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Nulls: March-April 2021

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Here’s a very quick summary of some randomized controlled trials (RCTs) and meta-analyses of RCTs that were published in March and April of 2021 that didn’t find evidence of an effect. This is known as a null effect.

Keep the following in mind when interpreting a null effect:

  • While one study can provide evidence that something doesn’t work, it doesn’t prove it. Similar, repeatable results from multiple studies make for stronger evidence, whether the finding is positive or negative.

  • Not all null effects are the same. A meta-analysis of low-quality studies or a small clinical trial usually won’t provide strong evidence, whether the finding is positive or negative.

  • The population matters. For instance, the lack of an effect in healthy young people doesn’t necessarily mean that an intervention wouldn’t work in people who are older and have a specific health condition.


Vitamin D supplementation does not seem to reduce the risk of breast cancer[1]

What was studied? Seven randomized controlled trials involving almost 20,000 women were meta-analyzed to explore the effect of vitamin D supplementation on the risk of developing breast cancer. Doses ranged from 400-3,000 IU daily or bolus doses of up to 100,000 IU monthly. Studies lasted from 20 weeks to over 5 years.

Why study it? Observational evidence suggests that higher vitamin D intake is correlated with reduced breast cancer risk. However, randomized controlled trials have yielded unclear results.

What was(n’t) found? No clear effect on the risk of developing breast cancer or on mammography density was found.

How null was it? The quality of evidence for breast cancer risk was rated as moderate, and the quality for mammography density was rated as low. Furthermore, while some of the included studies were long term, many were under a year, which isn’t a very long time period when it comes to detecting breast cancer. Longer-term studies could shed more light on this issue.

Cardiovascular disease

Montmorency tart cherry juice didn’t impact CVD risk markers in middle-aged people[2]

What was studied? Researchers investigated whether three months of drinking a Montmorency tart cherry juice providing a daily dose of around 70 mg of anthocyanins and 170 mg of total phenolics affected cardiovascular risk markers people in ages 40–60 at risk for diabetes. The population also was relatively sedentary and consumed under 5 servings of fruits and vegetables daily.

Why study it? While research has been done on the effects of tart cherry juice on risk factors for cardiovascular disease, none have focused on a disease-free middle-aged population at risk of diabetes.

What was(n’t) found? No statistically significant effect on any risk marker was found. Risk markers included blood pressure, glycemic control measures, and blood lipids.

How null was it? The study was well powered to detect a 5 mmHg difference in systolic blood pressure between the placebo and tart cherry groups, ruling out any big effects on this important risk factor. While there were no trends of note in the other markers, there is one major limitation to the study: compliance was assessed using self-reported tally sheets. This means that the lack of effect could possibly be chalked up to lack of compliance coupled with inaccurate participant record keeping.

Diets & foods

Giving people free tasty veggies doesn’t form a helpful veggie-eating habit[3]

What was studied? This study was designed to evaluate whether providing washed, portioned vegetables that participants especially liked for eight weeks would form a positive habit of eating vegetables in people with a BMI over 25 with low baseline vegetable intake.

Why study it? Repeated exposure to foods has been shown to increase affinity for the food, which may, in turn, form a positive habit of eating more of the food in lieu of other potentially higher-calorie, nutrient-poor foods. Given that a lot of Americans don’t meet their vegetable intake recommendations, and vegetables aren’t calorie-dense, the researchers wanted to see if they could influence the creation of a positive habit of eating vegetables by providing eight weeks of vegetables that participants especially liked and then see if they picked up the habit of eating vegetables afterwards.

What was(n’t) found? The researchers used an objective blood test to measure vegetable intake and found that intake increased only while the vegetables were provided. After the researchers stopped providing vegetables, the participants reverted back to their regular low vegetable intake.

How null was it? Somewhat null, but the study leaves some unanswered questions about whether varying duration and vegetable type may help improve the likelihood of participants picking up a vegetable-eating habit.

Anything else? This study can be spun another way: it provides evidence that providing free vegetables and monitoring intake in a clinical setting does increase vegetable intake, just not after provisioning ceased. So, it’s not completely null, but it does suggest that a lot of work and resources need to be poured into changing behaviors!

Healthy aging & longevity

Selenium supplementation didn’t help with musculoskeletal health in postmenopausal women[4]

What was studied? Researchers evaluated whether 50 mcg or 200 mcg of sodium selenite daily for 6 months affected bone turnover markers or physical functioning in postmenopausal women.

Why study it? Previous observational studies identified an association between selenium levels and bone health. Selenium’s potential role in bone health has also been bolstered by animal studies and mechanistic considerations. This is why selenium supplementation could help improve bone health in older people. This is especially important for this population because many older people are selenium deficient.

What was(n’t) found? The primary outcome of a marker of bone degradation (the urinary ratio of N-terminal cross-linking telopeptide of type I collagen, a bone breakdown byproduct, to creatine) was not affected by either dose of selenium compared to placebo. There were occasional small effects seen in some of the secondary outcomes, but they could have been statistical blips, and the effect sizes were clinically irrelevant.

How null was it? This was a particularly well-designed and well-reported study. While it used surrogate markers for bone health as opposed to what ultimately matters—fracture incidence—the markers used in the study were well validated. While this study didn’t have a sample size big enough to detect small effects, it strongly suggests that selenium supplementation didn’t make much of a difference.

Infants, children & teenagers

PUFAs many not help with with ADHD in children and adolescents[5]

What was studied? Researchers meta-analyzed a total of 31 randomized controlled trials. The studies looked at how either 8 weeks or 12 months of omega-3 or omega-6 fatty acid supplementation affected ADHD symptoms in children or adolescents.

Why study it? Observational evidence suggests that lower essential fatty acid levels are correlated with ADHD incidence, and there are mechanistic reasons to think raising levels may help. While a lot of clinical studies have been run examining the effects of PUFA supplementation on ADHD, the results are mixed, suggesting that meta-analysis may help clarify the situation.

What was(n’t) found? There was no clear effect of supplementation on any ADHD core symptom as rated by either parents or teachers. No improvement in quality of life was observed, either.

How null was it? While there’s a lot of studies looking at this subject, the authors rated the evidence base as being low or very low quality, mainly due to the included studies’ high risk of bias. Higher quality evidence is needed.

Muscle gain & exercise

Carbohydrate mouth rinse had no effect on strength or muscular endurance in resistance-trained women[6]

What was studied? This study was designed to evaluate whether 6%, 12%, or 18% carbohydrate mouth rinses before a strength test (bench press 1RM) or a muscular endurance test (40% of bench press 1RM to failure) improved outcomes for resistance trained women.

Why study it? Carbohydrate mouth rinses have been studied mostly for aerobic exercise, not strength training. The studies that have examined the issue have not focused exclusively on resistance-trained women.

What was(n’t) found? There was no effect on either muscular strength or endurance.

How null was it? No power calculation was done, making it difficult to interpret these results. While not statistically significant, the evidence lightly suggests that there may have been an effect on total repetitions, with slightly more reps being able to be cranked out with higher carbohydrate concentrations. However, this would need to be replicated.

Blueberry supplementation had no effect on hypoxic running performance[7]

What was studied? Researchers studied how supplementing 24 grams of blueberry powder three times per day for four days (rough equivalent of 500 grams of fresh blueberries per day) affected a 30-minute time trial under low-oxygen conditions (%O2 = 15.5%) in recreational runners.

Why study it? Fatigue during running may come about, in part, due to oxidative stress. Blueberry supplementation could help mitigate this, but past research has been mixed. Metabolic and oxidative stress could be increased under low-oxygen conditions, which could exaggerate the effect size of blueberry supplementation, raising the chances of actually seeing an effect.

What was(n’t) found? Blueberry supplementation didn’t affect time trial performance, nor did it affect most biomarkers that were measured after the time trial, including inflammatory markers and salivary cortisol levels.

How null was it? A power calculation wasn’t performed and many factors were measured, leaving room for false negatives. However, this study does not provide much, if any, evidence concerning the efficacy of short-term blueberry supplementation.

Anything else? The one possible exception to the null outcomes was post-exercise lactate levels, the rise of which were blunted by supplementation. Further research is needed to explore whether this effect is reproducible and whether it has any relevance to performance or recovery outcomes.

High-dose vitamin D supplementation didn’t boost beginner’s gains in vitamin D-deficient healthy young men[8]

What was studied? Whether supplementing 8,000 IU vitamin D3 for 12 weeks helped untrained young men boost their gains during a 12-week thrice-weekly whole body resistance training program. Participant baseline vitamin D levels were under 20 ng/mL (50 nmol/L).

Why study it? Two previous studies looked at the effects of vitamin D on physical performance improvements, and both came up empty-handed. However, both studies involved participants who had baseline levels above 20 ng/mL.

What was(n’t) found? There was no difference between vitamin D and placebo groups for any exercise, with the vitamin D group possibly having slightly weaker gains in the chest press and seated row, compared to the placebo group.

How null was it? Pretty null, although there’s some suggestive evidence that lower vitamin D doses could be more effective. Also, the participants were all given 20 grams of whey protein after each workout, possibly masking any effect vitamin D may have had.

A caffeine mouth was didn’t help with running performance in slow caffeine metabolizers[9]

What was studied? Researchers evaluated whether a mouth rinse containing 300 mg of caffeine used right before a 10-km run could improve running or jumping performance in well-trained runners who were slow caffeine metabolizers. All participants were regular caffeine users at baseline.

Why study it? Cytochrome P450 1A2 is the main enzyme that metabolizes caffeine. People who carry one more more “C” mutations in a part of the gene for that enzyme metabolize caffeine more slowly, which usually leads to a worse ergogenic response to caffeine. Previous studies have found that caffeine mouth rinses could activate parts of the brain related to sports performance, and could, in theory, help slow metabolizers perform better because this activity is independent of 1A2 caffeine metabolism.

What was(n’t) found? No effect on time trial performance or jump performance was found.

How null was it? The sample size was quite small and there was a lot of variability in the response, leaving room for more research with larger samples.

Transcranial alternating current stimulation does not seem to affect explosive power[10]

What was studied? This study tested whether the explosive power of healthy adults with at least three years of sports experience was affected by transcranial alternating current stimulation.

Why study it? Electrical oscillations in certain parts of the brain are necessary for movement. A relatively new method of electrical brain stimulation called transcranial alternating current stimulation has been found to modify simple motor performance. Other types of stimulation have been researched in the context of sports performance, but this was the first study on this kind of stimulation.

What was(n’t) found? Ten minutes of transcranial alternating current stimulation had no clear benefit on vertical jump, upper body power, or handgrip strength, compared to a sham treatment.

How null was it? This was a pilot study, so these null results should be taken as preliminary. Also, there are many variables to account for when applying transcranial alternating current stimulation, many of which may show benefits in the future. The authors also raise the possibility that younger people may benefit more from the technique than older people, which is noteworthy because the study population included a wide range of ages.

Pain, joints & bones

Vitamin D plus calcium supplementation didn’t help improve bone mineral density in people with cirrhosis[11]

What was studied? Researchers evaluated whether 60,000 IU of vitamin D3 per week for two months, followed by 60,000 IU per month for 10 more months, improved bone mineral density in people with cirrhosis of the liver. An additional 500 mg of daily elemental calcium was also given to participants in the form of calcium carbonate.

Why study it? The liver plays a pivotal role in activating vitamin D, and people with liver cirrhosis are almost universally vitamin D deficient. While some professional societies suggest supplementing with vitamin D to people with liver disease, there’s not much evidence that supplementation can help with bone health in this population.

What was(n’t) found? No effect on lumbar spine or hip bone mineral density was found.

How null was it? The study was powered for the change in vitamin D level, making it hard to interpret this null result. However, this study doesn’t suggest much of an effect.

Vitamins & minerals

Zinc supplementation has little apparent influence on brain-derived neurotrophic factor blood level[12]

What was studied? Five studies reporting how zinc supplementation affected blood levels of brain-derived neurotrophic factor (BDNF) were included in this meta-analysis. The participants of the included studies had depression, obesity, diabetic retinopathy, or premenstrual syndrome. All the studies lasted 12 weeks, and used 25–30 mg of daily elemental zinc, usually in the gluconate salt form.

Why study it? Zinc may play a role in several conditions related to neurological functioning, from depression to neurodegenerative diseases. However, how zinc could influence these conditions isn’t exactly clear. The authors of this meta-analysis decided to meta-analyze the literature to see whether BDNF is a possible candidate.

What was(n’t) found? There was no statistically significant influence of zinc supplementation on blood BDNF levels.

How null was it? Not very. For one, the 95% confidence interval was mostly consistent with an increase in BDNF. Second, the authors found only a small handful of trials reporting on how BDNF was affected by zinc supplementation. Finally, some studies involved other interventions, like antidepressants, that could influence BDNF levels and which the authors couldn’t adjust for due to the paucity of data. More research is needed before firm conclusions can be drawn.

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See other articles with similar topics: Nulls.

See other articles in Issue #81 (July 2021) of Study Deep Dives.

Other Articles in Issue #81 (July 2021)


  1. ^ Ziyi Li, et al. Effect of Vitamin D Supplementation on Risk of Breast Cancer: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Front Nutr. (2021)
  2. ^ Rachel Kimble, et al. The Influence of Tart Cherry ( Prunus cerasus, cv Montmorency) Concentrate Supplementation for 3 Months on Cardiometabolic Risk Factors in Middle-Aged Adults: A Randomised, Placebo-Controlled Trial. Nutrients. (2021)
  3. ^ Shanon L Casperson, et al. Consumption of a Variety of Vegetables to Meet Dietary Guidelines for Americans' Recommendations Does Not Induce Sensitization of Vegetable Reinforcement Among Adults with Overweight and Obesity: A Randomized Controlled Trial. J Nutr. (2021)
  4. ^ Jennifer S Walsh, et al. Effect of selenium supplementation on musculoskeletal health in older women: a randomised, double-blind, placebo-controlled trial. Lancet Healthy Longev. (2021)
  5. ^ Mina Nicole Händel, et al. Efficacy and Safety of Polyunsaturated Fatty Acids Supplementation in the Treatment of Attention Deficit Hyperactivity Disorder (ADHD) in Children and Adolescents: A Systematic Review and Meta-Analysis of Clinical Trials. Nutrients. (2021)
  6. ^ Raci Karayigit, et al. Different Doses of Carbohydrate Mouth Rinse Have No Effect on Exercise Performance in Resistance Trained Women. Int J Environ Res Public Health. (2021)
  7. ^ Jason P Brandenburg, Luisa V Giles. Blueberry supplementation reduces the blood lactate response to running in normobaric hypoxia but has no effect on performance in recreational runners. J Int Soc Sports Nutr. (2021)
  8. ^ Lauri Savolainen, et al. Vitamin D supplementation does not enhance resistance training-induced gains in muscle strength and lean body mass in vitamin D deficient young men. Eur J Appl Physiol. (2021)
  9. ^ Nayra Figueiredo, et al. Acute caffeine mouth rinsing does not improve 10-km running performance in CYP1A2 C-allele carriers. Clin Nutr ESPEN. (2021)
  10. ^ Andreina Giustiniani, et al. Transcranial Alternating Current Stimulation (tACS) Does Not Affect Sports People's Explosive Power: A Pilot Study. Front Hum Neurosci. (2021)
  11. ^ Indu Grover, et al. Effect of Vitamin D Supplementation on Vitamin D Level and Bone Mineral Density in Patients With Cirrhosis: A Randomized Clinical Trial. Am J Gastroenterol. (2021)
  12. ^ Fatemeh Jafari, Hamed Mohammadi, Reza Amani. The effect of zinc supplementation on brain derived neurotrophic factor: A meta-analysis. J Trace Elem Med Biol. (2021)