Soy Protein After 40: Benefits, Myths and Practical Use

After 40, maintaining muscle becomes a priority, and protein choices start to matter more. Soy protein is a popular plant option, but it is surrounded by concerns about hormones and oestrogen. Our editorial team looks at what research says about soy for people in midlife and beyond.

Why protein needs change after 40

From around the fourth decade, muscle mass and strength begin a gradual decline, a process that accelerates later in life and is known as sarcopenia when it becomes clinically significant. Reduced physical activity, hormonal changes and a phenomenon called anabolic resistance all contribute.

Anabolic resistance means that ageing muscle responds less strongly to the same dose of protein. Younger adults may maximise muscle protein synthesis with about 20 g of high-quality protein per meal, whereas older adults often need closer to 30 to 40 g to achieve a similar response.

Expert groups such as the PROT-AGE study group (Bauer et al., 2013) suggested that older adults aim for 1.0 to 1.2 g of protein per kilogram of body weight per day, and more if they are active or recovering from illness. This is higher than the general UK reference nutrient intake of 0.75 g per kilogram.

Resistance training remains the most powerful tool against muscle loss. Protein supports the adaptation, but it cannot replace the training stimulus itself.

How soy compares with whey

Soy is one of the few plant proteins that provides all essential amino acids in proportions suitable for human needs. Its digestibility score is high, close to that of animal proteins, which makes it a credible alternative for vegetarians, vegans and people who avoid dairy.

However, soy contains less leucine than whey, and it is digested at a moderate rate. Acute studies in older men (Yang et al., 2012) found that whey stimulated muscle protein synthesis more strongly than the same dose of soy, although higher doses of soy narrowed the gap.

Over longer training periods the differences shrink. A meta-analysis by Messina and colleagues (2018) found that soy and animal protein supplements produced similar gains in strength and lean mass when combined with resistance training. Total daily protein intake appears to matter more than the source.

Practically, people choosing soy may benefit from slightly larger servings, around 30 to 40 g per meal, to compensate for the lower leucine content.

Myth: soy lowers testosterone in men

Soy contains isoflavones, plant compounds that can bind weakly to oestrogen receptors. This has fuelled fears that soy "feminises" men or lowers testosterone, concerns often amplified by isolated case reports involving extremely high intakes.

Controlled research does not support these fears at normal intakes. A meta-analysis by Hamilton-Reeves and colleagues (2010) found no significant effects of soy protein or isoflavones on testosterone, free testosterone or SHBG in men. An expanded meta-analysis by Reed and colleagues (2021) reached the same conclusion, including for oestrogen levels.

Isoflavones are not the same as human oestrogen. They are much weaker and act selectively, behaving more like natural SERMs in some tissues. The amounts in a typical serving of soy protein isolate are also lower than in many traditional soya foods.

The few reported cases of hormonal effects involved consumption of very large quantities of soya products daily for long periods. Normal use as part of a varied diet is not comparable.

Soy for women in midlife

For women approaching or past the menopause, soy has attracted interest for hot flushes and bone health. Some trials suggest that isoflavones modestly reduce the frequency of hot flushes, although results are inconsistent and effects are smaller than with HRT.

A common concern is breast cancer risk. Large observational studies, particularly from Asia, have generally found that soya food intake is not associated with increased breast cancer risk and may be associated with lower recurrence. Women with a history of hormone-sensitive cancer should still discuss concentrated isoflavone supplements with their oncology team.

Soy protein isolates used in sports nutrition are primarily a protein source rather than an isoflavone treatment. For a woman in her forties or fifties who wants to maintain muscle with resistance training, soy can be a practical choice.

Calcium-set tofu and fortified soya drinks additionally contribute calcium, supporting bone health when combined with weight-bearing exercise and adequate vitamin D.

Heart health and practical tips

Soy protein can modestly lower LDL cholesterol, typically by a few per cent. In the past, the US FDA permitted a heart-health claim for soy protein, but in 2017 it proposed revoking it because evidence was less consistent than originally thought. In the UK and EU, no authorised health claim exists for soy protein and cholesterol.

Replacing processed red meat with soya foods may still support heart health indirectly, by lowering saturated fat intake and increasing fibre.

• Aim for about 30 to 40 g of protein per main meal after 40, spread over three or four meals.
• Choose soy protein isolate for higher protein content per serving.
• Combine with regular resistance training at least twice a week, as advised in UK physical activity guidelines.
• Consider blending soy with other plant proteins, such as pea, to improve the amino acid profile.

People with soya allergy should avoid these products, and those taking levothyroxine should separate soy intake from their tablet, as soy may reduce absorption. A pharmacist can advise on timing.

Editorial conclusions

Protein needs rise after 40 because of anabolic resistance, and soy is a complete plant protein that can help meet them.

Soy is slightly less potent than whey per gram for muscle protein synthesis, but long-term training outcomes are similar when total protein intake is adequate.

Robust meta-analyses show that soy does not lower testosterone in men, and it is a reasonable choice for midlife women too, combined with resistance training.

For further reading, our editorial team suggests articles on preventing age-related muscle loss, comparing plant protein powders and creatine for adults over 50.

References

1. Messina M, Lynch H, Dickinson JM, Reed KE. No difference between the effects of supplementing with soy protein versus animal protein on gains in muscle mass and strength in response to resistance exercise. Int J Sport Nutr Exerc Metab. 2018;28(6):674–685.
2. Hamilton-Reeves JM, Vazquez G, Duval SJ, et al. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010;94(3):997–1007.
3. Reed KE, Camargo J, Hamilton-Reeves J, Kurzer M, Messina M. Neither soy nor isoflavone intake affects male reproductive hormones: an expanded and updated meta-analysis of clinical studies. Reprod Toxicol. 2021;100:60–67.
4. Yang Y, Churchward-Venne TA, Burd NA, et al. Myofibrillar protein synthesis following ingestion of soy protein isolate at rest and after resistance exercise in elderly men. Nutr Metab (Lond). 2012;9(1):57.
5. Bauer J, Biolo G, Cederholm T, et al. Evidence-based recommendations for optimal dietary protein intake in older people: a position paper from the PROT-AGE Study Group. J Am Med Dir Assoc. 2013;14(8):542–559.
6. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376–384.
7. https://anabolizantes-espana.com/