Nulls: September-October 2020

Omega-6 fatty acid blood levels don’t affect pancreatic cancer risk one way or the other^[1]

What was studied? How genetic factors that influence omega-6 fatty acid blood levels are associated with pancreatic cancer risk.

Why study it? Omega-6 fatty acids, especially arachidonic acid, have been found to promote pancreatic cancer growth in test tube studies. Furthermore, some, but not all, observational studies have found an association between omega-6 fatty acid intake and pancreatic cancer risk.

What was(n’t) found? Genetic markers that naturally boost omega-6 fatty acid blood levels had practically no influence on the risk of developing pancreatic cancer, either positively or negatively. This implies that blood levels of omega-6 fatty acids from dietary or supplementary sources also won’t affect pancreatic cancer risk much.

How null was it? Quite null. The sample size was large, all measures were very close to zero, and the genes the authors chose accounted for a lot of change in omega-6 fatty acid levels, suggesting that the genes were well chosen.

Supplementing vitamin D or omega-3 fatty acids before a stroke didn’t clearly help with outcomes in people who had strokes^[2]

What was studied? Data from the VITAL trial was analyzed to determine if supplementing with 2,000 IU of vitamin D or 1 gram of omega-3 fatty acids before having a stroke improved stroke outcomes afterward.

Why study it? People with low vitamin D levels who have strokes tend to have worse outcomes, but it’s unclear if this is correlation or causation. Also, it’s been found that mice on a high omega-3 diet have improved stroke outcomes, but it’s unclear if this effect carries over to humans. Since there’s not much evidence around how supplementation before a stroke affects outcomes after a stroke, the authors decided to do a secondary analysis of the VITAL trial to see what they could find.

What was(n’t) found? Supplementation of vitamin D or omega-3s before a stroke didn’t clearly improve functional limitations or physical disability outcomes after people had a stroke.

How null was it? Due to limited data, the confidence intervals for these results suggest that the evidence is compatible with a wide range of outcomes. So, while there’s no clear strong effect, more evidence would be useful, especially for omega-3 supplementation, which was suggestive of a possible benefit, while vitamin D was less suggestive of a benefit.

Sugary drinks didn’t affect cognition, mood, or satiety in the short term^[3]

What was studied? The effect of three drinks with different glycemic indices (sucrose, isomaltulose, and non-caloric sucralose as the control) on measures of cognition, mood, and satiety 60 minutes after ingesting the beverage.

Why study it? Animal studies have found that boosting blood sugar leads to higher brain activity in areas related to cognition and memory, but human studies have yielded mixed results. One reason for this may be due to differences in glycemic response due to the type of sugar used and other meal components. This study aimed to address these concerns by manipulating glycemic response through the type of sugar given while the participants were fasting.

What was(n’t) found? There was no strong difference between the two caloric sweeteners and the control drink on any metric of cognition, mood, or satiety.

How null was it? This study was quite small and measured a lot of outcomes, leaving open the possibility for false negatives. Also, glycemic response was not measured before the cognitive tests because the researchers thought that the finger prick could affect performance.

Supplementing vitamin D may not lower type 2 diabetes risk in Europeans^[4]

What was studied? The relationship between vitamin D levels and the risk of developing type 2 diabetes was studied in two ways: through meta-analyzing observational studies to look at correlation, and through Mendelian randomization to explore causation in a large European population.

Why study it? Observational studies suggest that vitamin D levels are correlated with type 2 diabetes risk, but the limited number of clinical trials that have directly explored the matter haven’t found strong evidence that vitamin D causes lower risk. The topic has also been studied in previous Mendelian randomization studies, but with mixed results for two possible reasons: not looking at vitamin D metabolites, and having weaker predictive genes than currently exist. This study sought to address both of these problems in its Mendelian randomization analysis.

What was(n’t) found? While a clear association between vitamin D levels and diabetes risk was found by meta-analyzing observational studies, the Mendelian randomization analysis found no causal link between any vitamin D levels (or its metabolites) and diabetes risk. This suggests that the link the observational studies found is correlation, not causation, and that raising vitamin D levels through supplementation wouldn’t affect type 2 diabetes risk much.

How null was it? The evidence is pretty strong for European populations since that’s the population involved in the Mendelian randomization study. However, these results may not apply to other populations.

No clear evidence of ginseng’s effect on weight, fat, or waist circumference

What was studied? The effect of Panax ginseng supplementation on weight and other anthropometric measurements in randomized controlled trials was meta-analyzed.

Why study it? The effect of ginseng on anthropometric measurements has been measured in several studies, but there hasn’t been a meta-analysis of these studies to date.

What was(n’t) found? There was no clear effect of supplementation on weight, waist circumference, or body fat percentage.

How null was it? Most of the trials to date have been small, heterogeneous, and short-term (under three months). Longer, larger trials would be useful.

Long-chain omega-3 fatty acid supplementation for pregnant or breastfeeding women had no strong effect on their children’s cognition or birthweight^[5]

What was studied? The cognition and birthweight of children whose mothers supplemented fish oil while pregnant or breastfeeding, through meta-analyzing randomized, controlled trials.

Why study it? The long-chain omega-3 polyunsaturated fatty acids found in fish oil, especially docosahexaenoic acid (DHA), have been shown to be essential in learning and memory in primate experiments, and in neuronal development in other animal experiments. However, whether supplementation will help the cognition of developing human minds is still an open question.

What was(n’t) found? There was no statistically significant effect on any cognitive measure or birthweight.

How null was it? The result was not strongly null for three big reasons. The first was that most of the trials were small and had large dropout rates, making it difficult to see even moderate effects in many cases. Second, supplementation would probably have the greatest effect before 20 weeks of gestation, but none of the examined trials started supplementation that early (although this is a big logistical challenge and would be hard to do practically). Finally, while there were no statistically significant effects observed, the 95% confidence intervals suggest that the data were mostly compatible with small effects on attention and birthweight. Thus, there may be some small effect in these two domains.

DHA supplementation didn’t have a big effect on the executive function of school-aged children^[6]

What was studied? The effect of 300 milligrams of daily docosahexaenoic acid (DHA) supplementation on school-aged children’s executive function, which is the ability to exert self-control, shift focus consciously, and process information.

Why study it? DHA accumulates in parts of the brain that develop rapidly in childhood through mid-adolescence and are important for executive function. DHA levels in certain parts of the brain have also been found to be associated with stronger brain activation. However, it’s still unclear whether supplementation with DHA could actually improve functioning in school-aged children.

What was(n’t) found? No effect on any metric of executive function, including cognitive flexibility and working memory, was found.

How null was it? The trial was well designed and preregistered with a clear primary outcome. However, the dose may have been too small or the time frame too short to see a clear effect. Confirmation of these results at higher doses with a wider battery of cognitive tests for confirmation may be helpful.

Zinc supplementation, both with and without iron co-supplementation, has no clear effect on childhood motor or mental development^[7]

What was studied? Randomized controlled trials exploring the effect of zinc supplementation on development metrics in children up to 5 years old were meta-analyzed.

Why study it? Zinc is important for neuronal development, but about 7.5% of children in high-income countries and over 50% in lower-income countries may be zinc deficient. However, zinc can lower iron blood levels because the two elements interfere with each others’ absorption.

What was(n’t) found? No measure of childhood motor or mental development was strongly affected by zinc supplementation, whether or not it was co-administered with iron.

How null was it? The authors were unable to meta-analyze trials in which the children were older than one year due to major differences between the studies they found. They rated the quality of evidence for zinc’s effect on children under one year as being low to moderate. However, it was harder to evaluate zinc’s longer-term impact, although they did assess the longer-term data qualitatively and found no major effect.

Leucine supplementation doesn’t help keep muscle on immobilized limbs^[8]

What was studied? How 15 grams of leucine supplementation per day for seven days in young, healthy men affected leg strength and muscle mass when the leg was immobilized.

Why study it? There are mechanistic reasons for thinking that branched-chain amino acids like leucine could preserve muscle mass during times of immobilization (e.g., after injury or during illness). However, previous research at half the dose didn’t work in younger men, but higher doses of an amino acid cocktail containing 12 grams of leucine daily have been found to work. This led the authors to suspect that higher doses of leucine alone could preserve strength and muscle mass in younger people, too.

What was(n’t) found? There was no effect of high-dose leucine supplementation on fat-free mass preservation or strength in the immobilized leg compared to placebo.

How null was it? This was a somewhat small study involving 16 people. The study was powered to detect moderate changes in fat-free mass (the study’s primary outcome), but smaller effects cannot be ruled out. This problem may have been compounded by the use of DXA to measure fat-free mass, which isn’t quite as precise as MRI or computed tomography.

A shot of nitrate-rich beetroot juice didn’t help with repeated sprint performance^[9]

What was studied? How a shot of nitrate-rich beetroot juice containing about 6 mmol of nitrate three hours prior to sprinting affected performance in ten 40-meter shuttle sprints with 30 seconds of rest in between each sprint.

Why study it? Nitrate supplementation has been mostly studied in specific activities like cycling and kayaking. However, there’s less evidence looking at its effect on exercise patterns that carry across activities, like multiple short sprints.

What was(n’t) found? While plasma nitrate levels were boosted six-fold in the nitrate-rich condition compared to placebo, there was no clear effect on sprint time or performance degradation over time.

How null was it? The study was relatively small (involving 16 men) and didn’t involve a power calculation, leaving the possibility of whether or not this is a false negative open to question.

Vitamin D plus omega-3s don’t help much with AMD^[10]

What was studied? How 2,000 IU of vitamin D₃ and 1 gram of omega-3 fatty acids daily for over five years affected development of age-related macular degeneration (AMD; a progressive disease involving the retina in the eye that can lead to severe vision loss) or its progression in people who already had it.

Why study it? Observational evidence has found that people with lower blood levels of vitamin D or omega-3s may be at higher risk of AMD. However, there’s little experimental evidence addressing whether supplementation could help.

What was(n’t) found? Neither vitamin D nor omega-3 supplementation helped prevent AMD from occurring or progressing.

How null was it? This was a preplanned secondary study stemming from the VITAL trial. And even though the trial had a huge sample size and lasted years, there weren’t many AMD events that occurred. This led to pretty wide uncertainty in how helpful supplementation could be. The evidence from this study is roughly compatible with a 20–30% increase or decrease in AMD risk with supplementation. So, while this study provides solid evidence that huge effects don’t exist, the evidence it provides is compatible with smaller risk reductions (or increases!).

Adding MK-7 to calcium and vitamin D didn’t improve bone health in postmenopausal women^[11]

What was studied? Postmenopausal women with osteopenia were all given about 1,500 IU of vitamin D₃ and 800 milligrams of calcium daily for three years. They were also randomized to either placebo or 375 mcg MK-7 (a type of K₂ vitamin) on top of the first two supplements. Bone mass, structure, and markers of bone turnover were measured.

Why study it? While calcium and vitamin D supplementation is recommended by the World Health Organization for people with osteopenia and osteoporosis, studies looking at MK-7’s effect on bone mass have been less clear, warranting a longer-term study.

What was(n’t) found? Bone mineral density at several sites declined to roughly the same extent in the MK-7 and placebo groups. Bone turnover markers also weren’t clearly different between the two groups. Bone microstructure was better in the MK-7 group at one year after supplementation, but not at three years.

How null was it? This study was originally planned for one year, but was extended when a Dutch study using a lower dose (180 mcg) of MK-7 found improved bone density after three years and the researchers wanted to see if they could replicate the results. The sample size of the Dutch study was larger, which could explain why it found results, while this study did not. The women in the Dutch study also had a higher average starting bone mineral density and were younger on average (60, compared to 67 in this study).

Vitamin K^[12]~2^[12]~ didn’t help boost bone mass or reduce arterial calcification in people with diabetes^[12]

What was studied? The effect of 360 mcg of vitamin K~2 ~daily for six months on slowing bone loss and arterial calcification (arteries hardening due to calcium buildup) in people with diabetes and cardiovascular disease.

Why study it? People with diabetes have an increased risk of fractures and calcification, so finding ways to reduce the risk of these processes in people with diabetes would be helpful. Two proteins in the body could slow down these processes: Matrix Gla protein and osteocalcin. Both of these require vitamin K to be fully functional, though. This suggests that vitamin K supplementation, especially vitamin K₂ which lasts longer in the body and is absorbed more than vitamin K₁, could be beneficial.

What was(n’t) found? Vitamin K₂ supplementation did not slow bone density loss from the spine or slow arterial calcification in major arterial beds compared to placebo in people with diabetes and cardiovascular disease.

How null was it? Somewhat, but follow-up would be useful since this was a small study of 68 people. It was also a secondary analysis of a previous study designed to test the effects of supplementation on femoral artery calcification, as opposed to calcification of several arterial beds. Secondary analyses normally should have less confidence placed in them, but confidence in this finding is bolstered a bit since the original study found little evidence of an effect on calcification and this analysis was pre-planned. It’s also possible that future studies involving participants with more severe vitamin K deficiency may find an effect.

Low-quality evidence suggests vitamin D supplementation doesn’t clearly improve blood lipids in people with the metabolic syndrome^[2]

What was studied? Trials measuring blood lipids in people with the metabolic syndrome were meta-analyzed to determine if supplementing with vitamin D affected lipid levels.

Why study it? Low levels of vitamin D have been linked to increased risk and severity of the metabolic syndrome in observational studies, but the limited number of controlled trials examining the issue yielded mixed results, warranting a meta-analysis.

What was(n’t) found? There was no clear impact of vitamin D supplementation on lipid levels in people with metabolic syndrome.

How null was it? Not particularly null. Only seven trials were found, of which four were meta-analyzed. The authors rated the quality of evidence as low or very low, depending on the outcome. More high-quality studies are needed to explore this issue.