bad’ LDL cholesterol. This may be beneficial towards cardiovascular health, and may also benefit weight loss through altering the way the lipids are handled in the body.

In addition to lowering cholesterol, there is also some evidence to suggest that it may be able to support weight loss; one study of a soy protein meal replacement showed that the product may have a slight benefit towards weight loss.

Protein supplements are often suggested to be able to support weight loss as they can help increase muscle mass, which can lead to increased energy expenditure, due to the higher energy requirement of muscle compared to fat tissue.

Having a greater amount of protein in the diet may help to reduce the appetite and has been linked with reduced energy intake, even when taking the same amount of carbohydrates as usual. Soy protein supplementation, either alongside meals or as a meal replacement, may also be beneficial for health more generally in humans. 

Clinical Studies on Soy Protein

There are several clinical studies published on the health benefits of soy protein. One study published from 1998 looked at how soy protein could help postmenopausal women. All subjects followed a similar diet aimed at reducing cholesterol, and were then supplemented with different types of protein, including soy protein. Improvements in the cholesterol profile in those who took the soy protein, such as higher levels of HDL and lower LDL cholesterol, were noted – suggesting that soy protein can help protect against cardiovascular disease.

There are several other studies available that further investigate the role of soy proteins in reducing cholesterol levels. A review has been published that looked at various papers published on the lipid-lowering effects of soy protein. Although there was not shown to be a consistent decrease in energy or fat intake in all the studies, there was shown to be reductions in LDL cholesterol, suggesting that soy protein can indeed be beneficial for cardiovascular health, and that this effect was greater than that seen from animal protein.

Another study has been published that looked at muscle protein synthesis. This small scale study had human volunteers undergo resistance training and were supplemented with soy, whey or casein protein. The study showed that soy protein supplementation increased muscle synthesis, however to a lesser extent that whey protein.

Weight loss clinical studies

As well as papers showing that soy protein can help reduce cholesterol, there are also studies that have investigated how soy protein could to help reduce body weight.

A 2004 study looked at subjects with a BMI between 27.5 and 35, who were given different weight loss programs, including a high soy protein diet, and their body weight and fat mass recorded at the end of the trial. All groups showed reduced BMI by the end of the trial, however those who had the soy protein supplementation lost a greater amount of weight, and there was also an improvement in the lipid profile of the treated subjects, suggesting that a high soy protein diet could help improve weight loss.

In another double-blind trial, overweight but otherwise healthy people were given a protein supplement coming either from whey or soy protein, or neither as a control. All participants continued to follow their usual diet throughout the study. Although lean body mass did not differ between the groups, those who took the whey protein showed reduced waist circumference, although this effect was not replicated in the soy protein, and soy protein was unable to give a significant weight loss effect.

This final study also found that soy protein did not benefit weight loss. Here, overweight women followed reduced calorie diets for 12 weeks, while half were also given 15 g soy protein daily. Although both the control and experimental group lost weight by the end of the study, there was no significant difference between the groups, and there were no biochemical differences between the groups. This paper also suggested that soy protein was unable to support weight loss at the doses used.

A review looking at soy protein on weight loss is freely available. It looked at a range of studies, and it was shown that taking soy protein was able to help give moderate weight loss, as well as a reduction in blood lipids and cholesterol. 

Soy Protein Side Effects

Although coming from a natural plant source, soy protein does have some possible side effects associated with its use.

Some mild side effects include constipation, bloating or nausea. There may also be concerns about it being associated with some allergies such as those who are allergic to cow’s milk may also have an allergy to soy products as well. There is also some evidence to suggests that soy could contribute to the development of of a peanut allergy in children, although more research is needed to confirm this.

Another health concern is associated with it reducing testosterone levels leading it to being shunned sometimes, particularly by males; not all papers agree with this with some showing no effect on testosterone levels.

Other possible adverse effects have included negative effects on thyroid function, however these effects may not be as serious as sometimes suggested, and although patients with hypothyroidism should ensure they have an adequate iodine supply, they do not need to avoid soy products entirely. 

Soy Protein Products      

There are several soy protein products available. One of the most common products is soy protein powder, costing around £15 for a 1kg bag, with soy protein isolate being slightly more expensive at around £19. Compared with whey protein, this is a slightly cheaper source of protein supplement.

Soy protein can be used in a similar way as other protein supplements. As a powder it can be mixed with milk or water and blended to a shake. Soy protein powders can be used as meal replacements, such as for breakfast or a snack, however it is important to remember to continue following a balanced diet for efficient and healthy weight loss.  Soy protein may be incorporated into a range of different foods and drinks in order to increase an individual’s protein intake.


Soy protein may well be a beneficial addition to the diet, with several health benefits associated with its use, mainly its potential benefit of reducing cholesterol. There is also some evidence supporting its use for weight loss, with the general consensus being that it can support weight loss when used in conjunction with a healthy diet. Although it may not be as effective as whey protein for weight loss, it can be a less expensive and vegetarian alternative, offering an easy way to supplement your diet with protein, which is understood to help support weight loss and improve muscle maintenance.