The cause of some people having high anxiety (without a cause) is typically believed to be a combination of genetics (DNA), environment and the microbiome. DNA and the microbiome evolved together. This is actually a good aspect, because while you cannot change your DNA, you can change your microbiome.
When a major illness or disability occurs, anxiety may occur — which appears to be environmentally caused (typically with depression). I will attempt to filter the studies to anxiety-prone individuals (and families), an area that is likely understudied. Studies dealing with illness/injury induced anxiety (and depression) were excluded, for example “mild asthma, there was a 43.5% increased risk of anxiety.”
Why am I looking at this on a CFS blog? The answer is simple. Anxiety prone people are more likely to get CFS. It has a role in the development of IBS
- “The findings suggest that self-reported anxiety and depression provide a twofold risk for IBS onset.” 
Are not anxiety and depression the same thing?
“The results of this study established that the development of anxiety and depressive disorder symptoms of adolescents from the general community occurs as two distinct disorders with parallel growth processes, each with their own unique growth characteristics.’ 
An estimated 15% of persons reported frequent (> or = 14 days in the past 30 days) anxiety symptoms. After adjusting for frequent depressive symptoms and sociodemographic characteristics, those with frequent anxiety symptoms were significantly more likely than those without to report fair or poor general health (vs. excellent, very good, or good general health), frequent physical distress, frequent activity limitations, frequent sleep insufficiency, infrequent vitality, frequent mental distress, and frequent pain.” 
DNA and Anxiety
Studies suggest that DNA plays a minor role for anxiety. This implies that families with a tendency to anxiety is likely due to microbiome (gut bacteria) and similar diet
- “evidence of modest heritability (7.2%; P = 0.03)… was also only detected for generalized anxiety disorder (GAD) symptoms,” 
- “GAD has the lowest diagnostic reliability of the anxiety disorders and is poorly recognized in clinical practice.”
- Don’t worry; be informed about the epigenetics of anxiety. Epigenetics is DNA being turned off or on due to environmental influences.
Competing targets of microRNA-608 affect anxiety and hypertension.“Additionally, minor allele heterozygote and homozygote subjects showed reduced cortisol and elevated blood pressure, predicting risk of anxiety and hypertension. Parallel suppression of the conserved brain CDC42 activity by intracerebroventricular ML141 injection caused acute anxiety in mice”
Food and Microbiome
Food influences the microbiome, so I will look at both. ” we review recent findings in the relationship between intestinal microbes and brain function, such as anxiety…. We highlight the advances in modulating brain development and behavior by probiotics, prebiotics, and diet through the gut microbiota-brain axis.” 
“Furthermore, higher intakes of n-6 fatty acids and linoleic acid were associated with decreased likelihood of self-reported diagnosed anxiety and higher intakes of n-9 fatty acids and oleic acid were associated with increased likelihood of self-reported diagnosed anxiety.” 
- Impaired fear extinction retention and increased anxiety-like behaviours induced by limited daily access to a high-fat/high-sugar diet in male rats: Implications for diet-induced prefrontal cortex dysregulation . ” Despite increasing evidence that over-consumption of palatable high-fat/high-sugar “junk” foods leads to adverse neurocognitive outcomes, little is known about the effects of palatable diets on emotional memories and fear regulation…Adverse effects of HFHS diets on the mechanisms of fear regulation may precipitate a vulnerability in obese individuals to the development of anxiety disorders.”
- High fat diet induced-obesity facilitates anxiety-like behaviors due to GABAergic impairment within the dorsomedial hypothalamus in rats.
- Carbohydrate-enriched diet predispose to anxiety and depression-like behavior after stress in mice 
- High-Fat Diet Induced Anxiety and Anhedonia: Impact on Brain Homeostasis and Inflammation .
“In conclusion, our study showed that hyper-anxiety associated with prediabetic condition is ameliorated[reduced] by resveratrol [grape seed extract] through modulation of sirtuins,”
- Grape Seed Proanthocyanidins Attentuate Anxiety-Like Behavior in an Experimental Model of Dietary-Induced Hypercholesterolemia in Rats.
“The adjusted mean of anxiety score in subjects with high dietary diversity score was significantly lower than those with low dietary diversity score. Dietary diversity score was found to be inversely associated with anxiety. However, the causality between anxiety and dietary diversity could not be determined.” 
” The mean heart rates were also significantly different between vitamin C group and placebo control group. Present study results not only provide evidence that vitamin C plays an important therapeutic role for anxiety but also point a possible use for antioxidants in the prevention or reduction of anxiety.” 
“lower anxiety in males was related to a vegan diet and daily fruit and vegetable intake.” 
“Studies on probiotic supplementation in mice and in humans provides compelling evidence of microbial regulation over stress and anxiety induced neuroendocrine signaling. In particular, strains of Lactobacillusand Bifidobacterium are found to exert a profound anxiolytic influence through the production of γ-aminobutryic acid (GABA), 5-HT, and SCFAs, and by dampening HPA adrenergic reaction [26,34,38,41,42].”
- Lactobacillus plantarum attenuates anxiety-related behavior and protects against stress-induced dysbiosis in adult zebrafish.
- Ingestion of Lactobacillus strain reduces anxiety and improves cognitive function in the hyperammonemia rat. – Lactobacillus helveticus strain NS8
- ” L. helveticus R0052 and B. longum R0175 taken in combination display anxiolytic [anti-anxiety]-like activity in rats and beneficial psychological effects in healthy human volunteers.”  — combination is available here. EU review
- Bifidobacteria modulate cognitive processes in an anxious mouse strain. “these data suggest that B. longum 1714 had a positive impact on cognition and also that the effects of individual Bifidobacteria strains do not generalise across the species.”
- “We found a significant rise in both Lactobacillus and Bifidobacteria in those taking the Lactobacillus casei strain Shirota(Yakult), and there was also a significant decrease in anxiety symptoms among those taking the probiotic vs controls (p = 0.01).” 
See also this general article.
- Grape seed extract contains substantial amounts of flavonoids(resveratrol), vitamin E and linoleic acid, among other important constituents.
- Reduce refined carbohydrates, increase fiber.
- Reduce Fat and Sugar intake significantly
- Increased Vitamin C – especially from different fruits (increased diversity of diet)
- Increased use of turmeric (curcumin) in cooking, increased Indian dishes/meals.
- Probiotics – where possible, the precise species cited. Two of those cited were B. Longum – so any probiotic containing B. Longum may be worth trying.
- Yakult is available in many Asian supermarkets.
- If you have CFS, try to keep the ratio of Bifidobacteria to Lactobacillus below 3:1 (i.e. 25%)
For moderate adult coffee drinkers: (but not sugar enhanced energy drinks!)
- Caffeine increases food intake while reducing anxiety-related behaviors .
- Modulatory effects of caffeine on oxidative stress and anxiety-like behavior in ovariectomized rats . “caffeine had anxiolytic effects”