Are diet soft drinks and sweeteners bad for you, and if so, why?
Studies where people were assigned to a sweetener or comparison drink show that diet soft drinks at moderate consumption are not demonstrably more harmful than water, and are probably more favourable than sugary soft drinks when you swap one for the other. Open questions remain about gut bacteria, so drinking less soft drink overall remains the most defensible goal.
Diet soft drinks contain no sugar but do contain artificial sweeteners such as aspartame, sucralose, saccharin or stevia. In studies where people were assigned to receive either a sweetener or a comparison drink, sweeteners had no measurable effect on body weight or BMI. They did not demonstrably cause weight gain, but they did not help with weight loss either.
When researchers compared sweeteners with regular sugar in controlled studies, they did see benefits: fewer calories consumed, slightly lower body weight and body fat, and a modest reduction in blood pressure. So if you are choosing between a regular cola and a diet version, the evidence suggests the diet version is the better choice.
In studies where large groups of people were followed for years, researchers found that regular sweetener consumers more often had overweight, high blood pressure and type 2 diabetes. But this does not mean sweeteners caused those problems: people who already have health concerns simply tend to choose diet products more often. When researchers accounted for this, the negative picture largely disappeared.
If you drink a glass of diet soft drink without eating anything alongside it, nothing measurable changes in your blood sugar, insulin or hunger signals. Sweeteners then behave much like water. Animal studies and experiments with cells do show that artificial sweeteners can change the composition of gut bacteria, and in mice this worsened blood sugar control. In a small group of people, researchers saw a similar pattern. Whether this is relevant at normal levels of consumption in people is not yet clear.
The honest summary is this: diet soft drinks are not demonstrably harmful at moderate consumption, and are probably better than sugary soft drinks if you are swapping one for the other. But there are biological questions that remain unanswered, and keeping both sugar and sweeteners as low as possible remains the most defensible advice.
This answer draws on many studies combined, along with hundreds of thousands of participants followed over decades. The studies in which people were assigned to a sweetener or a comparison drink are the most reliable for cause-and-effect questions, but they are relatively short in duration. The large population studies are broader in scale but cannot establish cause and effect. The gut bacteria evidence comes largely from animal studies and experiments with cells; the human data come from a single small study. The researchers themselves rate the evidence on sweeteners from large population studies as very low quality, and the evidence from controlled studies as moderate quality.
How harmful is too little daylight or sunlight?
Too little sunlight is demonstrably harmful, especially for your bones. For broader disease risks the associations are plausible but not yet proven causes, and a daily dose of 5 to 30 minutes of sunlight is already sufficient for most people.
Are smoothies and fresh juice just as healthy as whole fruit?
Whole fruit comes out ahead of fruit juice: more fibre, greater satiety, and fewer free sugars. Smoothies fall somewhere in between, but are not equivalent to whole fruit either.
Are seed oils like sunflower oil bad for you?
Seed oils like sunflower oil are probably not outright harmful, but cannot be given an unconditional clean bill of health either: ideally, also make sure your diet includes enough omega-3-rich foods such as oily fish or flaxseed to keep the ratio in balance.
Are ultra-processed products bad for your longevity?
Eating a lot of ultra-processed products is associated with a shorter lifespan and more chronic disease, but the effect varies considerably by product type and largely disappears when your overall diet is good. Eating less processed meat and improving your overall eating pattern gives you the greatest benefit.
How bad is ultra-processed food, and what does it do to you?
Eating a lot of ultra-processed food is consistently associated with higher risks of cardiovascular disease, diabetes, depression and premature death. The evidence is observational and not always of high quality, but the associations are broad enough to make limiting intake a sensible choice.
Is coffee healthy or unhealthy?
For most healthy adults, three to four cups of filter coffee per day, without sugar and cream, does more good than harm. Pregnant women are a clear exception: limit intake and discuss it with your doctor.